When carbon burns, the carbon atoms join onto oxygen atoms to make carbon dioxide.

Every carbon atom in that carbon dioxide is still carbon.

A chemical change rearranges how atoms are joined to one another.

Every atom keeps its identity through the change.

The reason lies in which parts of the atom take part.

A chemical change involves only the atoms' electrons — the outer parts that form the connections between atoms.

The nucleus of each atom is never touched by a chemical change.

Because the nucleus never changes, no chemical change can ever turn an atom of one element into an atom of another — which is why the alchemists always failed.

Some changes reach past the electrons entirely.

A **'nuclear change'** alters the nucleus itself.

When the nucleus changes, the proton count can change.

The proton count is the atomic number — the number that decides which element an atom is.

So a nuclear change can turn an atom of one element into an atom of a different element.

In one nuclear change found in nature, a uranium atom's nucleus goes from 92 protons to 90 protons.

An atom with 90 protons is thorium — the uranium atom has become a thorium atom.

To classify a change, ask one question: did any atom change its identity?

If every atom keeps its identity and only the connections between atoms change, the change is chemical.

If the nucleus changes so that a new element can appear, the change is nuclear.

Iron rusting is a chemical change: the iron atoms join onto oxygen atoms, but every iron atom is still iron.

Watch out for energy as a false clue — many chemical changes release plenty of heat and light too, so energy release alone settles nothing.

Compare equal masses: 1 g of uranium in a nuclear change, and coal burning in a chemical change.

The nuclear change of 1 g of uranium releases about as much energy as burning 3 tons of coal.

For the same mass of material, a nuclear change releases around a million times more energy than a chemical change.

The comparison only holds mass-for-mass — the rule compares the SAME mass of material.

Weigh everything before and after a nuclear change, and the books do not balance.

After a uranium nucleus changes, the pieces left behind have slightly less total mass than the original nucleus.

The missing mass did not leave as a particle — no proton or neutron has gone missing.

The missing mass was converted into energy.

A nuclear change can release so much energy because a small amount of the nucleus's mass is converted into a very large amount of energy during a nuclear change.

Chemical changes never tap this supply, because they involve only the electrons and leave every nucleus untouched.

Every nucleus is one of two kinds: stable or unstable.

A **'stable'** nucleus stays the same forever on its own.

An **'unstable'** nucleus will sooner or later change on its own.

Nothing needs to touch an unstable nucleus — the change comes from within.

Every carbon-12 nucleus stays carbon-12 forever: carbon-12 is stable.

Any carbon-14 nucleus will eventually change: carbon-14 is unstable.

Stability belongs to the specific isotope, not to the element — carbon-12 and carbon-14 are both carbon.

Inside every nucleus, the protons all carry a positive charge.

Like charges repel, so the protons in a nucleus constantly push one another apart.

Neutrons help hold the nucleus together.

Because protons repel one another, a nucleus needs a balanced number of neutrons to hold together, and a nucleus with too few or too many neutrons for its proton count is unstable.

Carbon-12, with 6 neutrons for its 6 protons, has a balanced count and is stable.

Carbon-14, with 8 neutrons for its 6 protons, has too many neutrons and is unstable.

What counts is the balance for that proton count — not simply whether neutrons outnumber protons.

Sooner or later, an unstable nucleus changes on its own into a more stable nucleus.

This event is called **'radioactive decay'**.

As the nucleus decays, it shoots out energy and particles.

The energy and particles shot out are called **'nuclear radiation'**.

An isotope whose nuclei decay on their own is **'radioactive'**.

Carbon-14 is a radioactive isotope, because its nuclei decay on their own.

Three words, three jobs: the decay is the event, the radiation is what flies out, and radioactive describes the isotope.

When nuclear radiation passes through an object, the object has been **'irradiated'**.

Irradiation does not make the object radioactive.

The radiation deposits energy as it passes, and then it is gone.

**'Contamination'** means radioactive atoms have been left on or in an object.

A contaminated object keeps giving off radiation, because the radioactive atoms on it keep decaying there.

Food sterilized by a beam of radiation has been irradiated — the food does not become radioactive.

To classify a situation, ask where the radioactive atoms are: still sealed in the source, or left on the object?

One kind of decay shoots out a whole chunk of the nucleus: 2 protons and 2 neutrons bound together.

This chunk is called an **'alpha particle'**, and the decay that shoots it out is **'alpha decay'**.

Two protons and two neutrons is exactly the makeup of a helium-4 nucleus.

The alpha particle is written ⁴₂He, in the same isotope notation used for atoms.

The 4 in ⁴₂He is the particle's mass number: 2 protons + 2 neutrons = 4.

The 2 in ⁴₂He is the particle's proton count.

A **'nuclear equation'** records a nuclear change in symbols.

The nucleus before the change sits on the left.

The arrow means 'changes into'.

The nucleus and radiation produced sit on the right.

Each species is written in isotope notation: mass number on top, atomic number below.

Read ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He as: a uranium-238 nucleus changes into a thorium-234 nucleus and an alpha particle.

The + on the right simply lists everything produced — the new nucleus and the particle both appear.

Rule 1: the mass numbers on the left add up to the same total as the mass numbers on the right.

Mass numbers balance because the total count of protons plus neutrons never changes in a nuclear change.

Rule 2: the atomic numbers on the left add up to the same total as the atomic numbers on the right.

Atomic numbers balance because electric charge never appears or disappears.

Check ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He: the mass numbers give 238 on the left and 234 + 4 = 238 on the right.

The atomic numbers give 92 on the left and 90 + 2 = 92 on the right.

Both totals match, so the equation is balanced.

In alpha decay, the nucleus shoots out an alpha particle: 2 protons and 2 neutrons.

Losing those 4 particles drops the mass number by 4.

Losing the 2 protons drops the atomic number by 2.

A new atomic number means a new element: the atom becomes the element two places earlier in the periodic table.

Uranium-238 shows the shift: ²³⁸₉₂U becomes ²³⁴₉₀Th plus ⁴₂He.

238 − 4 = 234, and 92 − 2 = 90 — and the element with atomic number 90 is thorium.

The two drops always come together, because every alpha particle carries exactly 2 protons and 2 neutrons.

Write the decaying nucleus on the left, in isotope notation.

Find the new mass number: subtract 4.

Find the new atomic number: subtract 2.

Use the periodic table to name the element with the new atomic number.

Write the new nucleus after the arrow, then add the alpha particle ⁴₂He.

Polonium-210 runs the routine: ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He.

Check the balance: 210 = 206 + 4, and 84 = 82 + 2.

The particle shot out in beta decay is an electron — chemists call it a **'beta particle'**.

The nucleus holds no electrons, so the beta particle must be created at the moment of decay.

It is created when one of the nucleus's neutrons changes into a proton plus an electron.

The proton stays in the nucleus; the electron shoots out as the beta particle.

So a nucleus with 6 protons and 8 neutrons becomes a nucleus with 7 protons and 7 neutrons.

In nuclear equations the beta particle is written ⁰₋₁e.

The top number is 0 because an electron is not a proton or a neutron, so it adds nothing to a mass-number count.

The bottom number is −1 because that slot records the particle's charge, and the electron's charge is 1−.

In beta decay, a neutron changes into a proton, and the new electron shoots out.

The total count of protons plus neutrons is unchanged, so the mass number stays the same.

The proton count grows by one, so the atomic number rises by 1.

A new atomic number means a new element: the atom becomes the element one place later in the periodic table.

Carbon-14 shows the shift: ¹⁴₆C becomes ¹⁴₇N plus ⁰₋₁e.

The mass number stays 14, and 6 + 1 = 7 — the element with atomic number 7 is nitrogen.

The rise of exactly 1 never varies, because every beta decay turns exactly one neutron into one proton.

Write the decaying nucleus on the left, in isotope notation.

Keep the mass number the same.

Add 1 to the atomic number.

Use the periodic table to name the element with the new atomic number.

Write the new nucleus after the arrow, then add the beta particle ⁰₋₁e.

Strontium-90 runs the routine: ⁹⁰₃₈Sr → ⁹⁰₃₉Y + ⁰₋₁e.

Check the balance: 90 = 90 + 0, and 38 = 39 + (−1).

(One honest footnote: beta decay also shoots out a second, tiny neutral particle — it changes none of the numbers here, so chemists at this level leave it out of the equation.)

The energy leaves as a burst of high-energy light called **'gamma radiation'**.

Gamma radiation sits in the highest-energy region of the electromagnetic spectrum.

A single burst of it is called a **'gamma ray'**.

A gamma ray is not a particle of matter — it carries away energy, not protons or neutrons.

So releasing a gamma ray changes neither the mass number nor the atomic number.

An energized technetium-99 nucleus that releases a gamma ray is still technetium-99.

The nucleus ends up calmer: same element, same isotope, less energy.

How far a radiation type can push into matter before it is stopped is its **'penetrating power'**.

A material placed in radiation's path to stop it is called **'shielding'**.

Alpha particles have the lowest penetrating power: a sheet of paper — or the outer layer of your skin — stops them.

Beta particles pass through paper but are stopped by a thin sheet of aluminum.

Gamma rays pass through paper and aluminum alike; only thick lead or thick concrete cuts them down.

The order never changes: alpha is stopped first, beta next, gamma last.

Read backwards, the shielding that stops a radiation identifies it: radiation stopped by paper must be alpha.

That is what makes the smoke detector safe: americium-241 gives off alpha particles, and the plastic case — even the air — stops them.

Each decay type leaves its own fingerprint in the two numbers.

Mass number down 4 with atomic number down 2 is alpha decay.

Mass number unchanged with atomic number up 1 is beta decay.

Both numbers unchanged is gamma emission.

To classify a decay, compare the nuclide before with the nuclide after, then match the fingerprint.

One decay shows the routine: the mass number stays 90 while the atomic number rises from 38 to 39 — an unchanged mass number with the atomic number up 1, so beta decay.

In every nuclear equation, the mass numbers on each side add to the same total, and so do the atomic numbers.

If one species is missing, those two totals tell you its two numbers.

Find the missing mass number: the number that makes the left and right mass totals equal.

Find the missing atomic number: the number that makes the left and right atomic totals equal.

Then name the species: ⁴₂He is an alpha particle, ⁰₋₁e is a beta particle, and anything else is a nucleus you identify by its atomic number.

One equation shows it: in ²²⁶₈₈Ra → ²²²₈₆Rn + ?, the missing numbers are 226 − 222 = 4 and 88 − 86 = 2, so the missing species is the alpha particle.

The missing piece can sit anywhere — the starting nucleus, the new nucleus, or the emitted particle — and the same two subtractions find it.

For any radioactive isotope, the time it takes for half of the nuclei in a sample to decay is always the same.

That fixed time is the isotope's **'half-life'**.

Iodine-131's half-life is 8 days: take any sample, and 8 days later half of its iodine-131 nuclei have decayed.

The half-life belongs to the isotope, not to the sample.

A large sample and a small sample of iodine-131 share the same 8-day half-life.

Heating the sample, cooling it, or binding the isotope into a different chemical compound does not change the half-life either.

Every radioactive isotope has its own half-life, from fractions of a second to billions of years.

After one half-life, 1/2 of the original radioactive nuclei remain.

After a second half-life, half of that half remains: 1/4 of the original.

After a third, half again: 1/8.

The fraction keeps halving because after each half-life, half of the nuclei that remain decay.

So the fraction remaining after any whole number of half-lives is a string of halvings: 1/2, 1/4, 1/8, 1/16, and so on.

The sample never jumps to zero — each half-life removes only half of what is left.

The question gives three things: a starting amount, the isotope's half-life, and how much time passes.

First count the half-lives: divide the time passed by the half-life.

Then halve the starting amount once for each half-life counted.

Iodine-131 shows the routine: an 80 g sample, a half-life of 8 days, and 24 days of waiting.

Count the half-lives: 24 ÷ 8 = 3.

Halve three times: 80 → 40 → 20 → 10.

10 g of iodine-131 remains — because after each half-life, half of the nuclei that remain decay.

Suppose a hospital sample of an energized form of technetium-99 — a bone-scan tracer — has decayed from 96 g down to 12 g, and you want to know how long that took.

Count how many halvings turn the starting amount into the remaining amount.

96 g → 48 g → 24 g → 12 g, so the sample has halved 3 times.

Each halving takes exactly one half-life, because after each half-life, half of the nuclei that remain decay.

So multiply the number of halvings by the half-life to get the time that has passed.

This energized technetium-99 has a half-life of 6 hours, so the decay from 96 g to 12 g took 3 × 6 = 18 hours.

Plot the amount of a radioactive isotope remaining against time, and you get a **'decay curve'**.

The curve starts at the full starting amount and keeps falling by half, because after each half-life, half of the nuclei that remain decay.

The figure shows the decay curve of a 60 g sample of sodium-24.

To read the half-life from a decay curve: find where the amount has fallen to half its starting value, and read the time directly below that point.

Sodium-24's curve falls from 60 g to 30 g at 15 hours, so sodium-24's half-life is 15 hours.

To read the amount remaining at any time: go up from that time to the curve, then straight across to the amount axis.

At 30 hours, the sodium-24 curve sits at 15 g.

**'Nuclear fission'** is the splitting of a large, heavy nucleus into two smaller nuclei.

Fission usually starts when the heavy nucleus absorbs a neutron.

The extra neutron makes the nucleus so unstable that it splits apart.

Along with the two smaller nuclei, the split releases several neutrons and a large amount of energy.

In one fission event, a uranium-235 nucleus absorbs a neutron and splits into a barium-141 nucleus, a krypton-92 nucleus, and 3 neutrons: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n.

The equation balances like any nuclear equation: 235 + 1 = 141 + 92 + 3, and 92 + 0 = 56 + 36 + 0.

**'Nuclear fusion'** is the merging of two small, light nuclei into one heavier nucleus.

The merge releases a large amount of energy.

In one fusion reaction, a hydrogen-2 nucleus and a hydrogen-3 nucleus merge into a helium-4 nucleus and release a neutron: ²₁H + ³₁H → ⁴₂He + ¹₀n.

Fusion happens only at extremely high temperatures — millions of degrees.

The extreme temperature is needed because the two nuclei are both positively charged and repel each other, and only nuclei colliding at enormous speed get close enough to merge.

To classify a nuclear reaction as fission or fusion, compare the nuclei before the reaction with the nuclei after.

One large, heavy nucleus splitting into two smaller nuclei is fission.

Two small, light nuclei merging into one heavier nucleus is fusion.

A plutonium-239 nucleus that absorbs a neutron and splits into a xenon-134 nucleus, a zirconium-103 nucleus, and three neutrons has gone from one heavy nucleus to two smaller ones — fission.

Two helium-3 nuclei that merge into one helium-4 nucleus, releasing two protons, have gone from two light nuclei to one heavier one — fusion.

The quickest tell is the starting nuclei: fission starts heavy, fusion starts light.

Nuclear power plants run on fission: the nuclei in the uranium fuel split, releasing the plant's energy.

The sun and other stars run on fusion: hydrogen nuclei in their cores merge into helium.

So sunlight is fusion energy arriving from the sun's core — even the energy a solar panel collects started as nuclear energy.

A living thing keeps a steady, known level of the radioactive isotope carbon-14 in its tissues; when it dies, the intake stops and the carbon-14 begins to halve away.

Estimating an object's age from the radioactive isotope remaining inside it is called **'radiometric dating'**.

Measure what fraction of the original carbon-14 remains in the object.

Count the half-lives: each halving from the original level is one half-life passed.

Multiply the count by carbon-14's half-life, 5,730 years, to get the age.

A wood fragment holding 1/4 of its original carbon-14 has halved twice, so its age is about 2 × 5,730 = 11,460 years.

The same routine works with other isotopes: dating rocks uses isotopes with far longer half-lives, such as potassium-40, whose half-life is 1.25 billion years.

A **'tracer'** is a small amount of a gamma-emitting radioactive isotope given to a patient.

The tracer travels through the body and collects in the tissue being examined — a bone-scan tracer collects in bone.

The tracer's gamma rays pass out through the body, because gamma radiation is the most penetrating of the three radiation types.

A detector outside the body catches the escaping gamma rays and builds an image of where the tracer has collected.

Tracer isotopes are chosen with short half-lives, so the radioactivity quickly fades away after the scan.

The bone-scan tracer — an energized form of technetium-99 — has a half-life of 6 hours: two days after the scan, 8 half-lives have passed, and almost none of the tracer remains.

Radiation therapy treats cancer by aiming beams of high-energy radiation at a tumor.

The radiation damages the cancer cells inside the tumor so they stop dividing.

The beams pass through healthy tissue on the way in, so a treatment is planned to hit as little healthy tissue as possible.

One planning trick: fire several weaker beams from different directions, all crossing at the tumor.

At the crossing point the doses add up, so the tumor receives the full dose while each patch of healthy tissue is crossed by only one weak beam.

Hospitals often use the gamma rays of cobalt-60 for these beams, because gamma radiation penetrates deep into the body.

A nuclear power plant turns fission energy into electricity through a chain of ordinary steps.

Fission in the uranium fuel releases energy as heat.

The heat boils water into steam.

The steam pushes through a **'turbine'** — a wheel of blades that spins when steam rushes past it.

The spinning turbine drives a generator, and the generator produces the electricity.

Every step after the fuel matches a coal-fired plant — only the source of the heat differs.

The fuel's energy is concentrated: a single fingertip-sized fuel pellet releases about as much energy as a ton of coal.

The danger of a radiation source depends on where the source sits: outside the body or inside it.

From outside, alpha radiation barely matters, because alpha particles are stopped by the skin's outer layer before they reach living tissue.

From outside, gamma radiation is the danger: gamma rays pass through the whole body, reaching every organ.

Inside the body, the ranking flips.

An alpha emitter inside the body does the most damage, because alpha radiation dumps all of its energy into the tissue right around it.

That is radon's trick: it is a gas, so it is breathed in, and its alpha particles then strike the lung tissue directly.