Question

Ordinary chlorine gas has two isotopes: ${}_{17}{}^{35}\mathrm{Cl}$, and ${}_{17}{}^{35}\mathrm{Cl}$ in the ratio 3:1.

Calculate the relative atomic mass (atomic weight) of chlorine.

Answer 1

• Atomic mass- Mass of an atomic particle is called the atomic mass.
• This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (amu).
• 1 amu = 1.66 ×10⁻²⁴ g
• Most of the elements have two or more isotopes.
• The atomic weight of such elements is an average of the mass numbers of various isotopes.
• Isotopes- Atoms of an element having the same atomic number but a different mass number are known as isotopes.
• Atomic number(Z)- The total number of protons in the nucleus of an atom gives us the atomic number of that atom.
• Mass number(A)- The number of protons and neutrons combine to give us the mass number of an atom.

Chlorine(Cl)-

• Chlorine has two isotopes ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{35}\mathrm{Cl}$ which are present in the ratio of 3:1.
• The atoms are written in this manner ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ where A denotes mass number and Z denotes the atomic number.
• Hence, the atomic mass of chlorine is the average of the chlorine atoms, considering its ratio.

$Atomicmass=\frac{3\times 35+1\times 37}{4}=35.5amu$