Atomic Structure and Periodicity - SS2 Chemistry Past Questions and Answers - page 3

The electronic configuration of an atom refers to the arrangement of electrons within its energy levels or electron shells. It follows a set of rules and principles, including the Aufbau principle, the Pauli exclusion principle, and Hund's rule.

The Aufbau principle states that electrons fill the lowest energy levels first before occupying higher energy levels. Each energy level can accommodate a specific number of electrons: the first energy level holds a maximum of 2 electrons, the second energy level holds up to 8 electrons, the third energy level holds up to 18 electrons, and so on.

The Pauli exclusion principle states that each electron within an atom must have a unique set of quantum numbers, including spin. This means that each orbital within an energy level can hold a maximum of two electrons with opposite spins.

Hund's rule states that electrons prefer to occupy separate orbitals within the same energy level before pairing up. This results in unpaired electrons, which contribute to the chemical properties of an element.

The electronic configuration determines the distribution of electrons in the atom's outermost energy level, known as the valence shell. The number and arrangement of valence electrons play a crucial role in determining the chemical properties of an element, including its reactivity, bonding behaviour, and ability to gain or lose electrons.

The atomic radius refers to the size of an atom, typically defined as half the distance between the nuclei of two adjacent atoms in a molecule or crystal lattice.

Across a period (from left to right) in the periodic table, the atomic radius generally decreases. This is primarily due to the increasing number of protons in the nucleus as you move across the period. The increased positive charge exerts a stronger pull on the electrons, bringing them closer to the nucleus and reducing the atomic radius.

Down a group (top to bottom) in the periodic table, the atomic radius generally increases. This is because each successive energy level or electron shell is farther from the nucleus. The addition of more energy levels increases the distance between the nucleus and the outermost electrons, resulting in a larger atomic radius.