Inside the Atom: Quick Revision

A brief note on Inside the Atom. Read it before a test. Try each question at the end before you open its answer.

The three particles

• Electron: where: outside the nucleus; charge: $-1$; mass: $\dfrac{1}{1836}\ \text{u}$; found by: J. J. Thomson, 1897
• Proton: where: in the nucleus; charge: $+1$; mass: about $1\ \text{u}$; found by: Goldstein's rays, then Rutherford
• Neutron: where: in the nucleus; charge: $0$; mass: about $1\ \text{u}$; found by: Chadwick, 1932

Three models

• Thomson: a sphere of positive charge with electrons set in it, like seeds in a watermelon. There is no nucleus.
• Rutherford: from the gold foil experiment. The atom is mostly empty space, with a tiny, heavy, positive nucleus. Electrons move round it.
• Bohr: electrons move only in fixed shells, K, L, M, N, and lose no energy there. A jump between shells takes in or gives out a fixed amount of energy.

A worked example

A particle has $13$ protons, $14$ neutrons and $10$ electrons. Find its charge and its mass.

1. Charge. $13 - 10 = +3$. It is an ion, $\text{Al}^{3+}$.
2. Mass. $13 + 14 = 27\ \text{u}$. The $10$ electrons add only $\dfrac{10}{1836}\ \text{u}$, about $0.005\ \text{u}$.

Practice with new numbers

A program to try

The program finds the charge and the mass of three particles, one of them the worked example.

Mistakes to avoid

• Giving Thomson a nucleus. The nucleus came from Rutherford.
• Counting electrons in the mass. They add almost nothing.
• Saying that an electron gives out light all the time. In Bohr's model, it does so only when it falls to a lower shell.
• Forgetting the neutron's mass. It has no charge, but its mass is about $1\ \text{u}$.

Check yourself

1. A particle has $9$ protons, $10$ neutrons and $10$ electrons. What is its charge?

2. Why did a very few alpha particles bounce straight back from the gold foil?

3. In hydrogen, the M shell is at $-1.51\ \text{eV}$ and the L shell at $-3.4\ \text{eV}$. How much energy is given out when an electron falls from M to L?

4. How many electrons can the O shell, $n = 5$, hold at most?