Today we zoom into the tiny core of the atom — the nucleus — and find out what atoms are actually made of, how chemists describe different versions of the same element, and why some nuclei are unstable enough to be radioactive.
What's Inside an Atom?
Every atom is built from three subatomic particles:
- Protons – positive charge, found in the nucleus
- Neutrons – no charge (neutral), also found in the nucleus
- Electrons – negative charge, found in the space outside the nucleus
The nucleus itself is tiny but extremely dense, and it carries a positive charge because it contains the protons and neutrons (together called nucleons) (Talbot, p. 34). The electrons occupy the much larger space around the nucleus, arranged in shells and subshells (which you'll explore further when we look at electron configuration).
Nuclear Notation: A over Z
Chemists describe the composition of an atom using a standard notation:
A
ᴢX
Where:
- Z = atomic number = number of protons
- A = mass number (nucleon number) = protons + neutrons
From this, we get a key relationship (Talbot, p. 42):
number of neutrons = A − Z
Worked Example 1: For aluminium-27, written as ²⁷₁₃Al:
- Z = 13, so there are 13 protons and (in a neutral atom) 13 electrons
- A = 27, so neutrons = 27 − 13 = 14 neutrons
Isotopes: Same Element, Different Mass
Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons — and therefore different mass numbers. For example, carbon-12 (¹²C) and carbon-14 (¹⁴C) both have 6 protons, but ¹²C has 6 neutrons while ¹⁴C has 8.
A common trap to avoid: two different elements can share the same mass number. For instance, ⁴⁰₁₈Ar, ⁴⁰₁₉K and ⁴⁰₂₀Ca all have A = 40, but they are different elements because their proton numbers (Z) differ (Talbot, p. 42).
Ions: When Electrons Come and Go
Atoms are neutral because protons = electrons. An ion forms when electrons are gained or lost:
- Losing electrons → positive ion (cation)
- Gaining electrons → negative ion (anion)
Worked Example 2: An Al³⁺ ion still has 13 protons (that never changes for a given element), but it has lost 3 electrons, so it has only 10 electrons. The number of neutrons is unaffected.
Radioactivity: Unstable Nuclei
Some combinations of protons and neutrons make a nucleus unstable. To become more stable, these nuclei release energy and particles — a process called radioactivity. Isotopes that do this are called radioisotopes, and ¹⁴C is a well-known example, used in radiocarbon dating of ancient organic material. The key idea is that radioactivity is a nuclear event — it changes the nucleus itself, not just the arrangement of electrons.
Worked Example 3: How many protons, neutrons and electrons are in a neutral atom of ²³⁵U (uranium-235)?
- Z = 92 → 92 protons, 92 electrons
- A = 235 → neutrons = 235 − 92 = 143 neutrons
Key Takeaways
- Atoms consist of protons and neutrons in a dense central nucleus, with electrons occupying the surrounding space.
- The nuclear symbol ᴬzX lets you calculate protons, neutrons and electrons using A − Z = neutrons.
- Isotopes share the same proton number but differ in neutron number and mass number.
- Ions form by gaining or losing electrons only — the proton number stays fixed.
- Radioactivity occurs when an unstable nucleus releases energy/particles to become more stable.