Atomic structure explains why elements behave the way they do. Master this topic and the periodic table, bonding and chemical reactions become far easier to understand.
The three subatomic particles
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | About 1/1840 (negligible) | Shells (energy levels) around the nucleus |
An atom is electrically neutral because it has the same number of protons and electrons.
Atomic number and mass number
- Atomic number (Z) is the number of protons. It identifies the element. Every carbon atom has 6 protons.
- Mass number (A) is the number of protons plus neutrons.
- Number of neutrons = A − Z.
Example: sodium has Z = 11 and A = 23. It has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers. Chlorine has two common isotopes, chlorine-35 and chlorine-37. Isotopes of an element have the same chemical properties because they have the same electron arrangement.
The relative atomic mass is the weighted average of the isotopes. For chlorine, about 75% is Cl-35 and 25% is Cl-37: (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5.
Electron arrangement
For the first 20 elements, electrons fill shells in a simple pattern: up to 2 in the first shell, then up to 8 in the second and 8 in the third.
- Oxygen (Z = 8): 2, 6
- Sodium (Z = 11): 2, 8, 1
- Chlorine (Z = 17): 2, 8, 7
- Calcium (Z = 20): 2, 8, 8, 2
In more advanced work you will write configurations using sub-shells, for example sodium as 1s² 2s² 2p⁶ 3s¹.
The periodic table
- Groups are vertical columns. Elements in the same group have the same number of outer (valence) electrons, so they have similar chemical properties. Sodium (2, 8, 1) is in Group 1 because it has one outer electron.
- Periods are horizontal rows. The period number equals the number of occupied electron shells. Sodium has three shells, so it is in Period 3.
Group 1 metals (alkali metals) are very reactive and lose one electron easily. Group 17 elements (halogens) need one electron to complete their outer shell. Group 18 elements (noble gases) have full outer shells and are very unreactive.
Periodic trends
- Atomic radius decreases across a period (more protons pull the electrons closer) and increases down a group (more shells).
- Ionisation energy, the energy needed to remove an electron, generally increases across a period and decreases down a group.
- Metallic character decreases across a period and increases down a group.
Exam tips
- Always show your working for neutron and relative atomic mass calculations.
- When explaining a trend, mention nuclear charge, the number of shells and shielding.