What element is this? how do you know?

Helium

Explanation:

We know that this atom has two positive particles on the nucleus (protons) with two neutral particles (neutrons). We can also see that it hass two electrons on the orbitals. Knowing this, we can say that the element is Helium.

Helium has a relative atomic mass of 4 and it's atomic number is 2. The atomic number is equal to the number of protons and electrons the atom has, and the atomic mass can be used to calculate the number of neutrons:

Relative atomic mass = Neutrons + atomic number

So we know that:

Neutrons = relative atomic mass - atomic number

Neutrons = 4 - 2 = 2.