When an atom loses a valence electron, it becomes a(n) ion.
a, positive b, negative c, neutral d, polyatomic

A) Positive

Explanation:

Let's start with a term, atom. An atom is an important piece of matter that consists of three subatomic particles: protons, neutrons and electrons. An atom is neutral because because the number of protons equals the number of electrons. So what would happen if you gained or lost an electron. That atom would turn into an ion since it now has a charge. So if an atom loses an electron, there are more protons than electrons making it a positive ion. Similarly, if an atom gains an electron, then the electrons outnumber the protons, making it a negative ion. In this case, since the atom loses a valence electron, we end up with a positive ion.

"the periodic table is arranged in order of atomic number."

it is the most commonly accepted one