The atomic number is the number of protons of the atom.
So, just count the number of red spheres (protons) and that is the atomic number of the atom.
The protons are in the nucleus of the atom. Every single element is formed by a representative number of protons (atomic number).
For example, every hydrogen atom has one proton and its atomic number is one.
Every helium atom has two protons and its atomic number is two.
And so on. This permits you to order the elements in the periodic table in increasing atomic number.
There are no two elements with the same number of protons (atomic number) and all the atoms of a same element have the same number of protons.
