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1.3.1Principles of Chemistry

Atoms- Definitions and Structure — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 50

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Part 1 of 5Structure of the Atom

1.3.1 Atoms- Definitions and Structure

All matter is built from atoms, each made of a central nucleus surrounded by electrons arranged in shells.

Proton
  • A particle in the nucleus with a relative mass of 1 and a relative charge of +1, which gives the nucleus its positive charge.
Neutron
  • A particle in the nucleus with a relative mass of 1 and no charge, which adds mass to the atom without changing the element.
Electron
  • A particle in shells around the nucleus with a very small mass and a relative charge of −1,
    • so a neutral atom has equal numbers of protons and electrons.
Nucleus
  • The dense central region containing protons and neutrons, where almost all of the mass of the atom is concentrated.
ParticleRelative massRelative chargeLocation
Proton1+1Nucleus
Neutron10Nucleus
ElectronAbout 1/1840−1Shells around the nucleus

Part 2 of 5Atomic Terms

Atomic Terms
Atomic number
  • The number of protons in the nucleus, which identifies the element and equals the number of electrons in a neutral atom.
Mass number
  • The total number of protons and neutrons in the nucleus, always the larger of the two numbers shown for an element.
Isotopes
  • Atoms of the same element with the same number of protons but different numbers of neutrons, so their mass numbers differ.
Relative atomic mass
  • The weighted average mass of the atoms of an element, compared with one twelfth of the mass of a carbon-12 atom.

Part 3 of 5Finding the Particle Numbers

1
Read the atomic number from the periodic table or the notation to find the number of protons in the atom.
2
In a neutral atom the number of electrons is equal to the number of protons, so the electron count is the same.
3
Subtract the atomic number from the mass number to find the number of neutrons.
4
Check that the protons plus neutrons add up to the mass number before writing the final answer.

Part 4 of 5Worked Example

Question Work out the numbers of protons, electrons and neutrons in potassium-39, which has atomic number 19 and mass number 39.

  1. Protons = atomic number = 19.
  2. Electrons = protons in a neutral atom = 19.
  3. Neutrons = mass number − atomic number = 39 − 19 = 20.

Answer 19 protons, 19 electrons and 20 neutrons.

Part 5 of 5Exam Checklist & Tips

1.3.1 – Key Points for the Exam

  • Atom The smallest particle of an element that keeps the chemical properties of that element.
  • Molecule A group of two or more atoms chemically combined to form an identifiable unit.
  • Atomic number The number of protons in the nucleus of an atom.
  • Mass number The number of protons plus neutrons in the nucleus of an atom.
  • Isotope Atoms of the same element with the same number of protons but different numbers of neutrons.
  • Relative atomic mass The weighted average mass of an atom of an element compared with one twelfth of a carbon-12 atom.

Common Mistakes

  • Swapping the atomic number and mass number;
    • the mass number is the larger value and includes protons plus neutrons.
  • Placing electrons inside the nucleus; electrons occupy shells outside the nucleus.
  • Saying that isotopes are different elements;
    • isotopes have the same number of protons and so are the same element.
  • Giving the atomic number as the number of neutrons; it is the number of protons.

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