- Describe the structure of an atom
- State the charges and masses of subatomic particles
- Explain what makes an element what it is
The smallest building blocks
An atom is the smallest particle of an element that still has that element's properties. Atoms are unimaginably small — about ten million would span a millimetre — but they have internal structure.
An atom consists of a central nucleus (protons and neutrons) with electrons arranged in shells around it. Atoms are electrically neutral overall.
The three particles
| Particle | Charge | Mass | Location | |----------|--------|------|----------| | Proton | +1 | 1 | Nucleus | | Neutron | 0 | 1 | Nucleus | | Electron | −1 | ~0 | Shells |
Protons and neutrons have nearly all the mass; electrons are ~1/2000th of a proton's mass — negligible for mass calculations, but they control chemistry: it's electrons that form bonds.
- Atomic (proton) number = number of protons. This defines the element: 6 protons is always carbon.
- Mass number = protons + neutrons. Neutrons can vary — same element, different mass.
- In a neutral atom, electrons = protons.
An atom has 8 protons and 10 neutrons. Identify it and count its electrons.
8 protons → atomic number 8 → oxygen. Mass number = 8 + 10 = 18. Neutral atom → 8 electrons.
Isotopes
Atoms of the same element with different numbers of neutrons are isotopes: carbon-12 (6 neutrons) and carbon-14 (8 neutrons) are both carbon — same protons, same chemistry, different mass. Some isotopes are unstable and radioactive.
Practice
Atomic number 11 → sodium. Mass number = 11 + 12 = 23.
The number of positive protons equals the number of negative electrons, so the charges cancel.
Quick check
Which particle has a negative charge?
What defines which element an atom is?
What are isotopes?
- Atoms: nucleus (protons + neutrons) with electrons in shells.
- Proton number defines the element; mass number = protons + neutrons.
- Neutral atoms have equal protons and electrons.
- Isotopes: same element, different neutron count.