IB Chemistry SLTopic 1 — The Nuclear AtomPaper 1 & 2Core idea~7 min read
The Nuclear Model
Atoms are built from three smaller particles — protons, neutrons and electrons. Once you know where each one sits and how heavy it is, the whole picture of the atom clicks into place: a tiny dense nucleus at the centre, with electrons in the space around it.
📘 What you need to know
Atoms are made of three subatomic particles: protons, neutrons and electrons.
Their masses and charges are so tiny that we compare them using relative mass and relative charge, not grams and coulombs.
The nucleus is a small, dense, positively charged centre containing protons and neutrons (together called nucleons).
Electrons are negatively charged and occupy the space outside the nucleus.
The atom is held together by electrostatic attraction between the positive nucleus and the negative electrons.
Relative mass and charge
The three subatomic particles are far too small to weigh in grams or measure in coulombs, so chemists compare them to each other instead. This gives simple, easy numbers to work with.
Protons and neutrons have almost the same mass, so each is given a relative mass of 1.
An electron is roughly 1836 times lighter, so its mass counts as negligible (near zero).
The relative mass and charge of the three subatomic particles — the values you’ll use again and again.
The actual charge on a proton is +1.602 × 10-19 C and on an electron −1.602 × 10-19 C — equal but opposite. You’ll almost always use the simple relative values (+1 and −1); the real numbers are in the IB data booklet if you need them.
Where the particles sit
The atom has two distinct regions: a tiny central nucleus and a much larger space around it.
The nucleus is positively charged because it holds the protons.
It’s incredibly dense — nearly all the atom’s mass is packed into it, since the heavy protons and neutrons live there. Together these are called nucleons.
Electrons are negatively charged and move around outside the nucleus, in a region often pictured as a “cloud” of negative charge.
Nearly all the mass sits in the tiny positive nucleus; electrons occupy the mostly empty space around it, held by electrostatic attraction.
What holds the atom together
Opposite charges attract. The nucleus is positive and the electrons are negative, so there’s an electrostatic attraction pulling the electrons towards the centre. That attraction is what keeps the atom held together as a single, stable unit.
💡 Exam tip
Learn the relative mass and charge table cold — it underpins the whole of atomic structure.
“Nucleons” = protons + neutrons. If a question mentions mass, it’s really asking about the particles in the nucleus.
Up next: Protons, Neutrons and Electrons — how to use atomic number and mass number to count exactly how many of each particle an atom or ion has.
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