Mass and charge of a nucleus

1. When an unstable nucleus decays, the original nucleus is called the parent nucleus and the nucleus left behind is called the daughter nucleus.
2. The change to the nucleus is described by what happens to its mass and its charge.
3. The mass number, $A$, measures the mass of the nucleus, because it counts the nucleons.
4. The atomic number, $Z$, measures the charge of the nucleus, because it counts the protons.
5. Any change in the atomic number changes the element, so the daughter nucleus belongs to a different element whenever the charge changes.
6. Each type of emission changes the mass, the charge, both or neither.

Alpha emission

1. An alpha particle carries away two protons and two neutrons.
2. The nucleus loses four nucleons, so the mass number falls by 4.
3. The nucleus loses two protons, so the atomic number falls by 2 and the charge of the nucleus falls by $+2$.
4. Alpha emission changes both the mass and the charge of the nucleus.
5. The daughter nucleus is an element two places earlier in the periodic table.
6. When uranium-235 emits an alpha particle, its mass number falls from $235$ to $231$ and its atomic number falls from $92$ to $90$, so it becomes thorium-231.

Beta emission

1. In beta decay, a neutron in the nucleus changes into a proton, and a fast electron is created and emitted as the beta particle.
2. The nucleus loses one neutron but gains one proton, so the total number of nucleons stays the same and the mass number does not change.
3. The nucleus gains one proton, so the atomic number rises by 1 and the charge of the nucleus increases by $+1$.
4. Beta emission changes the charge of the nucleus but not its mass number.
5. The daughter nucleus is an element one place later in the periodic table.
6. When thorium-234 emits a beta particle, its mass number stays at $234$ while its atomic number rises from $90$ to $91$, so it becomes protactinium-234.

Gamma emission

1. A gamma ray has no mass and no charge, so emitting it leaves the number of protons and the number of neutrons unchanged.
2. Gamma emission changes neither the mass number nor the atomic number, so the nucleus stays as the same isotope of the same element.
3. The nucleus loses energy instead, because the gamma ray carries away the extra energy it had.
4. Gamma emission often follows alpha or beta emission, because the daughter nucleus is frequently left with excess energy, as happens to the protactinium-234 nucleus above.

Neutron emission

1. When a nucleus emits a neutron, it loses one nucleon, so its mass number falls by 1.
2. The number of protons is unchanged, so the atomic number and the charge stay the same.
3. Neutron emission changes the mass but not the charge of the nucleus.
4. The daughter nucleus is a lighter isotope of the same element, so beryllium-13 becomes beryllium-12 when it emits a neutron.