Why do nonmetals and halogens tend to become anions?

[aniseed]

TL;DR Summary

Generally speaking the elements in the top right side of the periodic table (the nonmetals and halogens) tend to form anions. Why is this? Is there some fundamental underlying reason?

My niece is in her first year of public high school in Japan, and her science textbook introduces the concept of elements, atomic number and charge, then shows the periodic table. It states that elements in the upper right hand side of the periodic table tend be become anions, and elements in the bottom left hand side tend to become cations, without giving any explanation about why this is.

So my niece asked me 'why' and the best I can come up with is 'because that's probably a side-effect of how the periodic table is structured'. I am not a chem major, my background is engineering. The only chem book I've read is Theodore Gray's excellent coffee table book, 'The Elements'.

I'm attaching a photo of the relevant section from her textbook in case it's of any use.

 

[Borek]

Periodic table reflects the electron configuration of the atoms and how their orbitals are filled. Some configurations are more stable than others and the trend she asks about has its source in the way electron configurations and the periodic table are related. I suppose she will later learn some basic ideas of quantum chemistry and then things will become more clear.

That being said "why?" question is dangerous, we often can't answer it - science is much better at answering the question "how?".

 

[DrDu]

The electrons in atoms are ordered in shells, which reflects in the ordering of the periodic system.
While atoms of elements to the left (typically metalls, like sodium, calcium or aluminium) contain only few electrons in an open shell, for elements on the right (typically non-metals, like nitrogen, oxygen or chlorine) of the periodic table, the shells are nearly full. Electrons within one shell are not very efficient in screening each other from the nuclear charge. Hence the outer electrons of non-metals are more tightly bound to the nucleus than in metals and this will also hold true for additional electrons in anions.
On the other hand, electrons of metals are more easily removed so that these elements form preferentially cations.
Also take in mind that cations and anions are usually not more stable in an absolute sense (i.e. sodium does not ionize spontaneously) but only in compounds containing both elements from the left and right of the table.