Which is a characteristic of atoms?

a)they come in ten major types
b)they are the smallest units of matter
c)they can be divided
d)they cannot combine together

They are the smallest units of matter.

The ionization energy is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule.The first ionisation energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+. I hope this helps :3