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.