# Selenium – Atomic Number – Atomic Mass – Density of Selenium

## Atomic Number of Selenium

Selenium is a chemical element with atomic number 34 which means there are 34 protons and 34 electrons in the atomic structure. The chemical symbol for Selenium is Se.

Since the number of electrons is responsible for the chemical behavior of atoms, the atomic number identifies the various chemical elements.

How does the atomic number determine the chemical behavior of atoms?

## Atomic Mass of Selenium

Atomic mass of Selenium is 78.96 u.

Note that each element may contain more isotopes. Therefore this resulting atomic mass is calculated from naturally-occurring isotopes and their abundance.

The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10-24 grams. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.

For 12C, the atomic mass is exactly 12u, since the atomic mass unit is defined from it. For other isotopes, the isotopic mass usually differs and is usually within 0.1 u of the mass number. For example, 63Cu (29 protons and 34 neutrons) has a mass number of 63, and an isotopic mass in its nuclear ground state is 62.91367 u.

There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:

1. The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
2. The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has lower total energy, and therefore a lower mass according to Einstein’s mass-energy equivalence relation E = mc2. For 63Cu, the atomic mass is less than 63, so this must be the dominant factor.

The atomic mass number determines especially the atomic mass of atoms. The mass number is different for each different isotope of a chemical element.

How does the atomic mass determine the density of materials?

## Density of Selenium

Density of Selenium is 4.819g/cm3.

Typical densities of various substances 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

In other words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).

## Selenium – Properties Summary

Element Selenium
Atomic Number 34
Symbol Se
Element Category Non Metal
Phase at STP Solid
Atomic Mass [amu] 78.96
Density at STP [g/cm3] 4.819
Electron Configuration [Ar] 3d10 4s2 4p4
Possible Oxidation States +4,6/-2
Electron Affinity [kJ/mol] 195
Electronegativity [Pauling scale] 2.55
1st Ionization Energy [eV] 9.7524
Year of Discovery 1817
Discoverer Berzelius, Jöns Jacob
Thermal properties
Melting Point [Celsius scale] 221
Boiling Point [Celsius scale] 685
Thermal Conductivity [W/m K] 2.04
Specific Heat [J/g K] 0.32
Heat of Fusion [kJ/mol] 6.694
Heat of Vaporization [kJ/mol] 37.7