Why are some elements in the periodic table heavier than the ones after?
In: Chemistry
They have a greater number of neutrons in their core to keep the protons together. Some configurations of protons are less stable than others, and need more neutrons to fill in that stability than some of the later ones in the periodic table.
When you get up to the really heavy elements, the amount of neutrons gets weird because isotopes are more common and plentiful.
For example, Uranium has 92 protons and (usually) 143 or 146 neutrons and an average atomic weight of 238.03. The next one over, Neptunium, has 93 protons but 144 neutrons for an average atomic weight of 237.05.
If you had pure U-235 (143 neutrons), it would be lighter than Neptunium.
The varying number of neutrons slightly messes up the progression of atomic weights.
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