GCSE Chemistry (Combined) CCEA

Atomic Structure

Atomic Structure

The Basics

• An atom, the smallest part of an element, consists of a nucleus surrounded by electrons in orbit.
• Protons and neutrons constitute the nucleus, with protons carrying a positive charge and neutrons being uncharged.
• Electrons, located in shells surrounding the nucleus, carry a negative charge.
• The atomic number, or proton number, identifies an element.
• The mass number, or nucleon number, equals the total number of protons and neutrons in an atom's nucleus.
• Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.

Electronic Structure

• Electrons inhabit energy levels or shells around the atomic nucleus.
• The lowest energy level (closest to the nucleus, called 1st level) is populated before the higher levels (2nd level, 3rd level, etc.).
• An electron shell can hold a specific maximum number of electrons.
• Energy levels nearer to the nucleus are filled first with electrons.
• The outermost energy level of an atom determines its chemical properties and reactivity.

Quantum Mechanics and Atomic Structure

• Each electron is described by four quantum numbers--principal, azimuthal, magnetic, and spin--which identify its state.
• Two electrons cannot occupy the same quantum state simultaneously, indicated by Wolfgang Pauli's Exclusion Principle.
• Valence electrons are the electrons present in an atom's outer shell.
• The arrangement of electrons in different energy levels, or shells, is known as the electron configuration. It is crucial for understanding an atom's reactivity and bonding behaviour.

Understanding the Periodic Table through Atomic Structure

• A period in the periodic table is a horizontal row. All the atoms in a period have the same number of atomic orbitals.
• Every element in a group has the same number of valence electrons. Examples can be seen in groups such as alkali metals and noble gases.
• The transition metals, located in the middle of the periodic table, contain their valence electrons in two shells instead of just one.
• The lanthanides and actinides at the bottom of the table include elements with their valence electrons in f orbitals.