AQA Chemistry 5.1.1.4–5.1.1.5 · Combined and Triple
Subatomic particles
Protons, neutrons and electrons: their charge, their mass, where they sit, and how to count them in any atom or ion from two numbers.
Combined Science · Higher tier
Start here
A balloon on a wall.
Rub a balloon on your hair and it sticks to the wall. Something has moved between the hair and the balloon.
If you had to guess, is the balloon charged because it gained something, or because it lost something?
The three particles
Charge, mass, place.
Atoms are made of three types of particle. Hydrogen-1, the commonest atom of all, has one proton, one electron and no neutron. Tap each particle.
Why does an atom have no overall charge?
Pick one.
Size and scale
Very small, and mostly empty.
Atoms are very small: a radius of about 0.1 nm, which is 1 × 10−10 m. The radius of a nucleus is less than 1/10 000 of that of the atom, about 1 × 10−14 m. Almost all the mass is in the nucleus.
If the atom were a football stadium, how big would the nucleus be?
Pick one.
Counting in atoms and ions
Two numbers, two subtractions.
The nuclear symbol gives the mass number (top) and the atomic number (bottom). The atomic number is the number of protons. Ions form when atoms lose or gain electrons: the number of protons never changes. Worked case:
2713Al3+
protons = atomic number = 13
neutrons = mass number − atomic number = 27 − 13 = 14
electrons = protons − charge = 13 − (+3) = 10
A 3+ charge means three electrons were lost. A negative ion gained electrons, so subtracting a negative charge adds them: for Cl−, 17 − (−1) = 18.
Now the builder. Choose a species, read the counts, then say how many electrons it has.
Na atom: mass number 23, atomic number 11.
How many electrons?
Higher · writing the electron transfer
Half equations.
On Higher you write the loss or gain of electrons as a half equation. Metals lose electrons; non-metals gain them. Chlorine and oxygen are diatomic, so their half equations start from Cl2 and O2.
Na Na+ + e−
Mg Mg2+ + 2e−
Cl2 + 2e− 2Cl−
O2 + 4e− 2O2−
Check the charge balances on both sides. Complete: Ca Ca2+ + ?
Pick one.
Think again · what the charge tells you
“Mg2+ has 12 electrons, because magnesium is element 12.”
Your call
ChooseMagnesium has atomic number 12. How many electrons are in a Mg²⁺ ion?
Sort
Which particle?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Calculate
- Protons, neutrons and electrons from the atomic number, mass number and charge.
- Explain
- Why atoms are neutral; why an ion has its charge.
- State
- The relative charge and mass of each particle; the radius comparison.
Key fact
Proton +1, neutron 0, electron −1. Protons and neutrons have relative mass 1; the electron’s is very small. Protons = atomic number; neutrons = mass number − atomic number; electrons in an ion = protons − charge.
Key note · AQA 5.1.1.4–5.1.1.5 · Combined and Triple
Subatomic particles
- Proton: +1, relative mass 1, nucleus. Neutron: 0, relative mass 1, nucleus. Electron: −1, relative mass very small, shells around the nucleus.
- Atoms are neutral because protons equal electrons. The atomic number is the number of protons; the mass number is protons plus neutrons.
- Atoms have a radius of about 0.1 nm (1 × 10⁻¹⁰ m). The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 × 10⁻¹⁴ m). Almost all the mass is in the nucleus.
- Ions: atoms lose electrons to become positive, gain electrons to become negative. Protons never change. Neutrons = mass number − atomic number; electrons in an ion = protons − charge (²⁷13Al³⁺: 13 p, 14 n, 10 e).
- Higher: half equations such as Mg → Mg²⁺ + 2e⁻ and Cl2 + 2e⁻ → 2Cl⁻ show the electrons lost or gained; the charge must balance on both sides.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Losing track of which number is which?
Atom and nucleus sizes are the spec’s approximate values; the figures are not to scale.