GCSE Physics - Atomic Structure, Isotopes & Electron Shells (2026/27 exams)

Summary

Key Points

• An atom’s nucleus contains positively charged protons and neutral neutrons, each with a relative mass of about one. 0:35
• Electrons orbit the nucleus in shells, are roughly 2,000 times lighter than protons, and carry a negative charge of one. 0:54
• In a neutral atom the number of electrons equals the number of protons, so lithium has three electrons as it has three protons. 2:15
• A periodic‑table box displays the element’s symbol, atomic number (the number of protons), and mass number (the total of protons and neutrons). 2:37
• Isotopes are atoms of the same element that have identical proton counts but different neutron counts, giving them different mass numbers (e.g., lithium‑6, lithium‑7, lithium‑8). 2:37
• Most isotopes are unstable and decay by emitting alpha, beta, gamma radiation, or neutrons, a process called radioactive decay. 3:27
• Electrons occupy discrete energy levels called shells; when they absorb the right amount of electromagnetic energy they jump to a higher shell and become excited. 4:08
• After a brief excitation, the electron returns to a lower energy level, releasing the absorbed energy as electromagnetic radiation. 4:31
• If an outer electron gains enough energy to leave the atom entirely, the atom becomes a positively charged ion, and ionizing radiation is defined as radiation capable of causing this electron loss. 5:04