A full electron configuration describes how electrons are distributed among the shells and subshells of an atom or ion. The shells, or energy levels, are numbered from 1 to 4, with shell 1 being the closest to the nucleus and shell 4 being the furthest. Electrons are filled into these shells from the innermost to the outermost.

• Shell 1 can hold a maximum of 2 electrons.
• Shell 2 can hold up to 8 electrons.
• Shell 3 can hold up to 18 electrons.
• Shell 4 can hold up to 32 electrons (though shell 4 is generally not considered at A Level).

Electron subshells are designated as s, p, d, and f. For A Level, the highest subshell you’ll encounter is d. The electron capacities for each subshell are:

• s subshell: 2 electrons,
• p subshell: 6 electrons,
• d subshell: 10 electrons.

When filling electron subshells, we follow the order: s → p → d, with a few exceptions to this rule, which will be discussed later.

For example, carbon (with 6 electrons) has the following electron configuration: 1s² 2s² 2p². This means it has 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 2 electrons in the 2p subshell.

Hydroxynitriles are synthesized through a nucleophilic addition reaction involving ketones and cyanide. The cyanide ion (CN⁻), which is negatively charged, is attracted to the electrophilic carbonyl carbon in the ketone, which has a partial positive charge. This leads to the breaking of the carbon-oxygen double bond, resulting in a negatively charged oxygen atom that possesses a lone pair of electrons. The lone pair on the oxygen then acts as a nucleophile, attacking a proton (H⁺) from the hydrochloric acid (HCl). This protonation process yields the final product: a hydroxynitrile.

When a reaction at equilibrium is disturbed by changes in concentration, pressure, or temperature, the system will adjust in a way that opposes the disturbance. This response shifts the position of equilibrium to counteract the effect of the change, restoring balance.