Question

${}_{12}{}^{24}\mathrm{Mg}$ and ${}_{12}{}^{26}\mathrm{Mg}$ are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to:

(i) the composition of their nuclei.

(ii) their atomic configuration.

(iii) give the reason why the two isotopes of magnesium have different mass numbers.

Answer 1

• 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.
• Isotopes- Atoms of an element having the same atomic number but a different mass number are known as isotopes.
• The atoms are written in this manner ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ where A denotes mass number and Z denotes the atomic number.
• Number of neutrons = Mass number - atomic number = A - Z

(i)

• ${}_{12}{}^{24}\mathrm{Mg}$ denotes atomic number = 12, mass number = 24
• Number of neutrons = 24-12 = 12
• ${}_{12}{}^{26}\mathrm{Mg}$ denotes atomic number = 12, mass number = 26
• Number of neutrons = 26-12 = 14
• Hence, the composition of the nuclei of both atoms is-

Isotopes-	Number of protons	Number of neutrons
${}_{12}{}^{24}\mathrm{Mg}$	12	12
${}_{12}{}^{26}\mathrm{Mg}$	12	14

(ii)

• Since both the atoms have an equal number of protons, they have the same atomic number.
• Atomic configuration is written based on the number of electrons.
• Both the atoms are neutral.
• Hence, they have equal numbers of electrons i.e.12.
• Atomic configuration for both will be 2,8,2.

(iii)

• The two isotopes of magnesium have different mass numbers because they have different numbers of neutrons in their atoms.