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Atomic number is the number of protons. Mass number counts protons plus neutrons. A neutral atom has equal proton and electron counts, while an ion has gained or lost electrons. An atom is mostly empty space with a small positive nucleus; electrons occupy energy levels. Evidence from scattering replaced the plum-pudding model because a few alpha particles were strongly deflected by a concentrated positive region.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate particle counts in atoms and ions
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which particle defines an element?
Proton. Changing proton number changes the element.
After watching 2: What differs between isotopes?
Neutron count. Their nuclei have different masses.
What you'll be able to do
Atomic number is the number of protons. Mass number counts protons plus neutrons. A neutral atom has equal proton and electron counts, while an ion has gained or lost electrons. An atom is mostly empty space with a small positive nucleus; electrons occupy energy levels. Evidence from scattering replaced the plum-pudding model because a few alpha particles were strongly deflected by a concentrated positive region.
Isotopes have the same proton number and different neutron numbers, so they are the same element with different masses. Their chemical behaviour is very similar because their electron arrangements are the same. Relative atomic mass is an abundance-weighted mean, so it need not be a whole number. Losing two electrons creates a 2+ ion without changing the atom’s proton or neutron count.
For the first twenty elements, use the school shell model with capacities 2, 8, 8 for the first occupied shells in these examples. Sodium is 2,8,1 and chlorine 2,8,7. The group indicates the outer-electron pattern for these main-group elements, while the period indicates occupied shells. This is a simplified model rather than a drawing of tiny electrons following planetary tracks.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: An element is 80% isotope 10 and 20% isotope 11. Find its relative atomic mass.
10.2.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.