An atom is a tiny, dense, positively charged nucleus wrapped in a cloud of electrons about ten thousand times wider. Three particles, two numbers on the symbol, and every count you will ever need falls out.
Protons (relative charge +1, relative mass 1) and neutrons (0, 1) sit in the nucleus, so the nucleus carries nearly all the mass and all the positive charge. Electrons (−1, 1/1836) fill the space around it. Proton number Z counts protons; nucleon number A counts protons plus neutrons. Neutrons = A − Z, and electrons = Z − (charge on the ion).
Build any atom or ion in the simulator: pick a proton number, add neutrons, then add or remove electrons. Watch the symbol update and check that you can predict A, Z and the charge before the display shows them.
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| proton | 1 | +1 | nucleus |
| neutron | 1 | 0 | nucleus |
| electron | 1/1836 (about 0.0005) | −1 | shells around the nucleus |
The nucleus is so small that if the atom were a football stadium the nucleus would be a pea on the centre spot. Nearly all the mass is in that pea. The electrons are what one atom touches when it meets another, which is why the electron arrangement, not the nucleus, decides the chemistry.
Fire a beam of each particle between two charged plates. Predict which way each beam bends and by how much before you switch the beam on.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
The relative charge and relative mass of a neutron are:
Which particle has a relative mass of about 1/1836?
Nearly all the mass of an atom is found in:
An atom of 2713Al contains how many neutrons?
Two numbers on the symbol give you everything. The nucleon number A sits top left, the proton number Z bottom left, and any charge sits top right.
The charge on an ion changes the electron count only. Protons and neutrons are untouched: an ion is still the same element with the same nucleus. And the nucleon number on a symbol is a whole-number count for one atom; it is not the relative atomic mass in the Periodic Table, which is an average over isotopes.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Beams of protons, neutrons and electrons travelling at the same speed pass between two charged plates. Which statement is correct?
How many electrons are there in one 5626Fe3+ ion?
Which species has the same number of electrons as an argon atom?
An ion contains 12 protons, 12 neutrons and 10 electrons. It is:
This skill is now recorded on your profile. Keep the chain going.