Atoms, elements and the periodic table — AQA GCSE Chemistry revision

Inside an atom

• Protons (+1, mass 1) and neutrons (0, mass 1) sit in the nucleus; electrons (−1, almost no mass) orbit in shells.
• Atomic number = number of protons. Mass number = protons + neutrons.
• Atoms are neutral, so protons = electrons.

Isotopes and the table

• Isotopes are atoms of the same element with different numbers of neutrons — same chemistry, different mass.
• Groups are columns and tell you the number of outer electrons; periods are rows and tell you the number of shells.
• Group 1 metals get more reactive down the group; Group 7 non-metals get less reactive down the group.

Key terms

Isotope

Same element, same protons, different number of neutrons.

Relative atomic mass

Average mass of an element's isotopes compared to carbon-12.

Electron shell

An energy level where electrons are found (2, 8, 8...).

Nucleon

A proton or neutron — a particle found in the nucleus.

Ion

A charged particle formed when an atom gains or loses electrons.

Relative isotopic abundance

The percentage of each isotope present in a sample of an element.

Common exam traps

• Saying isotopes have different chemical properties — they don't, only physical ones like mass.
• Reading the periodic table numbers the wrong way round: the bigger number is the mass number.
• Forgetting that ions are charged, so protons no longer equal electrons.

Practice questions with answers

1. 1. An atom has 17 protons and 18 neutrons. What is its mass number?

   • • 17
   • • 18
   • • 35
   • • 1

   Answer: 35

   Mass number = protons + neutrons = 17 + 18.

2. 2. Why are isotopes of an element chemically identical?

   • • They have the same number of neutrons
   • • They have the same electron arrangement
   • • They have the same mass
   • • They are in different groups

   Answer: They have the same electron arrangement

   Chemistry is decided by outer electrons, which isotopes share.

3. 3. State what the group number tells you about an atom.

   Answer: The number of electrons in its outer shell.

   Group 2 = 2 outer electrons, and so on (except the transition block).

4. 4. Explain why Group 1 metals get more reactive down the group.

   Answer: The outer electron is further from the nucleus and shielded, so it is lost more easily.

   Reactivity of metals = how easily the outer electron is lost.

5. 5. Which particle has no electrical charge?

   • • Proton
   • • Neutron
   • • Electron
   • • Ion

   Answer: Neutron

   Protons are +1, electrons are −1, neutrons are neutral (0).

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