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

1 Answer
Truong-Son N.
Dec 6, 2015

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/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.

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