2.2 Atomic Structure and the Periodic Table

2026 Syllabus Objectives

  1. Core: Describe the structure of the atom as a central nucleus containing neutrons and protons surrounded by electrons in shells
  2. Core: State the relative charges and relative masses of a proton, a neutron and an electron
  3. Core: Define proton number / atomic number as the number of protons in the nucleus of an atom
  4. Core: Define mass number / nucleon number as the total number of protons and neutrons in the nucleus of an atom
  5. Core: Determine the electronic configuration of elements and their ions with proton number 1 to 20, e.g. 2,8,3
  6. Core: State that: (a) Group VIII noble gases have a full outer electron shell (b) the number of outer shell electrons is equal to the group number in Groups I to VII (c) the number of occupied electron shells is equal to the period number

Structure of the Atom ⚛️

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Historical Development

Our modern understanding of the atom is based on atomic theory put forward by English chemist John Dalton in 1807. Dalton proposed that:

  • Atoms are the basic building blocks of elements
  • Atoms are indivisible particles that join together to make molecules
  • This theory explained many natural processes in chemistry

Although parts of Dalton's theory have changed based on later discoveries, it represented one of the great leaps of understanding in chemistry.

Modern Understanding

Research since Dalton's time has revealed that atoms are made up of subatomic particles:

  • 1897: Discovery of the electron
  • Early 1900s: Discovery of the proton
  • 1932: Discovery of the neutron

Crucial experiments showed that:

  • An atom is mostly empty space occupied by negatively charged electrons
  • Electrons surround a very small, positively charged nucleus
  • The nucleus contains almost all the mass of the atom
  • The nucleus is at the centre of the atom

Key Point: Modern techniques such as scanning tunnelling microscopy can now directly reveal the presence of individual atoms, confirming what Dalton only theorized.

The Three Subatomic Particles

All atoms are made from three universal subatomic particles:

🔑 Electron: A subatomic particle with negligible mass and a relative charge of −1-1; electrons are present in all atoms, located in the shells (energy levels) outside the nucleus

🔑 Proton: A subatomic particle with a relative atomic mass of 1 and a charge of +1+1 found in the nucleus of all atoms

🔑 Neutron: An uncharged subatomic particle present in the nucleus of atoms - a neutron has a mass of 1 relative to a proton

🔑 Nucleus: The central region of an atom that is made up of the protons and neutrons; the electrons orbit around the nucleus in different shells or energy levels

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