Isotopes, Isobars & Isosters | Structure of Atom Class 11

Atomic Species & Terminology

Isotopes, Isobars, Isotones & More | Structure of Atom Class 11

1. Basic Notation

An atom is represented as $_Z^A\text{X}$:

• Z (Atomic Number): Number of Protons ($p$). ($p = e^-$ in neutral atom).
• A (Mass Number): Sum of Protons and Neutrons ($p + n$).
• Neutrons ($n$): $A - Z$.

2. Isotopes and Isobars

Isotopes

Atoms of the same element having the same Atomic Number ($Z$) but different Mass Number ($A$).

• Same: Chemical properties (same no. of electrons).
• Different: Physical properties (mass, density).
• Examples: $_1^1\text{H}$ (Protium), $_1^2\text{H}$ (Deuterium), $_1^3\text{H}$ (Tritium).

Isobars

Atoms of different elements having the same Mass Number ($A$) but different Atomic Number ($Z$).

• Different: Chemical and Physical properties.
• Examples: $_{18}^{40}\text{Ar}$ and $_{20}^{40}\text{Ca}$.

3. Isotones

Atoms of different elements having the same number of neutrons ($n = A - Z$).

$$ A_1 - Z_1 = A_2 - Z_2 $$

Example: $_6^{13}\text{C}$ and $_7^{14}\text{N}$.
Neutrons in C: $13-6=7$. Neutrons in N: $14-7=7$.

4. Isoelectronic Species

Atoms or ions containing the same number of electrons.

Example Series: $\text{N}^{3-}, \text{O}^{2-}, \text{F}^{-}, \text{Ne}, \text{Na}^+, \text{Mg}^{2+}$. (All have 10 electrons).

Size Trend: For isoelectronic species, size decreases as nuclear charge ($Z$) increases. $$ \text{Size: Anion} > \text{Neutral} > \text{Cation} $$

5. Advanced Terms (Isodiaphers & Isosters)

Isodiaphers

Atoms having the same difference between the number of neutrons and protons (isotopic number).

$$ n - p = \text{Constant} \quad \text{or} \quad A - 2Z = \text{Constant} $$

Example: $_{92}^{235}\text{U}$ and $_{90}^{231}\text{Th}$. (Often seen in $\alpha$-decay).

Isosters

Molecules or ions having the same number of atoms and the same number of valence electrons.

• Properties: Often exhibit similar physical properties.
• Example: $\text{CO}_2$ and $\text{N}_2\text{O}$.
  • Atoms: 3 in both.
  • Valence $e^-$: $\text{CO}_2 (4+6+6=16)$, $\text{N}_2\text{O} (5+5+6=16)$.

Practice Quiz

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