Electronic configuration, orbitals and atomic radius

Why the periodic table has its shape

Mendeleev arranged elements by their properties, and the table came out in blocks. Two columns on the left, a block of ten transition columns, and six on the right. Those block sizes are not accidents. They are the numbers of electrons that fit in s, d and p sub-shells.

Shells, sub-shells and orbitals

Electrons are in energy levels called shells, numbered one, two, three. Each shell is divided into sub-shells: s, p, d and f. A sub-shell is made of orbitals. An orbital is a region of space that can hold at most two electrons, of opposite spin. An s sub-shell has one orbital, so it holds 2 electrons. A p sub-shell has three orbitals: 6 electrons. A d sub-shell has five: 10 electrons. The first shell has only 1s. The second has 2s and 2p. The third has 3s, 3p and 3d.

Orbital shapes

An s orbital is spherical. Its size grows with the shell number. A p orbital is a dumbbell, with two lobes, along an axis. The three p orbitals point along x, y and z, at right angles. The 2p orbitals are all at the same energy: they are degenerate.

Filling the orbitals

Aufbau: fill the lowest energy sub-shell first. The order runs 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Pauli: an orbital holds at most two electrons, with opposite spins. Hund: electrons fill orbitals of equal energy singly first, with parallel spins, before pairing. Calcium is 1s two, 2s two, 2p six, 3s two, 3p six, 4s two: twenty electrons.