What is the bond

Ionic Bonding : Ionic bonding occurs when one element donates an electron (or electrons) to another so that both elements will have a full outer shell.
Example: Here is an example showing lithium (which has 3 electrons and 1 in the outer shell) and fluorine (which has 9 electrons and 7 in the outer shell) donating an electron to form LiF or lithium fluoride. This is called an ionic bond.

Covalent Bonding: In covalent bonding electrons are shared between atoms rather than donated in order for the atoms of both elements to gain full outer shells. Electrons are always shared in pairs.
Example: An example of covalent bonding is the molecule of carbon dioxide. In this example carbon has 4 of 8 electrons in its outer shell and oxygen has 6 of eight electrons. By combining two oxygen atoms with one carbon atom, the atoms can share electrons such that each atom has a full outer shell.