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1.3.1Principles of Chemistry: Part 1

Atoms: Definitions and Structure — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 10 of 23

📚 Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Hub / Topical Revision
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Section 1 of 7Atoms and molecules

Atom
  • Atom An atom is the smallest particle of an element that can exist, made of a nucleus surrounded by electrons.
Molecule
  • Molecule A molecule is two or more atoms joined together by chemical bonds into one unit.
  • Examples are O2, H2O and CO2.
Building blocks
  • Building blocks Every substance is made of atoms, and different elements have atoms with different numbers of protons.

Section 2 of 7Inside the atom

Atom
Nucleus
  • Nucleus A tiny, dense centre containing protons and neutrons, and so positively charged overall.
  • Almost all the mass of the atom is in the nucleus.
Electrons
  • Electrons Very small, negatively charged particles that move quickly around the nucleus in shells (energy levels).
Neutral overall
  • Neutral overall An atom has equal numbers of protons and electrons, so the positive and negative charges cancel.

Section 3 of 7Subatomic particles

ParticleRelative massRelative chargeLocation
Proton1+1Nucleus
Neutron10Nucleus
ElectronVery small (about 1/1840)-1Shells around the nucleus

Section 4 of 7Key definitions

Atomic number
  • Atomic number The number of protons in the nucleus.
    • It identifies the element and, for a neutral atom, equals the number of electrons.
Mass number
  • Mass number The total number of protons and neutrons in the nucleus.
Isotopes
  • Isotopes Atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.
  • Isotopes have the same chemical properties because they have the same electron arrangement.
Relative atomic mass, Ar
  • Relative atomic mass, Ar The weighted average mass of the isotopes of an element compared with 1/12 of the mass of a carbon-12 atom.

Section 5 of 7Finding the numbers of particles

1
Read the atomic number from the Periodic Table or the question. This is the number of protons.
2
For a neutral atom, write the same number for electrons.
3
Subtract the atomic number from the mass number to find the neutrons: neutrons = mass number - atomic number.
4
Worked example: lithium-7 has atomic number 3 and mass number 7, so it has 3 protons, 3 electrons and 7 - 3 = 4 neutrons.

Section 6 of 7Key terms

TermMeaning
AtomThe smallest particle of an element, with a nucleus surrounded by electrons.
MoleculeTwo or more atoms chemically bonded together.
NucleusThe central part of the atom containing protons and neutrons.
Atomic numberThe number of protons in an atom.
Mass numberThe sum of the protons and neutrons in an atom.
IsotopeAtoms of the same element with different numbers of neutrons.

Section 7 of 7Exam technique

1
Saying the electron has a mass of 0. Its mass is very small or negligible, but not zero.
2
Using the mass number as the number of protons. The number of protons is the atomic number.
3
Forgetting that the relative masses and charges are compared with each other and are not actual values in grams or coulombs.
4
Saying isotopes have different numbers of electrons or protons. Only the neutron number differs.

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