Positive ions usually form from and are called cations.

Positive ions usually form from losing electrons and are called cations.

Explanation

An atom can lose one or more valence electrons and it may also gain one or more valence electrons leading to the formation of ions.

They are electrically charged particles unlike atoms which are neutrally charged.

The atoms can be converted to positive ion on removal of electrons from the valence shell.

Similarly the atoms can also be converted to negative ions when extra electrons get added in the valence shell.

So the losing of electrons leads to the formation of positive ions which are termed as cation while the gaining of electrons will form negative ions which are termed as anions.

it's c. elements are placed in rows based on how may valencies electrons have.

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-