Question

How do we know what atomic number an element has???

Answer 1

If you know what the element is called... look it up.

You can always get the atomic number, as long as you know what element you're looking at. Always.

![http://www.ptable.com/]()

You can see the atomic number directly on each square. The relevant information from each square is:

`"Z"`
`"X"`
`"Name"`
`"Atomic Mass"`

`"Z"` is the atomic number.
`"X"` is the atomic symbol.

For example, Phosphorus (`"P"`) is denoted as element number `15`. So of course, the atomic number must be `15`. Its atomic mass is about `"30.97 g/mol"`, which is close to `31`, so the most common isotope has a mass number of `31`.

If the element is generally known to be stable, the most common isotope is the most stable.

So, we have:

`"number of protons + number of neutrons = mass number"`

`"atomic number = number of protons"`

`"mass number - atomic number = number of neutrons"`

`31 - 15 = "16 neutrons"`

Finally, the number of electrons has to be equal to the number of protons for an element in its elemental form, i.e. its natural, neutral state. So it must have `15` electrons as `"P"`.