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Asked by anon-240763 to Tom, Rebecca, Emily, Elspeth, Ben, Antoine on 6 Mar 2020.
Question: why is hydrogen all alone in the Periodic Table?
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Ben Cropper answered on 6 Mar 2020:
The answer is that it isn’t (quite)! Helium is kind of next to it.
It’s to do with the electrons orbiting the nucleus. They fit in shells around the nucleus. Each full shell represents a row in the periodic table.
Each shell can hold a certain amount of electrons, and electrons in each shell have the same amount of energy as each other. When a shell is filled, the next one starts filling up and you go to the next row.
The first shell can have two electrons in it. The element with one electron in the first shell is hydrogen and the element with two electrons is helium. They then make up the top row.
But why is there a massive gap between them? That’s the weird bit.
In the periodic table, things are grouped together by how closely they behave to each other. This is to do with how close to full, or how close to empty, the last shell is. It’s not always to do with how many electrons are in the last shell. This means that helium (2 electrons in last shell, full last shell) behaves more similarly to neon (8 electrons in last shell, full last shell) than it does to beryllium (2 electrons in last shell, mostly empty last shell)
That’s why you get the weird gaps, because the periodic table fills up from both sides!
I hope this makes sense, the word ‘shell’ has stopped looking like a real word to me with the amount of times I wrote it!
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Emily Goddard answered on 9 Mar 2020:
Elements are grouped in the periodic table by the number of electrons they have in their outermost shell (think of shells as rings around the nucleus of an atom, or in 3D like the layers of an onion). Hydrogen only has one electron, so it’s in Group 1.
As you go down the table, you’re filling up the shells with the electrons, so Li has one full shell and one electron on the outside, Na has two full shells and one with only one electron in etc. Hydrogen is at the top because it’s the smallest.
Helium is the element that would be next to hydrogen, but it’s actually right over on the other side. Helium only has one shell, with two electrons in it. The first shell can only hold two electrons, but the next can hold eight. When the outside shell is completely full, the element isn’t very reactive.
When the periodic table was being put together, the actual number of electrons weren’t necessarily know — the elements were grouped by their similarities in behaviour. Because its shell is full, He behaves much more like Ne, Ar etc than it does Be, Mg , Ca, so it makes more sense to put it in the full-outer-shell group than the two-electrons-in-the-outer-shell group. That’s why it’s ended up so far away from hydrogen!
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