Intuition The one core ideaA nucleus is a tight clump of protons and neutrons, and pulling that clump apart costs energy — which means the clump weighs slightly less than its loose pieces. Everything in this topic is just measuring that tiny missing mass, turning it into energy with E = m c 2
Before you can read the parent note, you need the vocabulary it quietly assumes. This page builds every single symbol from nothing — plain words, then a picture, then why the topic needs it . Read top to bottom; each block leans on the one above.
Definition Proton and neutron — the nucleonsAt the centre of every atom sits a tiny dense ball called the nucleus . It is built from two kinds of particle:
a proton — carries a positive electric charge,
a neutron — carries no charge (neutral).
Together, protons and neutrons are called nucleons ("things in the nucleus").
Look at the figure: the red balls are protons, the grey balls are neutrons, all crammed together. The parent note talks about "separated free nucleons" versus "the nucleus" — that is exactly the difference between the scattered balls on the right and the clumped balls on the left of the picture.
Intuition Why the clump doesn't fly apartSame-charge things repel. All those positive protons should shove each other away. Something stronger must be holding them — the Strong nuclear force . That force is the "glue" the whole topic is really about.
Definition The three counting numbers
Z atomic number = how many protons the nucleus has. It also names the element (Z=2 is always helium).N neutron number = how many neutrons .A mass number = the total count of nucleons.
Definition Reading the symbolZ A X Z A X Z A 1 2 H → Z = 1 , A = 2 , N = 2 − 1 = 1 ( 1 proton, 1 neutron ) 2 4 He → Z = 2 , A = 4 , N = 2 ( 2 protons, 2 neutrons )
Recall Quick self-checkFor 92 235 U N = A − Z = 235 − 92 = 143
Definition The mass symbols
m p proton (in practice the parent uses m 1 H m n neutron .M nucleus M atom whole nucleus (or atom).
Intuition The picture: a balance scalePut all the loose nucleons on the left pan and the assembled nucleus on the right pan. The topic's central surprise is that the pans do not balance — the assembled nucleus is lighter. The next figure shows this.
The gap between the two pans is the star of the whole chapter. We give it a name in §5.
Definition Why a special mass unitNucleons are absurdly tiny (a proton is about 1.67 × 1 0 − 27 atomic mass unit , symbol u 1 u = 12 1 of the mass of one 12 C atom ≈ 1.6605 × 1 0 − 27 kg 1 u 1 u 1.007825 u
u u Atomic mass unit (u) . Why the topic needs it: every table value and every worked example is quoted in u
Δ Δ "how much something differs." So Δ m difference in mass."
Common mistake "Defect" does not mean brokenWrong feels right: "defect = flaw = something went wrong." Fix: here "defect" just means deficit / shortfall of mass. Nothing is broken — the mass turned into energy (next section).
E = m c 2 mass Δ m energy (the "glue"). We need a bridge that converts mass into energy. That exact bridge is Einstein's Mass–energy equivalence ($E=mc^2$) : it is the only law that says mass and energy are two currencies for the same thing, with a fixed exchange rate c 2
Definition The symbols inE = m c 2
E m c c ≈ 3 × 1 0 8 m/s c 2
Definition Binding energyBinding energy E B pour in to rip a nucleus back into free nucleons (equally, the energy released when they first snapped together). By §5 and §6:
E B = Δ m c 2
Intuition The energy-well pictureIn the figure, free nucleons sit at the top (high energy). As they fall together into the nucleus they drop into a well — the depth of that well is E B E B
Definition Binding energy per nucleonThe bar over a symbol means "average." So
E B = A E B per nucleon " — total glue shared out over all A Why the topic needs it: total E B A one average nucleon?" — the real stability score.
BE per nucleon E_B over A
Stability and the BE curve
Fusion and Fission release energy
Read it top to bottom: counts and masses give the mass defect , the mass–energy bridge turns it into binding energy , dividing by A per-nucleon score, and that score explains the stability curve — and therefore Nuclear fusion and Nuclear fission .
Test yourself — you are ready for the parent note only if each reveal comes instantly.
What are nucleons, and which one is charged? Protons and neutrons; the proton is positively charged, the neutron is neutral.
What do Z N A Z N A A = Z + N N = A − Z
How do you read Z A X Bottom-left Z A = A − Z
What is 1 u One-twelfth of a 12 ≈ 1.66 × 1 0 − 27
What does Δ Δ m Δ Δ m
Why is the assembled nucleus lighter than its parts? Assembling it releases binding energy that leaves the system, so by E = m c 2
State the mass→energy conversion the topic uses. 1 u c 2 = 931.5 E B = Δ m ( in u ) × 931.5
What is E B The energy needed to break a nucleus into free nucleons (or released when they combine).
What does the bar in E B A "Average"; dividing total E B A
