What does the atomic number of an element mean?

Apr 7, 2016

The periodic table is organised by the atomic number. If you look at it, you will see it begin at hydrogen with $1$, then $2$, helium, $3$, lithium, $4$, beryllium, and so on. The atomic number is what number atom it is.
For example, in lithium, the atomic number is $3$, which means it has $3$ protons in the nucleus and $3$ electrons around the outside. In ionic lithium, it loses an electron and ends up with only $2$ electrons, though the number of protons remains the same at $3$.