Material Properties

The original unit for measuring the amount of radioactivity was the curie (symbol Ci), which is a non-SI unit of radioactivity defined in 1910. Rutherford (symbol Rd) is also a non-SI unit defined as the activity of a quantity of radioactive material in which one million nuclei decay per second. This unit was introduced in 1946, but after the becquerel was introduced in 1975, the rutherford became obsolete.

Currently, one rutherford is equivalent to one megabecquerel:

1Rd = 1 x 10⁶ Bq = 1 MBq

One rutherford is quite large amount of activity. The typical human body contains roughly 0.0037 Rd (14 mg) of naturally occurring potassium-40. As well, a human body containing 16 kg of carbon would also have about 0.0037 Rd of carbon-14 (24 nanograms).

Rutherford – Examples

The relationship between half-life and the amount of a radionuclide required to give an activity of one rutherford is shown in the figure. This amount of material can be calculated using λ, which is the decay constant of certain nuclide:

The following figure illustrates the amount of material necessary for 37 kilorutherford of radioactivity. It is obvious, that the longer the half-life, the greater the quantity of radionuclide needed to produce the same activity. Of course, the longer lived substance will remain radioactive for a much longer time. As can be seen, the amount of material necessary for 37 kRd of radioactivity can vary from an amount too small to be seen (0.00088 gram of cobalt-60), through 1 gram of radium-226, to almost three tons of uranium-238.

Example – Calculation of Radioactivity

A sample of material contains 1 mikrogram of iodine-131. Note that, iodine-131 plays a major role as a radioactive isotope present in nuclear fission products, and it a major contributor to the health hazards when released into the atmosphere during an accident. Iodine-131 has a half-life of 8.02 days.

Calculate:

1. The number of iodine-131 atoms initially present.
2. The activity of the iodine-131 in curies.
3. The number of iodine-131 atoms that will remain in 50 days.
4. The time it will take for the activity to reach 0.1 mCi.

Solution:

1. The number of atoms of iodine-131 can be determined using isotopic mass as below.

N_I-131 = m_I-131 . N_A / M_I-131

N_I-131 = (1 μg) x (6.02×10²³ nuclei/mol) / (130.91 g/mol)

N_I-131 = 4.6 x 10¹⁵ nuclei

2. The activity of the iodine-131 in curies can be determined using its decay constant:

The iodine-131 has half-live of 8.02 days (692928 sec) and therefore its decay constant is:

Using this value for the decay constant we can determine the activity of the sample:

3) and 4) The number of iodine-131 atoms that will remain in 50 days (N_50d) and the time it will take for the activity to reach 0.1 mCi can be calculated using the decay law:

As can be seen, after 50 days the number of iodine-131 atoms and thus the activity will be about 75 times lower. After 82 days the activity will be approximately 1200 times lower. Therefore, the time of ten half-lives (factor 2¹⁰ = 1024) is widely used to define residual activity.

See also:

Units of Radioactivity

Stable Nuclei – Unstable Nuclei

A nuclear stability is determined by the competition between two fundamental interactions. Atomic nuclei consist of protons and neutrons, which attract each other through the nuclear force, while protons repel each other via the electromagnetic force due to their positive charge. These two forces compete, leading to various stability of nuclei. There are only certain combinations of neutrons and protons, which forms stable nuclei. Neutrons stabilize the nucleus, because they attract each other and protons , which helps offset the electrical repulsion between protons. As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus. If there are too many (neutrons also obey the Pauli exclusion principle) or too few neutrons for a given number of protons, the resulting nucleus is not stable and it undergoes radioactive decay. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture. Many other rare types of decay, such as spontaneous fission or neutron emission are known.

The Pauli exclusion principle also influences the critical energy of fissile and fissionable nuclei. For example, actinides with odd neutron number are usually fissile (fissionable with slow neutrons) while actinides with even neutron number are usually not fissile (but are fissionable with fast neutrons). Heavy nuclei with an even number of protons and an even number of neutrons are (due to Pauli exclusion principle) very stable thanks to the occurrence of ‘paired spin’. On the other hand, nuclei with an odd number of protons and neutrons are mostly unstable.

Magic Numbers of Protons and Neutrons

A magic number is a number of nucleons in a nucleus, which corresponds to complete shells within the atomic nucleus. Atomic nuclei consisting of such a magic number of nucleons have a higher average binding energy per nucleon than one would expect based upon predictions such as the mass formula of von Weizsaecker (also called the semi-empirical mass formula – SEMF) and are hence more stable against nuclear decay. Magic numbers are predicted by the nuclear shell model and are proved by observations that have shown that there are sudden discontinuities in the proton and neutron separation energies at specific values of Z and N. These correspond to the closing of shells (or sub-shells). Nuclei with closed shells are more tightly bound than the next higher number. The closing of shells occurs at Z or N = 2, 8, 20, 28, (40), 50, 82, 126. It is found that nuclei with even numbers of protons and neutrons are more stable than those with odd numbers. Nuclei which have both neutron number and proton number equal to one of the magic numbers can be called “doubly magic“, and are found to be particularly stable.There are further special propertis of nuclei, which have a magic number of nucleons:

1. Higher abundance in nature. For example, helium-4 is among the most abundant (and stable) nuclei in the universe.
2. The stable elements at the end of the decay series all have a “magic number” of neutrons or protons. The nuclei He-4, O-16, and Pb-208 (82 protons and 126 neutrons) that contain magic numbers of both neutrons and protons are particularly stable. The relative stability of these nuclei is reminiscent of that of inert gas atoms (closed electron shells).
3. Nuclei with N = magic number have much lower neutron absorption cross-sections than surrounding isotopes.
4. These nuclei appear to be perfectly spherical in shape; they have zero quadrupole electric moments.
5. Magic number nuclei have higher first excitation energy.

Unstable Nuclei – Decay Modes

Nuclear decay (Radioactive decay) occurs when an unstable atom loses energy by emitting ionizing radiation. Radioactive decay is a random process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. During radioactive decay an unstable nucleus spontaneosly and randomly decomposes to form a different nucleus (or a different energy state – gamma decay), giving off radiation in the form of atomic partices or high energy rays. This decay occurs at a constant, predictable rate that is referred to as half-life. A stable nucleus will not undergo this kind of decay and is thus non-radioactive. There are many modes of radioactive decay:

• Alpha radioactivity. Alpha decay is the emission of alpha particles (helium nuclei). Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it heavy ionizes material and has a very short range.
• Beta radioactivity. Beta decay is the emission of beta particles. Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles have greater range of penetration than alpha particles, but still much less than gamma rays.The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay.
• Gamma radioactivity. Gamma radioactivity consist of gamma rays. Gamma rays are electromagnetic radiation (high energy photons) of an very high frequency and of a high energy. They are produced by the decay of nuclei as they transition from a high energy state to a lower state known as gamma decay. Most of nuclear reactions are accompanied by gamma emission.
• Neutron emission. Neutron emission is a type of radioactive decay of nuclei containing excess neutrons (especially fission products), in which a neutron is simply ejected from the nucleus. This type of radiation plays key role in nuclear reactor control, because these neutrons are delayed  neutrons.

The rate of nuclear decay is also measured in terms of half-lives. The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. Half lives range from millionths of a second for highly radioactive fission products to billions of years for long-lived materials (such as naturally occurring uranium). Notice that short half lives go with large decay constants. Radioactive material with a short half life is much more radioactive (at the time of production) but will obviously lose its radioactivity rapidly. No matter how long or short the half life is, after seven half lives have passed, there is less than 1 percent of the initial activity remaining.

See also:

Atomic Nucleus

Antiatom

Antimatter is a term referring to material that would be made up of “antiatoms” in which antiprotons and antineutrons would form the nucleus around which positrons (antielectrons) would move. The term is also used for antiparticles in general.

Antimatter particles bind with one another to form antimatter, just as ordinary particles bind to form normal matter. For example, a positron and an antiproton can form an antihydrogen atom. Physical principles indicate that complex antimatter atomic nuclei are possible, as well as anti-atoms corresponding to the known chemical elements.

Our visible Universe is almost entirely composed of matter, and very little antimatter has existed since the Big Bang. This problem is known as the baryon asymmetry. This asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics.

In special theory of relativity certain types of matter may be created or destroyed, but in all of these processes, the mass and energy associated with such matter remains unchanged in quantity. As a result of the conservation of lepton and baryon numbers,  antimatter (antiparticles) can be created out of energy, but only if a particle counterpart for every antiparticle is created as well.

See also: Matter – Antimatter Creation and Annihilation

See also:

Atom

Volume of Atom

The volume of an atom is about 15 orders of magnitude larger than the volume of a nucleus. For uranium atom, the Van der Waals radius is about 186 pm = 1.86 ×10⁻¹⁰m. The Van der Waals radius, r_w, of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom.  Assuming spherical shape, the uranium atom have volume of about  26.9 ×10⁻³⁰ m³. But this “huge” space is occupied primarily by electrons, because the nucleusoccupies only about 1721×10⁻⁴⁵ m³ of space. These electrons together weigh only a fraction (let say 0.05%) of entire atom.

It may seem, that the space and in fact the matter is empty, but it is not. Due to the quantum nature of electrons, the electrons are not point particles, they are smeared out over the whole atom. The classical description cannot be used to describe things on the atomic scale. On the atomic scale, physicists have found that quantum mechanics describes things very well on that scale. Particle locations in quantum mechanics are not at an exact position, they are described by a probability density function. Therefore the space in an atom (between electrons and an atomic nucleus) is not empty, but it is filled by a probability density function of electrons (usually known as  “electron cloud“).

See also:

Atom

Atomic Theory

As was written, all matter except dark matter is made of molecules, which are themselves made of atoms. The atoms are the smallest constituents of ordinary matter, which can be divided without the release of electrically charged particles. The atoms consist of two parts. An atomic nucleus and an electron cloud. The electrons are spinning around the atomic nucleus. The nucleus itself is generally made of protons and neutrons.

This model is today quite clear and proven. But the path to this model was complex and full of many surprising findings. A scientific theory, which deals with the nature of matter is known as atomic theory. Atomic theory began as a philosophical concept in ancient Greece and India. It must be added, atomism was one of a number of competing theories on the nature of matter.  These ancient philosophers speculated that the earth was made up of different combinations of basic substances, or elements.

The word “atom” was coined by the ancient Greek philosophers Leucippus, Epicurus and Democritus. They supposed that the properties of materials were determined by the different shapes and forms of their atoms. This word comes from the Ancient Greek adjective atomos, meaning “indivisible“.

Other ancient Greek philosophers (Empedocles, Heraclitus and Aristotle) considered these basic elements to be earth, air, water, and fire. According to their philosophy, lead differs from gold only in the proportions of the these four elements it contains and so the problem of converting lead into gold is just a matter of correctly blending the right combination of elements. Unlike the atomism of Democritus and Epicurus, the Aristotelian “natural minimum” was not conceptualized as physically indivisible.

Modem ideas about the structure of matter originated in the 17th century. In 1661 the English chemist Robert Boyle laid down the modern criterion of an element published the modern criterion for an element:

“a basic substance that cannot be broken down into any simpler substance after it is isolated from a compound, but can be combined with other elements to form compounds”

To date, 105 different elements have been confirmed to exist. Of the 105 confirmed elements, 90 exist in nature and 15 are man-made.

See also:

Atomic Physics

Atomism

Atomic theory began as a philosophical concept in ancient Greece and India. It must be added, atomism was one of a number of competing theories on the nature of matter.  These ancient philosophers speculated that the earth was made up of different combinations of basic substances, or elements.

The word “atom” was coined by the ancient Greek philosophers Leucippus, Epicurus and Democritus. They supposed that the properties of materials were determined by the different shapes and forms of their atoms. This word comes from the Ancient Greek adjective atomos, meaning “indivisible“.

Other ancient Greek philosophers (Empedocles, Heraclitus and Aristotle) considered these basic elements to be earth, air, water, and fire. According to their philosophy, lead differs from gold only in the proportions of the these four elements it contains and so the problem of converting lead into gold is just a matter of correctly blending the right combination of elements. Unlike the atomism of Democritus and Epicurus, the Aristotelian “natural minimum” was not conceptualized as physically indivisible.

Modem ideas about the structure of matter originated in the 17th century. In 1661 the English chemist Robert Boyle laid down the modern criterion of an element published the modern criterion for an element:

“a basic substance that cannot be broken down into any simpler substance after it is isolated from a compound, but can be combined with other elements to form compounds”

To date, 105 different elements have been confirmed to exist. Of the 105 confirmed elements, 90 exist in nature and 15 are man-made.

See also:

Atomic Theory

Dalton’s Theory

Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory.

• Law of Conservation of Matter. Historically, already the ancient Greeks proposed the idea that the total amount of matter in the universe is constant. The principle of conservation of mass was first outlined by Mikhail Lomonosov in 1748. However, the law of conservation of matter (or the principle of mass/matter conservation) as a fundamental principle of physics was discovered in by Antoine Lavoisier in the late 18th century. It was of great importance in progressing from alchemy to modern chemistry.
• Law of Definite Proportions. This law (proven by the French chemist Joseph Louis Proust) states that a given chemical compound always contains its component elements in fixed ratio, regardless of the quantity or source of the original substance. John Dalton studied and expanded upon this previous work and developed the law of multiple proportions.

In the early 19th century atomic theory entered the scientific mainstream when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of atoms. The modern proof for the atomic nature of matter was first proposed by the English chemist John Dalton. His theory shows that the early Greeks held the correct concept that matter consists of a combination of basic elements.

In 1803 John Dalton published the first comprehensive atomic theory in A New System of Chemical Philosophy. He proposed that the elements are comprised of indestructible atoms, each chemical element possesses a particular kind of atom and all the atoms of a particular element being identical. These atoms can combine to form more complex structures (chemical compounds or molecules).

See also:

Atomic Theory

Thomson Model of the Atom – Plum Pudding Model

For almost 100 years (from Dalton’s A New System of Chemical Philosophy) atoms were thought to be the smallest possible division of matter. All of the results of chemical experiments during this time indicated that the atom was indivisible. Eventually, experimentation into electricity and radioactivity indicated that particles of matter smaller than the atom did indeed exist.

In 1897, an English physicist J.J. Thomson showed that cathode rays were composed of previously unknown negatively charged particles, which he calculated must have bodies much smaller than atoms and a very large value for their charge-to-mass ratio. He showed that these charged particles could be obtained from just about any material. For example, by the action of ultra-violet light on metals, by heating metal wires or by the ionising action of X-rays. Thomson originally called these particles “corpuscles,” and the name electron was proposed for these particles by the Irish physicist George Johnstone Stoney. It was evident that electrons are a component of all neutral atoms, the counterbalancing positive charge being carried by one or more of their other constituent parts. Thomson thus concluded that atoms are divisible, and that the corpuscles are their building blocks.

In 1906, J. J. Thompson won the Nobel Prize in physics for establishing the existence of electrons. According to current state of knowledge, the electrons are negatively charged (-1e), almost massless particles that nevertheless account for most of the size of the atom. Their rest mass equal to 9.109 × 10⁻³¹ kg (510.998 keV/c²) (approximately 1/1836 that of the proton). Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom.

But the original idea was different. The original model of an atom, based on the discovery of the electron, was proposed by J. J. Thomson in 1904 and is known as the Plum pudding model or the Thomson model of the atom. It must be noted, this model was proposed before the discovery of the atomic nucleus.

The Plum pudding model represented an attempt to consolidate the known properties of atoms at the time:

1) Electrons are negatively-charged particles

2) Atoms are neutrally-charged.

According to this model, an atom consist of a sphere of positive matter within which electrostatic forces determined the positioning of the negatively charged corpuscles. This fact explains the overall neutral charge of the atom, Thomson proposed that the corpuscles were distributed in a uniform sea of positive charge, in which they are scattered like “the plums in a pudding”.

The plum pudding model usefully inspired his student, Ernest Rutherford, to further explore the composition of atoms. As well, the Thomson model of the atom were useful predecessors of the more correct Rutherford model of the atom and especially the solar-system-like Bohr model of the atom. There were several problems, that could not be explained by this model. The most important experiments, which lead to the need to formulate another model, was the Geiger-Marsden Experiments (also called the Rutherford gold foil experiment).

See also:

Atomic Theory

Discovery of Electron

For almost 100 years (from Dalton’s A New System of Chemical Philosophy) atoms were thought to be the smallest possible division of matter. All of the results of chemical experiments during this time indicated that the atom was indivisible. Eventually, experimentation into electricity and radioactivity indicated that particles of matter smaller than the atom did indeed exist.

In 1897, an English physicist J.J. Thomson showed that cathode rays were composed of previously unknown negatively charged particles, which he calculated must have bodies much smaller than atoms and a very large value for their charge-to-mass ratio. He showed that these charged particles could be obtained from just about any material. For example, by the action of ultra-violet light on metals, by heating metal wires or by the ionising action of X-rays. Thomson originally called these particles “corpuscles,” and the name electron was proposed for these particles by the Irish physicist George Johnstone Stoney. It was evident that electrons are a component of all neutral atoms, the counterbalancing positive charge being carried by one or more of their other constituent parts. Thomson thus concluded that atoms are divisible, and that the corpuscles are their building blocks.

In 1906, J. J. Thompson won the Nobel Prize in physics for establishing the existence of electrons. According to current state of knowledge, the electrons are negatively charged (-1e), almost massless particles that nevertheless account for most of the size of the atom. Their rest mass equal to 9.109 × 10⁻³¹ kg (510.998 keV/c²) (approximately 1/1836 that of the proton). Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom.

But the original idea was different. The original model of an atom, based on the discovery of the electron, was proposed by J. J. Thomson in 1904 and is known as the Plum pudding model or the Thomson model of the atom. It must be noted, this model was proposed before the discovery of the atomic nucleus.

The Plum pudding model represented an attempt to consolidate the known properties of atoms at the time:

1) Electrons are negatively-charged particles

2) Atoms are neutrally-charged.

According to this model, an atom consist of a sphere of positive matter within which electrostatic forces determined the positioning of the negatively charged corpuscles. This fact explains the overall neutral charge of the atom, Thomson proposed that the corpuscles were distributed in a uniform sea of positive charge, in which they are scattered like “the plums in a pudding”.

See also:

Atomic Theory

Rutherford Model of the Atom

The Rutherford model of the atom is a model of the atom devised by the British physicist Ernest Rutherford. Rutherford’s new model for the atom is based on the experimental results, which were obtained from Geiger-Marsden experiments (also called the Rutherford gold foil experiment). The Geiger–Marsden experiments were performed between 1908 and 1913 by Hans Geiger (of Geiger counter fame) and Ernest Marsden (a 20-year-old student who had not yet earned his bachelor’s degree) under the direction of Ernest Rutherford.

Rutherford’s idea was to direct energetic alpha particles at a thin metal foil and measure how an alpha particle beam is scattered when it strikes a thin metal foil. A narrow collimated beam of alpha particles was aimed at a gold foil of approximately 1 μm thickness (about 10,000 atoms thick).  Alpha particles are energetic nuclei of helium (usually about 6 MeV). Alpha particles, which are about 7300 times more massive than electrons, have a positive charge of +2e. Because of their relatively much greater mass, alpha particles are not significantly deflected from their paths by the electrons in the metal’s atoms.

According to Thomson’s model, if an alpha particle were to collide with an plum-pudding atom, it would just fly straight through, its path being deflected by at most a fraction of a degree. But in the experiment Geiger and Marsden saw that most of the particles are scattered through rather small angles, but, and this was the big surprise, a very small fraction of them are scattered through very large angles, approaching 180° (i.e., they were recoiled backwards).

In Rutherford’s words:

“It is almost as incredible as if you had fired a fifteen inch shell at a sheet of tissue paper and it came back and hit you.”

Rutherford assumed that, to deflect the alpha particle backward, there must be a very large force. This force could be provided only result from a collision with a massive target or from an interaction with an electric or magnetic field of great strength. In previous experiments, it was shown the deflections had to be electrical or perhaps possibly magnetic in origin.

As a result, Rutherford abandoned the Thomson model. Geiger-Marsden experiments were a landmark series of experiments by which scientists discovered that every atom contains a nucleus (whose diameter is of the order 10^-14m.) where all of its positive charge and most of its mass are concentrated in a small region called an atomic nucleus. In Rutherford’s atom, the diameter of its sphere (about 10^-10 m) of influence is determined by its electrons. In other words, the nucleus occupies only about 10^-12 of the total volume of the atom or less (the nuclear atom is largely empty space), but it contains all the positive charge and at least 99.95% of the total mass of the atom.

Based on these results, Ernest Rutherford proposed a new model of the atom. He postulated that the positive charge in an atom is concentrated in a small region (in comparison to the rest of the atom) called a nucleus at the center of the atom with electrons existing in orbits around it. Furthermore, the nucleus is responsible for most of the mass of the atom.

Is an atom an empty space?

The volume of an atom is about 15 orders of magnitude largerthan the volume of a nucleus. For uranium atom, the Van der Waals radius is about 186 pm = 1.86 ×10⁻¹⁰ m. The Van der Waals radius, r_w, of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom.  Assuming spherical shape, the uranium atom have volume of about  26.9 ×10⁻³⁰ m³. But this “huge” space is occupied primarily by electrons, because the nucleus occupies only about 1721×10⁻⁴⁵ m³ of space. These electrons together weigh only a fraction (let say 0.05%) of entire atom.

It may seem, that the space and in fact the matter is empty, but it is not. Due to the quantum nature of electrons, the electrons are not point particles, they are smeared out over the whole atom. The classical description cannot be used to describe things on the atomic scale. On the atomic scale, physicists have found that quantum mechanics describes things very well on that scale. Particle locations in quantum mechanics are not at an exact position, they are described by a probability density function. Therefore the space in an atom (between electrons and an atomic nucleus) is not empty, but it is filled by a probability density function of electrons (usually known as  “electron cloud“).

Failure of Rutherford Model

It must be noted, the Rutherford model of the atom was not the first model, which proposed a nuclear structure. The idea of a nuclear structure was previously proposed in 1903 by a Japanese physicist Hantaro Nagaoka, who rejected Thomson’s model on the grounds that opposite charges are impenetrable.His model was known as the Saturnian model of the atom and according to this model the atom consists of a massive positive centre surrounded by a number of orbiting electrons, in the manner of Saturn and its rings.

Nagaoka’s planetary model had two predictions:

• a very massive atomic center (in analogy to a very massive planet)
• electrons revolving around the nucleus, bound by electrostatic forces

Both predictions were successfully confirmed by Ernest Rutherford in 1911, however, other details of the model was incorrect. Notwithstanding the Rutherford model of the atom was really very close to modern concept of the atom, it was based on classical physics. But neither classical physics nor the Rutherford model can explain the following problem. Why the negatively charged electrons are prevented from falling into the positively charged nucleus? Forces between static electrically charged particles are governed by the Coulomb’s law:

Rutherford suggested that perhaps the electrons revolve in orbits about the nucleus. In classical mechanics, a condition for the dynamic stability of such systems of rotating particles is that only forces of attraction act between its components.

Classical physics also states that any accelerating electric charge (either oscillating or revolving) emits electromagnetic radiation. As a result, an electron orbiting inside an atom should be continually losing energy and it should spiral into the nucleus within a fraction of a second. In fact, physicists calculated that the electron should lose all of its energy and spiral down into the proton in only about 0.000000000001 second, which means that every atoms should not exist longer than a mere 10^-12 seconds.

The solution to the problem of the stability of the electrons in an atom was provided by young Danish physicist Niels Bohr working with Ernest Rutherford at the University of Manchester in 1913. His model adopted Planck’s quantum hypothesis and he proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits. He postulated that an atom emits or absorbs energy only in discrete quanta corresponding to absorption or radiation of a photon.

See also:

Atomic Theory