GCSE Chemistry (Combined) AQA

Atomic Structure: Atoms

Fundamental Theories of Atoms

• Every element is made up of very small particles called atoms; this is the atomic structure.
• An atom is made up of an even smaller core called the nucleus, which contains protons and neutrons. Surrounding the nucleus are energy levels (or shells) which contain the electrons.
• Protons are positively charged particles, whereas electrons are negatively charged. Neutrons hold no charge, they are neutral.

Atomic Structure and Composition

• The atomic number of an atom (also known as the proton number) represents the number of protons within an atom's nucleus.
• The mass number is the total number of protons and neutrons in an atom's nucleus.
• Atoms are electrically neutral overall because they contain equal numbers of protons and electrons.

Electron Arrangement and Energy Levels

• An atom's energy levels can hold varying number of electrons: the first energy level can hold up to 2, and the second and third can hold up to 8 each.
• When an atom loses or gains electrons, it forms an ion. Loss of electrons results in a positive ion, or cation. Gain of electrons forms a negative ion, or anion.
• The electron arrangement of an atom determines how it will interact or bond with other atoms.

Periodic Table and Elemental Properties

• Elements are listed in the Periodic Table according to their atomic number. Each unique element has a different number of protons.

Isotopes and Relative Atomic Mass

• Isotopes are atoms of the same element with different numbers of neutrons, hence different mass numbers. Despite this, they have similar chemical properties as they have the same number of electrons.
• Comparing two atoms of the same element, the one with more neutrons is heavier and therefore is an isotope of that element.
• The relative atomic mass of an element takes into account the masses and proportions of its different isotopes.