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.

Genes are segments of DNA molecules that carry genetic information essential for various biological processes. This genetic information is passed from parents to offspring. Each gene is a specific sequence of DNA that performs a particular function, such as directing the production of proteins necessary for the development and functioning of organs.

Red blood cells are specialized for transporting oxygen from the lungs to all cells in the body. Their structure is uniquely adapted to perform this function efficiently:

• Haemoglobin Content: Red blood cells contain a high concentration of haemoglobin, a protein that can reversibly bind to oxygen. This allows them to pick up oxygen in the lungs and release it where it is needed in the body.
• Lack of Nucleus: Red blood cells lack a nucleus, which creates additional space for haemoglobin. This increases their oxygen-carrying capacity.
• Biconcave Shape: Their biconcave disc-like shape (thinner in the middle and thicker at the edges) increases their surface area relative to their volume. This facilitates more efficient oxygen diffusion into and out of the cell.
• Flexibility: Red blood cells are highly flexible, enabling them to squeeze through narrow capillaries without rupturing. This ensures oxygen is delivered to even the smallest and most remote tissues.

Each of these features is closely linked to the red blood cell's role in oxygen transport, optimizing their efficiency in delivering oxygen throughout the body.