The nuclear atom
Mostly empty space
- If an atom were the size of a stadium, its nucleus would be a pea in the centre.
- Almost all the atom is empty space — a surprising fact proved by a clever experiment.
- Let's build up the nuclear model of the atom.
The nuclear atom
- An atom has a tiny central nucleus (positive) with electrons (negative) moving around it.
- The whole atom is neutral — equal positive and negative charge.
- Almost all the mass is in the nucleus, which is tiny compared with the atom.
- Lose electrons → a positive ion; gain electrons → a negative ion.
Where is almost all of an atom's mass?
Almost all the mass is concentrated in the tiny, dense nucleus; the rest of the atom is mostly empty space.
An atom that loses an electron becomes a positive ion.
Losing a negative electron leaves the atom with more positive charge — a positive ion.
The alpha-scattering experiment
- Fast, positive alpha particles were fired at very thin gold foil.
- Most went straight through → the atom is mostly empty space.
- A few were deflected → the nucleus is positive (it repels the alpha particles).
- A very few bounced back → the nucleus is tiny, dense and heavy.
In the alpha-scattering experiment, most alpha particles passed straight through the foil. This shows that:
If most pass straight through, there must be very little in the way — the atom is mostly empty space.
Inside the nucleus
- The nucleus is made of nucleons: protons (charge $+1$) and neutrons (charge $0$).
- Proton number $Z$ = number of protons; nucleon number $A$ = protons + neutrons.
- So the number of neutrons is $A - Z$.
- We write a nucleus as $^{A}_{Z}\text{X}$ — for example $^{14}_{\ 6}\text{C}$ has $6$ protons and $14 - 6 = 8$ neutrons.
The proton number (Z) of a nucleus is:
Z is the proton number (atomic number). The nucleon number A is protons + neutrons.
How many neutrons are in a nucleus of $^{14}_{\ 6}\text{C}$?
Neutrons $= A - Z = 14 - 6 = 8$.
Isotopes
- Isotopes are atoms of the same element (same $Z$) with different numbers of neutrons (different $A$).
- They behave the same in chemistry but differently in the nucleus.
- Example: $^{12}_{\ 6}\text{C}$ and $^{14}_{\ 6}\text{C}$ are both carbon.
Isotopes of an element have:
Isotopes share the proton number Z (same element) but have different nucleon numbers A (different neutron counts).
You've got it
- atom = tiny positive nucleus + electrons; mostly empty space; almost all mass in the nucleus
- alpha-scattering: most pass through (empty), a few deflect (positive nucleus), a very few bounce back (small + dense)
- $Z$ = protons, $A$ = protons + neutrons, neutrons $= A - Z$; notation $^{A}_{Z}\text{X}$
- isotopes: same $Z$, different $A$