Questions: The Periodic Table and Atomic Electronic Structure

Question 1

Why are sodium (Na) and potassium (K) placed in the same group, and why do they have similar chemical properties?

Model answer: Both have a single s-electron outside a filled noble-gas core: Na is [Ne]3s¹ and K is [Ar]4s¹. Their valence electron configuration is the same type (one s-electron, relatively loosely held), so both metals readily lose that electron to form stable +1 cations, react vigorously with water, and form similar ionic compounds.

Group membership is determined by valence configuration, not total electrons. The key similarity is the single outer s-electron facing a shielded nuclear charge. K has that electron in a higher shell (farther from nucleus), so it is even more loosely held than Na's — potassium reacts more violently with water, consistent with lower ionization energy.

Question 2

Cobalt (Z=27) is in period 4 and the d-block. Write its electron configuration and explain which block and period it belongs to based on that configuration.

Model answer: Co: [Ar] 3d⁷ 4s². It belongs to period 4 (outer electrons are in n=4 shell: 4s²) and the d-block (the partially filled subshell being filled is 3d). It is element 9 in the d-block (3d⁷ means seven of the possible ten d-electrons are present, placing it in group 9).

Identifying block: look at which subshell type holds the last electron added (or the partially filled subshell): here it is 3d, so d-block. Identifying period: the outermost occupied principal quantum number shell is n=4 (the 4s electrons), so period 4. The d-block fills n−1 d-orbitals while n s-orbitals are also occupied, which is why transition metals are in the 4th period but fill 3d orbitals.