Lesson 1 of 13 · 6 min

Position and electronic configuration

NCERT §4.1; §4.2

Meera's first question is simple: where does iron sit, and what makes it and its neighbours a family? The periodic table on the lab wall has a wide middle block and two loose rows at the bottom.

The story this chapter follows: Meera's iron-tablet week

Meera, a Class 12 student, is curious about the iron tablets her grandmother takes, so she spends a week with them in the school lab. She dissolves one tablet in dilute sulphuric acid and makes it up to 100 mL. A 25.0 mL portion needs 5.80 mL of 0.0100 M potassium permanganate to turn pink. Around her are a shelf of coloured salt solutions, a strong magnet, a steel spatula, a gas-lighter flint and a jar of dark sand from a Kerala beach.

The lesson in notes

In short

The d-block is groups 3 to 12, where the d orbitals of the second-outermost shell fill. The f-block is the separate two-row panel at the foot of the table, where 4f and 5f orbitals fill. The elements are called transition and inner transition metals.

Four transition series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, then Hf to Hg) and 6d (Ac, then Rf to Cn). The 4f series (Ce to Lu) are the lanthanoids and the 5f series (Th to Lr) the actinoids.

IUPAC definition: a transition metal has an incomplete d subshell in the neutral atom or in its ions. Zn, Cd and Hg keep a full d¹⁰ set in the ground state and in their usual oxidation states, so they are not transition metals, though they are studied with them as the last members of each series.

Outer configuration in general: (n−1)d¹⁻¹⁰ ns¹⁻². Palladium is the exception to the ns part: 4d¹⁰ 5s⁰.

Because the (n−1)d and ns levels are so close in energy, half-filled and filled d sets win: Cr is 3d⁵ 4s¹ (not 3d⁴ 4s²) and Cu is 3d¹⁰ 4s¹ (not 3d⁹ 4s²).

Zn, Cd, Hg and Cn share the configuration (n−1)d¹⁰ ns².

Scandium (3d¹ 4s²) counts as a transition element because its atom has an incomplete 3d set; zinc does not, because it is d¹⁰ both as an atom and as Zn²⁺. Silver is 4d¹⁰ 5s¹ as an atom, but in the +2 state it is 4d⁹, so it is a transition element.

d orbitals reach further out towards the edge of the atom than s and p orbitals, so they feel the surroundings more and affect neighbouring atoms. Partly filled d orbitals lead to variable oxidation states, coloured ions, complex formation, catalytic activity and paramagnetism.

Elements along a transition row resemble one another more closely than the members of a main-group period do, though some group similarities exist too.