Notes : Define Isotopes, Isobars and Isotones with Example - Param Himalaya

Isotopes, Isobars and Isotones

Isotopes

Definition

Isotopes are atoms of the same element having the same atomic number (Z) but different mass numbers (A). Therefore, isotopes have the same number of protons and electrons but different numbers of neutrons.

Example

Hydrogen has three isotopes:

$$^{1}_{1}\mathrm{H}$$

$$^{2}_{1}\mathrm{H}$$

$$^{3}_{1}\mathrm{H}$$

For Protium:

$$p = 1,\quad e = 1,\quad n = 1 - 1 = 0$$

For Deuterium:

$$p = 1,\quad e = 1,\quad n = 2 - 1 = 1$$

For Tritium:

$$p = 1,\quad e = 1,\quad n = 3 - 1 = 2$$

Observation: All isotopes have the same number of protons and electrons but different numbers of neutrons.

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Isobars

Definition

Isobars are atoms of different elements having the same mass number (A) but different atomic numbers (Z).

Example

Argon-40:

$$^{40}_{18}\mathrm{Ar}$$

Calcium-40:

$$^{40}_{20}\mathrm{Ca}$$

For Argon-40:

$$p = 18,\quad e = 18,\quad n = 40 - 18 = 22$$

For Calcium-40:

$$p = 20,\quad e = 20,\quad n = 40 - 20 = 20$$

Observation: Both atoms have the same mass number $$A = 40$$ but different numbers of protons, electrons and neutrons.

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Isotones

Definition

Isotones are atoms of different elements having the same number of neutrons but different atomic numbers and mass numbers.

Example

Carbon-14:

$$^{14}_{6}\mathrm{C}$$

Nitrogen-15:

$$^{15}_{7}\mathrm{N}$$

For Carbon-14:

$$p = 6,\quad e = 6,\quad n = 14 - 6 = 8$$

For Nitrogen-15:

$$p = 7,\quad e = 7,\quad n = 15 - 7 = 8$$

Observation: Both atoms contain $$n = 8$$ neutrons. Therefore, they are isotones.

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Important Formula

The number of neutrons in an atom is given by:

$$n = A - Z$$

where:

• n = Number of neutrons
• A = Mass number
• Z = Atomic number

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Key Points

• Isotopes have the same atomic number but different mass numbers.
• Isobars have the same mass number but different atomic numbers.
• Isotones have the same number of neutrons but different atomic numbers and mass numbers.
• Number of protons = Atomic Number (Z).
• Number of electrons = Atomic Number (Z) for a neutral atom.
• Number of neutrons = Mass Number − Atomic Number.

Quick Revision

Isotopes: Same Z, Different A

Isobars: Same A, Different Z

Isotones: Same Number of Neutrons

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Frequently Asked Questions (FAQs)

What are isotopes?

Isotopes are atoms of the same element having the same atomic number (Z) but different mass numbers (A).

Why do isotopes have different mass numbers?

Isotopes have different mass numbers because they contain different numbers of neutrons while having the same number of protons.

What are isobars?

Isobars are atoms of different elements having the same mass number (A) but different atomic numbers (Z).

What are isotones?

Isotones are atoms of different elements having the same number of neutrons but different atomic numbers and mass numbers.

How do we calculate the number of neutrons in an atom?

The number of neutrons is calculated using the formula: $$n = A - Z$$ where A is the mass number and Z is the atomic number.

Give one example each of isotopes, isobars and isotones.

Isotopes: $$^{1}_{1}\mathrm{H},\;^{2}_{1}\mathrm{H},\;^{3}_{1}\mathrm{H}$$

Isobars: $$^{40}_{18}\mathrm{Ar},\;^{40}_{20}\mathrm{Ca}$$

Isotones: $$^{14}_{6}\mathrm{C},\;^{15}_{7}\mathrm{N}$$