The subatomic particles
An atom has a tiny central nucleus containing protons and neutrons, with electrons moving around it in shells (also called energy levels). Almost all of the atom is empty space, and almost all of its mass is in the nucleus. The three particles differ in mass and charge:
| Particle | Relative mass | Relative charge | Where it is |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1840 (almost 0) | -1 | Shells around the nucleus |
A neutral atom has equal numbers of protons and electrons, so the positive and negative charges cancel and the overall charge is zero.
- Nucleus
- The small, dense, positively charged centre of an atom, made of protons and neutrons, holding nearly all of the atom's mass.
Atomic number and mass number
Two numbers describe any atom. The atomic number (proton number) is the number of protons, and it is what makes an element that element. The mass number is the total number of protons plus neutrons.
Because a neutral atom has as many electrons as protons, the atomic number also tells you the number of electrons.
An atom of sodium has atomic number 11 and mass number 23. How many protons, electrons and neutrons does it have?
Electron configuration
Electrons fill shells from the inside out, and each shell holds a limited number before the next one starts. For the first twenty elements the pattern is: 2 in the first shell, then 8, then 8. We write the configuration as the number in each shell separated by commas.
| Element | Atomic number | Configuration |
|---|---|---|
| Oxygen | 8 | 2, 6 |
| Sodium | 11 | 2, 8, 1 |
| Chlorine | 17 | 2, 8, 7 |
| Argon | 18 | 2, 8, 8 |
| Calcium | 20 | 2, 8, 8, 2 |
Two patterns fall straight out of this and are worth memorising: the number of the outer shell electrons equals the group number, and the number of shells used equals the period number (the row).
Read the periodic table backwards
If you know an element is in Group 2, Period 4 (like calcium), you already know its outer shell has 2 electrons and it uses 4 shells: 2, 8, 8, 2. Configuration and table position tell you the same story.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because the proton number is unchanged they are still the same element and behave the same chemically; only the mass differs.
- Isotopes
- Atoms of the same element that have the same number of protons but different numbers of neutrons, so different mass numbers.
Chlorine, for example, exists as chlorine-35 (17 protons, 18 neutrons) and chlorine-37 (17 protons, 20 neutrons). Both have 17 electrons, so both react in exactly the same way.
Group 1 and Group 7 trends
The vertical columns are groups, and elements in the same group react similarly because they have the same number of outer electrons.
Group 1, the alkali metals (lithium, sodium, potassium), each have 1 outer electron, which they lose easily when they react. Reactivity increases as you go down the group: the outer electron is further from the nucleus and less strongly held, so it is lost more easily. Potassium reacts with water far more violently than lithium.
Group 7, the halogens (fluorine, chlorine, bromine, iodine), each have 7 outer electrons and need to gain 1 to fill the shell. Reactivity decreases as you go down: the outer shell is further out, so the atom pulls in an extra electron less strongly. Fluorine is the most reactive halogen.
Where this is assessed
Explaining a reactivity trend by talking about outer electrons and their distance from the nucleus is strong Criterion A understanding. Do not just say "potassium is more reactive", say why: the outer electron is further out and less strongly held, so it is lost more easily.
Check yourself
1. An atom has 17 protons and 18 neutrons. Give its atomic number, mass number, and electron configuration. +
Atomic number = protons = 17. Mass number = protons + neutrons = \(17 + 18 = 35\). With 17 electrons the configuration is 2, 8, 7. This is chlorine.
2. Explain why chlorine-35 and chlorine-37 are chemically identical. +
They are isotopes: both have 17 protons and 17 electrons, so the same electron arrangement (2, 8, 7). Chemical behaviour depends on the electrons, and only the number of neutrons differs, so the two isotopes react in exactly the same way.
3. Which is more reactive, lithium or potassium, and why? +
Potassium is more reactive. It is lower in Group 1, so its single outer electron is in a shell further from the nucleus and is less strongly held. That electron is lost more easily, making the reaction faster.
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