The atomic number is how many protons an element's atoms have. Look up the atomic number.
It is what makes an element that element, since changing it makes a different one entirely. The periodic table is arranged by atomic number, which is why it works as well as it does.
1nounatomic numberHow many protons an element's atoms have.
“Carbon has an atomic number of six.”
atomic number is a number.
Practised in Chemistry & Elements