In a neutral atom they are both equal, and their even quantities makes the atom neutral...
The first part of Dalton's model is that all matter is made of atoms, and these atoms are indivisible.
The second part of this model is that atoms of an element have identical masses and identical propertes.
At last, the third part of Dalton's model is that all compounds are formed by two or more different type of atoms combined.
Number of moles :
500 / 180.16 => 2.775 moles
Volume = 0.72 L
M = n / V
M = 2.775 / 0.72
M = 3.85 M
hope this helps!
Any of the Halogens (second to last vertical column going down)