Isotopes

1. Every atom of a particular element has the same number of protons, but not every atom of that element has the same number of neutrons.
2. Atoms of the same element that have different numbers of neutrons are called isotopes.
3. Isotopes of an element have the same proton number but different mass numbers, because the mass number includes the neutrons.
4. An isotope is named by writing the name of the element followed by its mass number, such as carbon-12 or chlorine-37.
5. Carbon has three naturally occurring isotopes.
   1. Carbon-12 has 6 protons and 6 neutrons.
   2. Carbon-13 has 6 protons and 7 neutrons.
   3. Carbon-14 has 6 protons and 8 neutrons.
6. Hydrogen also has three isotopes, containing 0, 1 and 2 neutrons, but every hydrogen atom has just 1 proton.
7. Most elements exist naturally as a mixture of isotopes, so a sample of chlorine gas contains both chlorine-35 atoms and chlorine-37 atoms.
8. Neutral atoms of different isotopes of an element have the same number of electrons, because they have the same number of protons.

Atomic symbols

1. An atom can be shown in a short form called an atomic symbol, written as ${}_Z^{A}X$, where $X$ is the chemical symbol of the element.
2. The mass number, $A$, is written at the top left of the chemical symbol.
3. The proton number, $Z$, is written at the bottom left of the chemical symbol.
4. The mass number is always the larger of the two numbers, because it counts the neutrons as well as the protons.
5. The number of neutrons is found from the atomic symbol by subtracting the bottom number from the top number, $A-Z$.
6. A neutral atom has the same number of electrons as protons, so the bottom number also gives the number of electrons.
7. For the atomic symbol ${}_6^{12}C$, the atom has 6 protons, 6 electrons and $12-6=6$ neutrons.
8. Isotopes of the same element have atomic symbols with the same bottom number but different top numbers, as in ${}_{17}^{35}Cl$ and ${}_{17}^{37}Cl$.

Symbols for ions

1. The atomic symbol of an ion has its charge written at the top right, as in ${}_{17}^{35}{Cl}^{-}$ or ${}_{12}^{24}{Mg}^{2+}$.
2. Forming an ion changes only the number of electrons, so the proton number and mass number of the ion are the same as those of the atom.
3. The number of neutrons in an ion is still found using $A-Z$.
4. A positive charge shows that electrons have been lost, so you subtract the size of the charge from the proton number to find the number of electrons.
5. A negative charge shows that electrons have been gained, so you add the size of the charge to the proton number to find the number of electrons.
6. A single $+$ or $-$ sign with no number means a charge of one.