What’s Inside an Atom
An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are far too small to see even with the most powerful microscopes, but we know they’re built from subatomic particles — mainly protons, neutrons and electrons. Protons and neutrons sit in the atom’s nucleus (together they’re called nucleons), while electrons move around the nucleus in circular paths called shells.
| Subatomic particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Inside the nucleus |
| Neutron | 1 | 0 | Inside the nucleus |
| Electron | 1/1840 (negligible) | −1 | Outside the nucleus, in shells |
Atomic Number and Mass Number
The atomic number (Z) equals the number of protons — and, in a neutral atom, the number of electrons too. All atoms of a given element share the same atomic number. The mass number (A), or nucleon number, is the number of protons plus neutrons, and is always larger than the atomic number: A = protons + neutrons.
An atom is written with its mass number as a superscript and atomic number as a subscript before its symbol — e.g. carbon, sodium and fluorine are written ¹²₆C, ²³₁₁Na and ¹⁹₉F.
| Atom | Atomic number (Z) | Mass number (A) | Protons | Electrons | Neutrons (A−Z) |
|---|---|---|---|---|---|
| ¹²₆C | 6 | 12 | 6 | 6 | 6 |
| ²³₁₁Na | 11 | 23 | 11 | 11 | 12 |
| ¹⁹₉F | 9 | 19 | 9 | 9 | 10 |
Electronic Configuration
Electronic configuration is how an atom’s electrons are arranged into shells. The maximum number of electrons a shell can hold is given by 2n², where n is the shell number: the K-shell (n=1) holds up to 2, the L-shell (n=2) up to 8, the M-shell (n=3) up to 18, and the N-shell (n=4) up to 32. Configurations are written in order K, L, M, N — for example, phosphorus (atomic number 15) is 2, 8, 5.
| Element | Symbol | Atomic number | Configuration (K,L,M,N) |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Helium | He | 2 | 2 |
| Lithium | Li | 3 | 2, 1 |
| Beryllium | Be | 4 | 2, 2 |
| Boron | B | 5 | 2, 3 |
| Carbon | C | 6 | 2, 4 |
| Nitrogen | N | 7 | 2, 5 |
| Oxygen | O | 8 | 2, 6 |
| Fluorine | F | 9 | 2, 7 |
| Neon | Ne | 10 | 2, 8 |
| Sodium | Na | 11 | 2, 8, 1 |
| Magnesium | Mg | 12 | 2, 8, 2 |
| Aluminium | Al | 13 | 2, 8, 3 |
| Silicon | Si | 14 | 2, 8, 4 |
| Phosphorus | P | 15 | 2, 8, 5 |
| Sulphur | S | 16 | 2, 8, 6 |
| Chlorine | Cl | 17 | 2, 8, 7 |
| Argon | Ar | 18 | 2, 8, 8 |
| Potassium | K | 19 | 2, 8, 8, 1 |
| Calcium | Ca | 20 | 2, 8, 8, 2 |
Ion Formation
An ion is a charged particle formed when an atom loses or gains electrons. Losing electrons leaves a positive charge — this is called a cation; gaining electrons leaves a negative charge — an anion. The size of the charge tells you how many electrons were lost or gained.
Examples:
- Magnesium ion, ²⁴₁₂Mg²⁺: 12 protons, 12 neutrons (24−12), and only 10 electrons (12 minus the 2 lost).
- Aluminium ion, ²⁷₁₃Al³⁺: 13 protons, 14 neutrons (27−13), and 10 electrons (13 minus the 3 lost).
- Nitride ion, ¹⁴₇N³⁻: 7 protons, 7 neutrons (14−7), and 10 electrons (7 plus the 3 gained).
- Sulphide ion, ³²₁₆S²⁻: 16 protons, 16 neutrons (32−16), and 18 electrons (16 plus the 2 gained).
Isotopy
Isotopes are atoms of the same element with the same number of protons and electrons, but different numbers of neutrons — and so different mass numbers. Isotopes of an element share its chemical properties but can differ in physical properties like density and rate of diffusion. Hydrogen’s three isotopes — ¹₁H (no neutrons), ²₁H (one neutron), ³₁H (two neutrons) — are a classic example, as are chlorine’s ³⁵₁₇Cl and ³⁷₁₇Cl.
Relative Atomic Mass
Most elements occur naturally as a mixture of isotopes, so the relative atomic mass (RAM) used for an element is a weighted average across all its naturally occurring isotopes, based on their abundance — formally, the average mass of an atom of the element compared to 1/12th the mass of a carbon-12 atom.
RAM = [(mass of isotope A × % abundance of A) + (mass of isotope B × % abundance of B) + …] ÷ 100
Example: chlorine occurs as 75% chlorine-35 and 25% chlorine-37. RAM = (35×75 + 37×25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
Example: boron occurs as 20% boron-10 and 80% boron-11. RAM = (10×20 + 11×80) ÷ 100 = (200 + 880) ÷ 100 = 10.8.
