77. Atomic Structure, Isotopes & Electron Shells

GCSE Physics: Atomic Structure, Isotopes & Electron Shells

This guide covers the fundamental structure of the atom, how to interpret nuclear symbols, the nature of isotopes, and how electrons behave within energy levels.

1. The Structure of the Atom

Atoms are composed of three subatomic particles:

  • Protons: Located in the nucleus; they have a positive charge (+1) and a relative mass of 1.

  • Neutrons: Also located in the nucleus; they are neutral (no charge) and have a relative mass of 1.

  • Electrons: Orbit the nucleus in shells; they have a negative charge (-1) and are roughly 2,000 times smaller than protons or neutrons.

2. Reading the Nuclear Symbol

A nuclear symbol (like those found on the Periodic Table) provides the specific identity and makeup of an atom:

  • Elemental Symbol: The shorthand for the element (e.g., Li for Lithium).

  • Atomic Number (Bottom Left): The number of protons. This is the most important number because it determines which element the atom is.

  • Mass Number (Top Left): The total number of protons and neutrons added together.

  • Calculating Electrons: In a neutral atom, the number of electrons is always equal to the atomic number (protons).

3. Isotopes and Radioactive Decay

  • Isotopes: These are atoms of the same element that have the same number of protons but a different number of neutrons. Because they have different neutron counts, they have different mass numbers.

  • Stability: Most elements have only one or two stable isotopes. Unstable isotopes tend to decay into other elements by emitting radiation (alpha, beta, or gamma) or neutrons. This process is called radioactive decay.

[Image comparing isotopes of carbon such as Carbon-12 and Carbon-14]

4. Electron Energy Levels

Electrons are arranged in shells, with each shell representing a specific energy level. The further a shell is from the nucleus, the higher its energy.

  • Excitation: An electron can "jump" to a higher energy level if it absorbs electromagnetic radiation with the exact amount of energy needed to reach the next shell. We then call the electron excited.

  • De-excitation: Usually, the electron quickly falls back down to its original lower energy level. When it does this, it emits the energy back out as electromagnetic radiation.

5. Ionization

If an outer electron absorbs enough energy, it can leave the atom entirely.

  • Positive Ion: Once an electron leaves, the atom has more protons than electrons, giving it an overall positive charge. It is now called a positive ion.

  • Ionizing Radiation: This is radiation that carries enough energy to knock electrons off atoms, turning them into ions.