Answers (1)
You do have a printed periodic table, right? You can get the information at www.chemicalelements.com/index.html but you really need your own printed copy to carry with you. It will be a primary reference for everything you do.
There is a diagonal line, B-C, Si-P, Ge-As, Sb-Te, and Po-At. Elements to the left of the line are metals, meaning they lose an electron easily, elements to the right of the line are non-metals, and elements on the line are semiconductors.
Chlorine has seven electrons in the outer shell, making it one electron short of a completed shell. So it is easy to share a bonding electron in that position, but quite difficult to remove one or share two. That is what valence means. (Not "valency.")
While checking my info I discovered that "valency" is in fact accepted by a lot of sources. But I never heard anybody use that word before you.
va·lence
noun Chemistry
noun: valence; plural noun: valences
the combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.
"carbon always has a valence of 4"
relating to or denoting electrons involved in or available for chemical bond formation.
modifier noun: valence
"molecules with unpaired valence electrons"
Linguistics
the number of grammatical elements with which a particular word, especially a verb, combines in a sentence.
Origin
late Middle English: from late Latin valentia ‘power, competence,’ from valere ‘be well or strong.’
google.com ("valency" has no etymology.)