An atomic change happens in the electrons around the nucleus. A nuclear change happens inside the nucleus, where protons and neutrons are. Exam questions often describe an event in words and expect you to say which kind it is, then use that to answer about the element, charge or radiation.
This is the first skill in atomic and nuclear models, and every later lesson depends on it.
What does the atom look like in this model?
A small, dense nucleus sits at the centre. It holds protons (positive) and neutrons (no charge). Electrons (negative) move around it, and in an atom the number of electrons equals the number of protons, so the atom is neutral.
Two numbers describe a nucleus. The proton number Z is the count of protons, and it decides the element.
The nucleon number A is protons plus neutrons. Neutrons = A − Z.
Take carbon-14. Z = 6, A = 14, so it has 14 − 6 = 8 neutrons.
How do you tell the two kinds of change apart?
Ask two questions about any event.
- Did the proton number change? If yes, it is nuclear, and a new element has formed.
- Did only the electrons change? If yes, it is atomic. The element stays the same.
| Event | Where | What changes |
|---|---|---|
| An electron jumps to a higher energy level | Atomic | Electron energy; atom is excited |
| A neutral atom loses one electron | Atomic | Becomes a positive ion; same element |
| An unstable nucleus emits an alpha particle | Nuclear | Z and A both change; new element |
| A neutron in a nucleus changes to a proton, emitting a beta particle | Nuclear | Z rises by 1; new element |
| A nucleus emits gamma radiation | Nuclear | Energy leaves the nucleus; Z and A unchanged |
Nuclear changes are not affected by heating, pressure or which chemicals the atom is bonded to. That is why radioactive decay cannot be switched on or off by ordinary means.
Worked example
A sodium atom has Z = 11 and A = 23. Decide which kind of change each event is, and state the element afterwards.
Event 1: the atom loses one electron.
The proton number stays at 11, so it is still sodium. It now has 10 electrons and 11 protons, so the charge is +1. This is atomic, and the result is a sodium ion.
Event 2: the nucleus of a different atom, radon-222 (Z = 86), emits an alpha particle.
The nucleus loses 2 protons, so Z becomes 84. A new element forms (polonium). This is nuclear.
Event 3: an electron in the sodium atom absorbs energy and moves to a higher level.
Z is unchanged, only the electron’s energy changed. This is atomic, and the atom is excited, not a new element.
The mistake to watch for
Mistaken answer: “When a sodium atom loses an electron it becomes a different element, because it has changed.”
The mistake treats any change as if it could make a new element. Losing an electron changes the charge, not the proton number.
The element is decided by protons only. A correct answer names the ion, sodium with a +1 charge, and says the element is unchanged.
The reverse slip also appears: writing that an alpha emission “just makes the atom positive”. It changes the nucleus and the element, and the charge of the atom is a separate matter.
Check yourself
1. An atom of chlorine gains one electron. Atomic or nuclear, and does the element change?
Show answer
Atomic, because only the electrons changed. The proton number is unchanged, so the element is still chlorine. It becomes a negative ion with charge −1.
2. A nucleus has Z = 27 and A = 60. How many neutrons does it have? It then emits only gamma radiation. What are Z and A afterwards?
Show answer
Neutrons = 60 − 27 = 33. Gamma emission carries away energy only, so Z = 27 and A = 60 are unchanged. It is a nuclear change, but the element stays the same.
3. Why can heating a radioactive sample not make it decay faster?
Show answer
Decay is a change in the nucleus, which is not affected by heating. Heating mostly changes how the atoms and their electrons move and interact.
Where this leads next
Now that you can place a change, the next skill is to balance a simple nuclear equation. You can then test the whole topic with the mixed practice set.
Some students follow the idea in class but still move between “atom” and “nucleus” loosely in written answers. A teacher can catch that habit early in online one-to-one Physics tuition.