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)
A neon atom has 10 protons, 10 neutrons, and 10 electrons. The second level can hold up to eight electrons; thus, both the first and second energy levels are filled.
Atomic number: 10
Mass number: 20
(10 protons + 10 neutrons)
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