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A typical hydrogen atom has one proton and one electron. The electron orbiting the nucleus occupies the first energy level.
Mass number: 1
An atom of helium has two protons, tow neutrons and two electrons. The two electrons orbit in the same energy level.
Atomic number: 2
Mass number: 4
(2 protons + 2 neutrons)
A lithium atom has three protons, three neutrons and three electrons. The first energy level can hold only tow electrons, so the third electron occupies a second energy level.
Atomic number: 3
Mass number: 6
(3 protons + 3 neutrons)
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