Electron Structure

How many electrons can fill the first four shells?

Shell 1 holds a maximum of 2 electrons, shell 2 holds 8, shell 3 holds 18, and shell 4 holds 32. The general formula is 2n², where n is the shell number.

What is an atomic orbital?

An atomic orbital is a region around the nucleus that can hold a maximum of 2 electrons with opposite spins.

What are the four types of atomic orbital, and how many of each exist in the first four shells?

The four types are s, p, d, and f orbitals. There is 1 s orbital per shell (from shell 1 onwards), 3 p orbitals per shell (from shell 2 onwards). Each orbital holds a maximum of 2 electrons.

What is the shape of an s orbital and a p orbital?

An s orbital is spherical, centred on the nucleus. A p orbital is dumbbell-shaped (two lobes), and comes in three orientations at right angles to each other (px, py, pz).

Why do electrons in the same sub-shell occupy separate orbitals before pairing?

Because electrons are negatively charged and repel each other. It is energetically more favourable for electrons to occupy separate orbitals with parallel spins (Hund's rule), minimising electron-electron repulsion.

Why are the electron configurations of Cr (Z=24) and Cu (Z=29) exceptions to the expected filling order?

Cr is [Ar] 3d⁵ 4s¹ rather than the expected [Ar] 3d⁴ 4s². Cu is [Ar] 3d¹⁰ 4s¹ rather than the expected [Ar] 3d⁹ 4s². Both exceptions arise because a half-filled (3d⁵) or fully-filled (3d¹⁰) d sub-shell has extra stability, making these configurations lower in energy than the expected ones.

How do you classify elements as s-, p-, or d-block?

The block is determined by the sub-shell in which the highest energy (outermost) electron is placed. Elements filling the s sub-shell are in the s-block (Groups 1 and 2), those filling the p sub-shell are in the p-block (Groups 13–18), and those filling the d sub-shell are in the d-block (transition metals).