Nuclear notation

1. A nucleus is written in nuclear notation as ${}_Z^{A}X$.
   1. $X$ is the chemical symbol of the element.
   2. $A$, written at the top left, is the mass number, which is the number of protons plus neutrons.
   3. $Z$, written at the bottom left, is the atomic number, which is the number of protons.
2. For example, ${}_6^{12}C$ represents a carbon nucleus with a mass number of $12$ and an atomic number of $6$.
3. The mass number is always the larger of the two numbers, except for hydrogen-1, where both numbers are $1$.
4. The chemical symbol and the atomic number give the same information, because each element has its own atomic number.
5. Nuclear notation and the written name carry the same information, so ${}_6^{14}C$ and carbon-14 describe the same isotope.

Reading the notation

1. A single symbol in nuclear notation tells you the identity, charge and mass of a nucleus.
2. The identity of the element comes from the atomic number, $Z$, or from the chemical symbol.
3. The charge of the nucleus is $+Z$, because each of the $Z$ protons has a relative charge of $+1$.
4. The mass of the nucleus is given by the mass number, $A$, because each nucleon has a relative mass of $1$.
5. The number of neutrons, $N$, is found by subtracting the atomic number from the mass number, so $N=A-Z$.
6. A neutral atom of the element has $Z$ electrons, so that their charge balances the charge of the nucleus.
7. For ${}_{11}^{23}Na$, the nucleus has $11$ protons, a charge of $+11$, a mass number of $23$ and $23-11=12$ neutrons.

Comparing isotopes

1. Isotopes of an element have the same bottom number but a different top number in nuclear notation.
2. Oxygen has three stable isotopes, ${}_8^{16}O$, ${}_8^{17}O$ and ${}_8^{18}O$, which contain $8$, $9$ and $10$ neutrons.
3. The uranium isotopes ${}_{92}^{235}U$ and ${}_{92}^{238}U$ both have $92$ protons, but they contain $143$ and $146$ neutrons.
4. The difference between the mass numbers of two isotopes is equal to the difference in their numbers of neutrons.
5. Two nuclei with the same mass number but different atomic numbers are not isotopes, because they belong to different elements.
6. For example, ${}_6^{14}C$ and ${}_7^{14}N$ have the same mass number but different numbers of protons, so they are nuclei of carbon and nitrogen.