Amolecule of oxygen gas is made from:
a) one oxygen atom
b) the bonding of two oxygen atoms
c) the bonding of one oxygen atom with two hydrogen atoms
d) the bonding of a variable amount of oxygen atoms

An oxygen molecule contains only oxygen. Nothing else.

That makes C wrong.

A is wrong because there needs to be 2 oxygens. The bonding rules for the non metals vary, but in general they are even numbers. They are also fixed. Oxygen happens to bond 2 oxygens for 1 molecule.

D is wrong. See the explanation for C. The number is fixed. It is 2.

B is the correct answer.

allowed for the prediction of the properties of missing elements.

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