A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the electrostatic force of attraction between oppositely charged ions as in ionic bonds or through the sharing of electrons as in covalent bonds.
The mole is the unit of measurement for amount of substance in the International System of Units. It is defined as exactly 6.02214076×10²³ constitutive particles, which may be atoms, molecules, ions, or electrons.
remembering this number is really important but u can also just put 6.02×10²³ instead, it would still be the same.
In the water molecule oxygen combines with two hydrogens and so has a valency of 2. A simple diagram of a water molecule makes this plain. The hydrogen atoms are each sharing one pair of electrons - they have a valency of 1. The oxygen atom is sharing two pairs of electrons - it has a valency of 2.