IB Chemistry HLTopic 1 — The Nuclear AtomPaper 1 & 2Core idea~11 min read
The Nuclear Model
An atom is a tiny, ferociously dense nucleus with almost nothing around it. That “almost nothing” is where every chemical reaction you will ever study takes place, so it is worth getting the picture right before anything else is built on top of it.
📚 What you need to know
Atoms contain three subatomic particles: protons, neutrons and electrons.
Masses and charges are quoted as relative values, because the real ones are absurdly small.
Proton: relative charge +1, relative mass 1. Neutron: charge 0, mass 1.
Electron: relative charge −1, mass about 1/1836 of a proton, so treated as negligible.
Protons and neutrons sit in the nucleus and are together called nucleons.
The nucleus is positively charged and extremely dense; electrons occupy the space outside it.
The atom is held together by electrostatic attraction between the nucleus and the electrons.
Why everything here is “relative”
A proton has a mass of about 1.673 × 10−27 kg. Nobody wants to carry that number through a calculation, and it tells you nothing useful on its own. So chemistry compares the particles with each other instead, and the comparison turns out to be beautifully simple: protons and neutrons weigh essentially the same, and electrons weigh essentially nothing.
Particle
Relative charge
Relative mass
Where it is
Proton
+1
1
in the nucleus
Neutron
0
1
in the nucleus
Electron
−1
negligible (about 1/1836)
outside the nucleus
“Negligible” is not a polite way of saying “small”. In a carbon-12 atom the six electrons together account for about 0.03% of the mass. That is why the mass number counts only nucleons and ignores electrons entirely — and it is also why an ion has essentially the same mass as the atom it came from.
Where the particles are
The shells here are drawn as neat circles for clarity. In reality electrons occupy fuzzy regions of space, which is why the phrase “cloud of negative charge” is often used instead.
How empty is empty?
The word “mostly” undersells it. A typical atom has a radius of around 1 × 10−10 m, while its nucleus has a radius of around 1 × 10−15 m. That is a factor of one hundred thousand.
Put those two facts together and you get the definition of dense. Squeeze all the empty space out of a person and what remains would fit on a pinhead.
How anyone knows this
None of the above is obvious, and it was not always believed. The evidence came from firing alpha particles at a very thin sheet of gold foil and watching where they went.
Each observation forces one conclusion. The rebounding particles were the shock: something in that foil was heavy enough and hard enough to throw them straight back.
WORKED EXAMPLE
A proton has a mass of 1.673 × 10−27 kg and an electron 9.109 × 10−31 kg. Show that the electron’s mass is negligible, and state what follows for the mass of an ion.
Compare the two masses1.673 × 10−27 ÷ 9.109 × 10−31 = 1836a proton is about 1836 times heavierScale it up to a whole atomIn carbon-12 there are 6 protons, 6 neutrons and 6 electrons. The electrons contribute roughly 0.03% of the total mass, which is smaller than the rounding on most measurements.What follows for ionsForming an ion means gaining or losing electrons, and electrons weigh almost nothing.an ion has essentially the same mass as its atomThis is why mass spectrometry can measure ions and still tell you about atoms.
WORKED EXAMPLE
An atom has a radius of 1.0 × 10−10 m and a nuclear radius of 1.0 × 10−15 m. Calculate how many times wider the atom is, and estimate the fraction of the atom’s volume taken up by the nucleus.
Ratio of the radii1.0 × 10−10 ÷ 1.0 × 10−15 = 1 × 105the atom is 100 000 times widerRatio of the volumesVolume scales with the cube of the radius, so cube the ratio.(10−5)3 = 1 × 10−15the nucleus is about 10−15 of the volumeThe point of the calculationNearly all the mass is packed into a millionth of a billionth of the space. Cubing the ratio is the step people forget — answering 10−5 confuses a width with a volume.
WORKED EXAMPLE
Explain why a nucleus containing several protons does not simply fly apart, and state what force holds the electrons in the atom.
The problem with the nucleusProtons all carry a charge of +1, so they repel one another electrostatically, and at nuclear distances that repulsion is enormous.Why it holds togetherA separate and much stronger attraction acts between nucleons over very short ranges, and it outweighs the repulsion inside the nucleus.a short-range nuclear force, stronger than the repulsionHolding the electronsThat is a different question with a simpler answer: the positive nucleus attracts the negative electrons.electrostatic attractionKeep the two separate. Chemistry only ever needs the electrostatic answer; the nuclear force never appears in a bonding explanation.
💡 Exam tip
Learn the relative charge and mass table cold — it is free marks and it underpins everything.
Say the electron mass is negligible, not zero.
Use the word nucleons for protons and neutrons together.
Name electrostatic attraction as what holds the atom together.
Describe the nucleus as small, dense and positively charged — all three words earn credit.
When comparing volumes, remember to cube the ratio of the radii.
⚠️ Common mix-up
Giving the electron a relative mass of 0 instead of negligible.
Putting electrons in the nucleus, or neutrons outside it.
Saying the nucleus is neutral because it contains neutrons. Protons make it positive.
Confusing relative and absolute values — +1 is a comparison, not a charge in coulombs.
Explaining nuclear stability with electrostatic attraction, which is the wrong force entirely.
Up next: Protons, Neutrons and Electrons — you know what the particles are. The next page is about counting them from a chemical symbol, including the ions where the counting goes wrong most often.
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