Selenium is a nonmetal with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic. It rarely occurs in its elemental state or as pure ore compounds in the Earth’s crust.
Summary
| Element | Selenium |
| Atomic number | 34 |
| Atomic mass [amu] | 78.96 |
| Atomic mass [pm] | 120 |
| Density at STP [g/cm3] | 4.819 |
| Number of protons | 34 |
| Number of neutrons (typical isotopes) | 74; 76; 77; 78; 80 |
| Number of electrons | 34 |
| Electron configuration | [Ar] 3d10 4s2 4p4 |
| Oxidation states | +4,6/-2 |
| Electron affinity [kJ/mol] | 195 |
| Electronegativity [Pauling scale] | 2.55 |
| First ionization energy [eV] | 9.7524 |
Atomic Number – Protons, Electrons and Neutrons in Selenium
Selenium is a chemical element with atomic number 34 which means there are 34 protons in its nucleus. Total number of protons in the nucleus is called the atomic number of the atom and is given the symbol Z. The total electrical charge of the nucleus is therefore +Ze, where e (elementary charge) equals to 1,602 x 10-19 coulombs.
The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Neutron number plus atomic number equals atomic mass number: N+Z=A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N – Z = A – 2Z.
For stable elements, there is usually a variety of stable isotopes. Isotopes are nuclides that have the same atomic number and are therefore the same element, but differ in the number of neutrons. Mass numbers of typical isotopes of Selenium are 74; 76; 77; 78; 80.
Atomic Mass of Selenium
Atomic mass of Selenium is 78.96 u.
The atomic mass is the mass of an atom. The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance.
Atomic Radius of Selenium
The atomic radius of Selenium atom is 120pm (covalent radius).
It must be noted, atoms lack a well-defined outer boundary. The atomic radius of a chemical element is a measure of the distance out to which the electron cloud extends from the nucleus. However, this assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. Therefore, there are various non-equivalent definitions of atomic radius.
Electrons and Electron Configuration
The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. Therefore, the number of electrons in neutral atom of Selenium is 34. Each electron is influenced by the electric fields produced by the positive nuclear charge and the other (Z – 1) negative electrons in the atom.
Since the number of electrons and their arrangement are responsible for the chemical behavior of atoms, the atomic number identifies the various chemical elements. The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element’s electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. In the periodic table, the elements are listed in order of increasing atomic number Z.
Electron configuration of Selenium is [Ar] 3d10 4s2 4p4.
Possible oxidation states are +4,6/-2.
Density of Selenium
Density of Selenium is 4.819g/cm3.
Typical densities of various substances are at atmospheric pressure.
Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
Atomic Masses of Elements
Atomic Radii of Elements
Densities of Elements
Electron Affinity – Selenium
Electron affinity of Selenium is 195 kJ/mol.
In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as:
the change in energy (in kJ/mole) of a neutral atom or molecule (in the gaseous phase) when an electron is added to the atom to form a negative ion.
X + e– → X– + energy Affinity = – ∆H
In other words, it can be expressed as the neutral atom’s likelihood of gaining an electron. Note that, ionization energies measure the tendency of a neutral atom to resist the loss of electrons. Electron affinities are more difficult to measure than ionization energies.
Electronegativity of Selenium
Electronegativity of Selenium is 2.55.
Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract electrons towards this atom. For this purposes, a dimensionless quantity the Pauling scale, symbol χ, is the most commonly used.
The electronegativity of Selenium is: χ = 2.55
First Ionization Energy of Selenium
First Ionization Energy of Selenium is 9.7524 eV.
Ionization energy, also called ionization potential, is the energy necessary to remove an electron from the neutral atom.
X + energy → X+ + e−
where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed (positive ion), and e− is the removed electron.
A Selenium atom, for example, requires the following ionization energy to remove the outermost electron.
Se + IE → Se+ + e− IE = 9.7524 eV
Electronegativity of Elements
Ionization Energy of Elements
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Other properties of Selenium
