Atoms, isotopes and ions (AQA GCSE Physics): Revision Notes
Atoms, isotopes and ions
What are atoms and isotopes?
All matter is made up of tiny particles called atoms. These atoms can exist in different forms depending on their structure and charge.
An atom contains three types of particles:
- Protons - positively charged particles in the nucleus
- Neutrons - neutral particles (no charge) in the nucleus
- Electrons - negatively charged particles that orbit around the nucleus
The nucleus is the dense centre of an atom where protons and neutrons are located, while electrons move in the space around the nucleus in what we call electron shells or orbitals.
The key thing to remember is that atoms of the same element always have the same number of protons. However, they can have different numbers of neutrons.
Isotopes explained
Isotopes are atoms of the same element that contain different numbers of neutrons in their nucleus. Even though they have different neutrons, they still have the same number of protons.
Worked Example: Carbon Isotopes
All carbon atoms have 6 protons. But carbon isotopes can have:
- 6 neutrons (carbon-12) → Total mass = 6 + 6 = 12
- 7 neutrons (carbon-13) → Total mass = 6 + 7 = 13
- 8 neutrons (carbon-14) → Total mass = 6 + 8 = 14
Notice how the number after "carbon" represents the total mass (protons + neutrons).
This means isotopes of the same element behave chemically the same way, but they have different masses because neutrons add weight to the atom.
Common misconception: Students often think isotopes are different elements. Remember - isotopes are the SAME element with different masses due to varying neutron numbers.
How positive ions form
Normally, atoms have no overall charge because they contain equal numbers of protons and electrons. The positive and negative charges balance out.
Positive ions form when an atom loses one or more electrons. Since there are now fewer negatively charged electrons than positively charged protons, the atom becomes positively charged overall.
When we write positive ions, we use a plus sign. For example, if a sodium atom loses one electron, it becomes Na⁺. If it loses two electrons, it would be written as Na²⁺.
Electrons can leave an atom in two main ways:
- Electromagnetic radiation - high energy radiation like UV rays, X-rays, or gamma rays can knock electrons out
- Particle bombardment - being hit by particles like alpha or beta particles can remove electrons
How negative ions form
Negative ions form when an atom gains extra electrons. Now there are more negatively charged electrons than positively charged protons, giving the atom an overall negative charge.
Negative ions are written with a minus sign. For example, when chlorine gains one electron, it becomes Cl⁻. If it gained two electrons, it would be Cl²⁻.
When atoms gain or lose electrons, they become ions rather than neutral atoms. The number of protons never changes - this is what determines what element it is.
Critical point: The number of protons NEVER changes during ion formation. Changing the number of protons would create a completely different element. Only electrons are gained or lost.
Key differences summary
| Particle type | Protons | Electrons | Overall charge |
|---|---|---|---|
| Neutral atom | Same number | Same as protons | Zero (neutral) |
| Positive ion | Same number | Fewer than protons | Positive |
| Negative ion | Same number | More than protons | Negative |
| Isotope | Same as original element | Same as protons | Zero (neutral) |
Key Points to Remember:
- Isotopes have the same protons but different neutrons
- Positive ions form when atoms lose electrons
- Negative ions form when atoms gain electrons
- The number of protons never changes - this determines the element
- Electromagnetic radiation or particle bombardment can remove electrons from atoms