Inside the Atom's Nucleus — Nuclear Stability
Every atom in your body has a tiny core 100,000 times smaller than the atom itself — yet that core holds 99.9% of the mass and decides whether the atom lasts forever or falls apart.
Q1 · An atom is mostly empty space, yet almost all of its mass is in the nucleus. What two particles do you think make up that nucleus, and which one carries the positive charge?
Q2 · Protons all carry a positive charge, and like charges repel. So why doesn't a nucleus simply blow itself apart? What might be holding it together?
● Know
- That the nucleus contains protons and neutrons (nucleons), held together by the strong nuclear force
- What isotopes are and how to read nuclide notation (mass number, atomic number)
- That some nuclei are stable and others are unstable (radioactive)
● Understand
- Why the strong force must overcome the electrostatic repulsion between protons
- How the neutron-to-proton ratio determines whether a nucleus is stable
- Why all elements heavier than bismuth (and some lighter isotopes) are unstable
● Can do
- Read and write nuclide symbols such as $^{14}_{6}\text{C}$ and state the number of protons and neutrons
- Identify isotopes of an element from their mass numbers
- Predict whether a nucleus is likely to be stable using its neutron-to-proton ratio
An atom has a tiny central nucleus surrounded by a cloud of electrons. The nucleus contains two kinds of particle, together called nucleons: protons (charge +1) and neutrons (no charge). Electrons (charge −1) orbit outside. The nucleus is about 100,000 times smaller than the whole atom, yet it holds more than 99.9% of the atom's mass.
Two numbers describe any nucleus:
- Atomic number, $Z$ = number of protons. This defines the element. Every carbon atom has $Z = 6$; change $Z$ and you change the element.
- Mass number, $A$ = number of protons + neutrons (total nucleons). So the number of neutrons is $A - Z$.
We write a nucleus using nuclide notation: the mass number goes top-left and the atomic number bottom-left of the element symbol. For example, $^{14}_{6}\text{C}$ is carbon with 6 protons and $14 - 6 = 8$ neutrons.
The nuclide $^{23}_{11}\text{Na}$ (sodium-23) has:
- Atomic number $Z = 11$, so 11 protons — this is what makes it sodium.
- Mass number $A = 23$, so $23 - 11 = 12$ neutrons.
- In a neutral atom, 11 electrons balance the 11 protons.
ANSTO's OPAL reactor: At Lucas Heights in southern Sydney, the OPAL research reactor deliberately makes unstable nuclei. It fires neutrons into stable atoms to add neutrons to their nuclei, producing radioisotopes used in nuclear medicine. Understanding which nuclei are stable and which are not — exactly the content of this lesson — is the everyday work of Australian nuclear scientists.
The mass number $A$ is not the same as the atomic mass on the periodic table. The atomic mass (e.g. 12.01 for carbon) is a weighted average of all naturally occurring isotopes. The mass number is a whole-number count of nucleons in one specific nuclide. Carbon-12 and carbon-14 are both carbon, but they have different mass numbers (12 and 14).
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because the chemistry of an atom is controlled by its electrons (and therefore by its proton count), isotopes of an element are chemically almost identical — they react the same way and form the same compounds. They differ only in mass and, crucially, in nuclear stability.
Carbon is the classic example. Carbon-12 ($^{12}_{6}\text{C}$, 6 neutrons) and carbon-13 ($^{13}_{6}\text{C}$, 7 neutrons) are stable. Carbon-14 ($^{14}_{6}\text{C}$, 8 neutrons) is unstable — it is a radioisotope. All three are carbon and behave the same chemically, but carbon-14 slowly decays. That is why carbon-14 can be used as a clock for dating once-living things (you will meet half-life in Lesson 24).
Hydrogen has three named isotopes you should recognise: protium ($^{1}_{1}\text{H}$, no neutrons), deuterium ($^{2}_{1}\text{H}$, 1 neutron) and tritium ($^{3}_{1}\text{H}$, 2 neutrons, radioactive).
Are $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$ isotopes? Yes. Both have $Z = 17$ (17 protons → both are chlorine), but one has $35 - 17 = 18$ neutrons and the other has $37 - 17 = 20$ neutrons. Same element, different neutron count = isotopes. Natural chlorine is a mixture of these two, which is why its average atomic mass (35.45) is not a whole number.
Australian groundwater detective work: CSIRO and Australian universities use stable and radioactive isotopes of hydrogen and oxygen to trace where groundwater comes from and how old it is — vital for managing the Murray–Darling Basin and outback bore water. Because isotopes are chemically identical, the water looks and tastes the same, but its isotope "fingerprint" reveals its history.
Isotopes are not different elements. A common error is thinking carbon-14 is a "different substance" from carbon-12. They are the same element with identical chemistry; only the nucleus differs. Changing the neutron number gives an isotope; changing the proton number gives a brand new element.
Quick-fire true or false on nuclei and isotopes.
The atomic number equals the number of protons.
Isotopes of an element have different numbers of protons.
Number of neutrons equals mass number minus atomic number.
Almost all of an atom's mass is in the nucleus.
Neutrons carry a positive charge.
Carbon-12 and carbon-14 are isotopes of carbon.
The strong nuclear force holds nucleons together.
A radioisotope has a perfectly stable nucleus.
Here is the puzzle: protons are all positively charged, and like charges repel. Packed into a nucleus a few femtometres across, protons push each other apart with enormous electrostatic repulsion. So why don't nuclei fly apart?
The answer is the strong nuclear force — an attractive force that acts between all nucleons (proton–proton, proton–neutron and neutron–neutron). It is far stronger than electrostatic repulsion, but it only acts over an extremely short range. A nucleus is stable when the attractive strong force exactly balances the repulsive electrostatic force.
Neutrons are the peacekeepers. Neutrons add strong-force attraction without adding any repulsion (they have no charge). So as nuclei get bigger and contain more protons, they need extra neutrons to stay glued together. The conditions that cause a nucleus to be unstable are:
- Wrong neutron-to-proton ratio. Too few or too many neutrons for the number of protons throws off the balance. Light stable nuclei have roughly 1 neutron per proton; heavier stable nuclei need up to about 1.5 neutrons per proton.
- Too many protons (too big). Beyond $Z = 83$ (bismuth), the strong force simply cannot hold the pile of mutually repelling protons together. Every element heavier than bismuth is radioactive.
An unstable nucleus releases its excess energy by radioactive decay — emitting particles or energy to move toward a more stable arrangement. You will model exactly how it does this (alpha and beta decay) in Lesson 23.
Helium-4 ($^{4}_{2}\text{He}$, 2 protons + 2 neutrons) sits right in the stable band — its neutron-to-proton ratio is 1:1 and the strong force easily wins. Uranium-238 ($^{238}_{92}\text{U}$, 92 protons + 146 neutrons) has $Z = 92$, far beyond bismuth. No matter how many neutrons it carries, 92 mutually repelling protons cannot be held permanently, so uranium-238 is radioactive.
Australia's uranium: Australia holds roughly one-third of the world's known uranium reserves, mined at sites such as Olympic Dam in South Australia. Uranium is valuable precisely because its nuclei are unstable — that instability is the energy source for nuclear power and the starting point for the fission chain reactions you will study in Lesson 26.
Adding neutrons does not always make a nucleus more stable. Neutrons help hold larger nuclei together, but too many neutrons is also unstable (the nucleus then tends to undergo beta decay). Stability needs the right ratio — a balance, not a maximum.
Connect the key ideas about nuclear stability. Click two connected ideas to explain the link.
Wrong: "Isotopes of an element are different elements because they have different masses." No — isotopes share the same proton number, so they are the same element with identical chemistry. Only the neutron count (and therefore the mass) differs.
Right: Isotopes are atoms of the same element with the same atomic number but different mass numbers. Carbon-12, carbon-13 and carbon-14 are all carbon; they differ only in their number of neutrons.
Wrong: "The more neutrons a nucleus has, the more stable it is." No — both too few and too many neutrons cause instability. Stability depends on the ratio of neutrons to protons being in the right range.
Right: A nucleus is stable only when the neutron-to-proton ratio sits inside the band of stability, so the strong nuclear force balances the electrostatic repulsion between protons.
Wrong: "Gravity holds the nucleus together." No — gravity is far too weak at this scale. The nucleus is held together by the strong nuclear force, which is many times stronger than the electrostatic repulsion it must overcome.
Right: The short-range strong nuclear force attracts all nucleons and is what overcomes proton–proton repulsion. Gravity plays no meaningful role at the scale of a nucleus.
Nuclear Science in Australia
Australia has chosen not to build nuclear power stations, but it is a world leader in nuclear science. At ANSTO's Lucas Heights campus in Sydney, the OPAL reactor produces radioisotopes that are used in roughly 10,000 Australian medical procedures every week, and the Australian Synchrotron in Melbourne lets scientists probe the structure of matter. None of this is possible without understanding which nuclei are stable and which are not.
Australia's enormous uranium reserves mean these ideas also matter for trade and energy policy. Whether or not Australia ever generates nuclear electricity, every citizen benefits from being able to reason about nuclear stability, radiation and risk — the foundation laid in this lesson.
✍ Copy Into Your Books
▾The Nucleus
- Nucleus = protons (+1) + neutrons (0) = nucleons
- Atomic number Z = number of protons (defines the element)
- Mass number A = protons + neutrons
- Neutrons = A − Z
- Nuclide notation: mass number top-left, atomic number bottom-left
Isotopes
- Same element, same protons, different neutrons
- Chemically identical; differ in mass and stability
- e.g. carbon-12, carbon-13 (stable), carbon-14 (radioactive)
Stability
- Strong nuclear force attracts all nucleons (short range)
- Electrostatic force repels protons
- Stable = correct neutron-to-proton ratio (band of stability)
- All elements with Z > 83 (above bismuth) are radioactive
Reading Nuclide Symbols
Stable or Unstable?
At the start of this lesson, the hook asked why some atoms of the same element are perfectly stable while others spontaneously break apart.
Now that you understand the tug-of-war between the strong nuclear force and electrostatic repulsion, write a clearer answer than you could have at the start. Use the words neutron-to-proton ratio and band of stability in your response.
Q1. The nuclide oxygen-18 is written $^{18}_{8}\text{O}$. State its number of protons, neutrons and electrons (neutral atom), and explain why oxygen-16 and oxygen-18 are described as isotopes. (3 marks)
Q2. Using the ideas of electrostatic repulsion and the strong nuclear force, explain the conditions that cause a nucleus to be unstable. Refer to both the neutron-to-proton ratio and very large nuclei in your answer. (4 marks)
Q3. A student says: "Adding more neutrons always makes a nucleus more stable." Evaluate this claim, correcting it where necessary and giving a reason. (3 marks)
Revisit Your Thinking
Go back to your Think First answers. Has your understanding changed?
- Can you now name the two particles in the nucleus and say which is positive?
- Can you explain what stops a nucleus from blowing itself apart?
Model answers (click to reveal)
Answers
▾MCQ 1
C — The number of protons (the atomic number, Z) defines the element. Change the proton number and you change the element.
MCQ 2
B — Neutrons = mass number − atomic number = 31 − 15 = 16.
MCQ 3
A — Same protons, different neutrons means they are isotopes of the same element.
MCQ 4
D — The strong nuclear force attracts all nucleons over a very short range and is strong enough to overcome the electrostatic repulsion between protons. Gravity is far too weak at this scale.
MCQ 5
B — Beyond Z = 83 there are so many mutually repelling protons that the short-range strong force can no longer hold them together permanently, so every such nucleus is unstable (radioactive), regardless of neutron number.
Short Answer 1
Model answer: Oxygen-18 has 8 protons (Z = 8), 18 − 8 = 10 neutrons, and 8 electrons in a neutral atom. Oxygen-16 and oxygen-18 are isotopes because they are both oxygen (both have 8 protons) but have different numbers of neutrons (8 and 10), and therefore different mass numbers, while remaining chemically the same element.
Short Answer 2
Model answer: Protons all carry a positive charge and repel each other through the electrostatic force. The strong nuclear force attracts all nucleons but acts only over a very short range. A nucleus is unstable when these forces are out of balance. If the neutron-to-proton ratio is wrong — too few or too many neutrons — there is not the right amount of strong-force attraction to balance the repulsion, so the nucleus decays. In very large nuclei (atomic number greater than 83), there are so many repelling protons that the short-range strong force cannot reach across the whole nucleus to hold it together, so all of these nuclei are unstable.
Short Answer 3
Model answer: The claim is incorrect as a general rule. Neutrons do add strong-force attraction without adding repulsion, so heavier nuclei do need extra neutrons to be stable. However, too many neutrons also makes a nucleus unstable — it then tends to undergo beta decay. Stability requires the neutron-to-proton ratio to lie within the band of stability, so it is a matter of the correct balance, not simply "more neutrons is better."