# Caesium – Atomic Number – Atomic Mass – Density of Caesium

## Atomic Number of Caesium

Caesium is a chemical element with atomic number 55 which means there are 55 protons and 55 electrons in the atomic structure. The chemical symbol for Caesium is Cs.

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 Caesium

Atomic mass of Caesium is 132.9055 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 Caesium

Density of Caesium is 1.879g/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).

## Caesium – Properties Summary

Element Caesium
Atomic Number 55
Symbol Cs
Element Category Alkali Metal
Phase at STP Solid
Atomic Mass [amu] 132.9055
Density at STP [g/cm3] 1.879
Electron Configuration [Xe] 6s1
Possible Oxidation States +1
Electron Affinity [kJ/mol] 45.5
Electronegativity [Pauling scale] 0.79
1st Ionization Energy [eV] 3.8939
Year of Discovery 1860
Discoverer Kirchhoff, Gustav & Bunsen, Robert
Thermal properties
Melting Point [Celsius scale] 28.4
Boiling Point [Celsius scale] 669
Thermal Conductivity [W/m K] 36
Specific Heat [J/g K] 0.24
Heat of Fusion [kJ/mol] 2.092
Heat of Vaporization [kJ/mol] 67.74