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So far we have treated particles as featureless circles. Now we look inside the particle. The smallest particle of an element is an atom, and an atom is itself made of even smaller subatomic particles: protons, neutrons and electrons. Understanding how these are arranged — a tiny dense nucleus surrounded by electrons in shells — is the key to the whole of chemistry, because it explains why atoms are neutral, where almost all of an atom's mass is concentrated, and just how empty an atom really is. This lesson is part of Topic C1 of OCR Gateway Science A.
By the end of this lesson you should be able to describe the structure of an atom, state the relative masses and charges of protons, neutrons and electrons, explain why atoms have no overall charge, and use standard form to compare the size of an atom with the size of its nucleus.
An atom has two parts:
The nucleus is extremely small compared with the whole atom, but it contains almost all of the atom's mass, because protons and neutrons are far heavier than electrons. The electrons occupy the relatively huge space around the nucleus, which means most of an atom is empty space.
Exam Tip: A labelled atom needs the nucleus (containing protons and neutrons) at the centre and electrons in shells around it. State that the mass is concentrated in the nucleus and that most of the atom is empty space.
Protons, neutrons and electrons differ in two key ways: their mass and their charge. Because the real masses and charges are tiny, we use relative values (compared with a proton).
| Particle | Where it is | Relative mass | Relative charge |
|---|---|---|---|
| Proton | In the nucleus | 1 | +1 |
| Neutron | In the nucleus | 1 | 0 (neutral) |
| Electron | In shells around the nucleus | 18361 (≈ negligible) | −1 |
The important facts to learn are:
Because protons and neutrons (in the nucleus) each have a relative mass of 1, while electrons have almost no mass, the mass of an atom is concentrated in its nucleus.
The reason we use relative masses and charges, rather than the true values, is that the real figures are far too small and awkward to work with — a single proton has a mass of roughly 1.7×10−27kg, a number that would be cumbersome to carry through every calculation. By comparing each particle with a proton and giving the proton a mass of 1, we get a simple set of whole numbers (1, 1 and negligible) that capture everything we need: a proton and a neutron weigh the same as each other, and an electron weighs almost nothing by comparison. The relative charges work the same way: instead of quoting the charge in coulombs, we say the proton is +1 and the electron −1, because what matters chemically is that they are equal and opposite.
Exam Tip: Learn the table cold. The two facts most often tested are that the electron's mass is negligible (about 1/1836, not zero — but treated as zero for mass calculations) and that the neutron is neutral (charge 0). Mixing up neutron and proton charges is a common error.
A complete atom has no overall electric charge — it is neutral. This is because it contains equal numbers of protons and electrons:
For example, a carbon atom has 6 protons (6×(+1)=+6) and 6 electrons (6×(−1)=−6); the total charge is +6+(−6)=0. Neutrons make no difference to the charge because they are neutral.
This is why, in a neutral atom, the number of electrons always equals the number of protons. (If an atom gains or loses electrons it becomes a charged ion, which you meet in the next lesson and in C2.)
It is worth pausing on why the charges are exactly equal and opposite. The charge on a single proton is the same size as the charge on a single electron — they are equal in magnitude but opposite in sign. So one proton is precisely balanced by one electron. As long as the number of each is the same, every positive charge is matched by a negative one, and the atom as a whole has no charge that the outside world can detect. This balance is the normal, stable state of an atom, and it is the starting point for understanding how atoms form bonds: when atoms lose or gain electrons in C2, it is exactly this balance that is upset, creating charged ions.
A fluorine atom has 9 protons, 10 neutrons and 9 electrons. Show that it is neutral.
Step 1 — total positive charge from protons: 9×(+1)=+9.
Step 2 — total negative charge from electrons: 9×(−1)=−9.
Step 3 — neutrons contribute 0. Add up: +9+(−9)+0=0.
Answer: the charges cancel, so the atom is neutral.
Atoms are astonishingly small, and their nuclei are smaller still. The standard figures to know (in standard form) are:
In other words, if an atom were scaled up to the size of a large sports stadium, its nucleus would be about the size of a pea at the centre. This is why we say an atom is mostly empty space: the tiny nucleus holds nearly all the mass, but the electrons occupy a volume tens of thousands of times wider.
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