A Level Chemistry CAIE

Atomic Structure

Atomic Structure

Subatomic Particles

• An atom consists of a centrally located nucleus which is surrounded by electrons.
• The nucleus is made up of protons and neutrons, which are collectively referred to as nucleons.
• Protons have a positive electric charge, neutrons have no charge, whereas electrons carry a negative charge.
• A key characteristic of an atom, the atomic number (also known as proton number), represents the number of protons in the nucleus of an atom.

Atomic Mass and Isotopes

• The atomic mass, or atomic weight, of an atom is essentially the sum of protons and neutrons in the nucleus.
• Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons, hence varying atomic masses.
• The relative atomic mass of an element is the weighted average of the masses of the isotopes, factoring in their natural abundance.

Atomic Energy Levels

• Electrons in an atom occupy spaces known as energy levels, or shells, which surround the nucleus.
• Each energy level can accommodate a specific number of electrons: the first level can hold up to 2, the second level can hold up to 8, and so on - guided by the 2n^2 rule.
• Orbitals are regions within an energy level where there's a high probability of finding an electron. They come in different shapes: s, p, d, and f.

Electronic Configuration

• The way electrons are arranged in various orbitals of an atom is known as the atom's electronic configuration.
• Electrons fill up the orbitals in a way that minimizes the energy of the atom, following the Aufbau principle, Hund's rule, and Pauli Exclusion principle.
• Aufbau principle states that electrons fill the lower energy levels first, followed by higher energy levels.
• Hund's Rule suggests that every orbital in a subshell is singly occupied before any orbital is doubly occupied.
• The Pauli Exclusion Principle states that no two electrons in an atom can have identical quantum numbers, meaning an orbital can hold at most 2 electrons, with opposite spins.

Ion Formation

• Atoms can lose or gain electrons to form charged particles known as ions.
• Cations are positively charged ions formed when atoms lose one or more electrons.
• Anions are negatively charged ions formed when atoms gain one or more electrons.
• The formation of ions leads to a stable electronic configuration, usually resembling that of a noble gas.

Periodic trends

• As one moves from left to right across a period in the periodic table, atomic radius decreases due to increased nuclear charge pulling electrons tighter.
• Ionisation energy, electron affinity and electronegativity generally increase across a period due to increasing effective nuclear charge.
• As one moves down a group in the periodic table, atomic size increases due to the addition of energy levels.
• Ionization energy, electron affinity and electronegativity decrease down a group as the electrons are further from the nucleus and thus less tightly held.