Simulation · Chemistry · Class 11
How big is an atom?
From the lesson Atomic and ionic radii in Classification of Elements and Periodicity in Properties. Change the values and watch what happens.
The idea behind it
NCERT §3.7.1(a), (b)
- Periodic trends: across a period reactivity is high in group 1, lower towards the middle and highest in group 17. Down the alkali metals reactivity rises; down the halogens it falls. Trends are explained through nuclear charge and energy levels.
- An atom is tiny (about 1.2 Å = 1.2 × 10⁻¹⁰ m) and its electron cloud has no sharp edge, so its radius is estimated from distances between bonded atoms.
- Covalent radius (non-metals): half the single-bond length, e.g. Cl–Cl in Cl₂ is 198 pm, so r(Cl) = 99 pm. Metallic radius: half the distance between neighbouring metal cores, e.g. Cu–Cu 256 pm, so r(Cu) = 128 pm. NCERT calls either one the atomic radius.
- Across a period the radius falls: electrons enter the same shell while effective nuclear charge rises and pulls them in. Period 2 (pm): Li 152, Be 111, B 88, C 77, N 74, O 66, F 64. Period 3: Na 186, Mg 160, Al 143, Si 117, P 110, S 104, Cl 99.
- Down a group the radius grows: n increases, and filled inner shells shield the outer electrons. Group 1 (pm): Li 152, Na 186, K 231, Rb 244, Cs 262. Group 17: F 64, Cl 99, Br 114, I 133, At 140.
- Noble gases are left out: they are monoatomic, so only non-bonded (van der Waals) radii exist, and these are much larger than covalent radii.
- A cation is smaller than its atom (fewer electrons, same nuclear charge); an anion is larger (extra repulsion, lower effective nuclear charge). F 64 pm but F⁻ 136 pm; Na 186 pm but Na⁺ 95 pm.
- Isoelectronic species have the same number of electrons, e.g. O²⁻, F⁻, Na⁺ and Mg²⁺ (10 each). The more protons, the smaller the ion: O²⁻ > F⁻ > Na⁺ > Mg²⁺.
- Among Mg, Mg²⁺, Al and Al³⁺ the largest is Mg and the smallest is Al³⁺.
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