How do you determine if a bond is ionic or covalent?
In an ionic bond, an electron is given from one atom to another. In a covalent bond, two atoms share a pair of electrons.
In an ionic bond, an atom (sodium, for example) gives an electron to an atom that will accept it (chlorine). When sodium gives an electron to chlorine, it makes sodium positively charged and chlorine negatively charged. The two atoms are attracted to each other and from an ionic bond between each other.
In a covalent bond, two atoms share a pair of electrons. A good example of this is a water molecule. Two hydrogen molecules, with one electron each in its valence shell, join an oxygen molecule with six electrons in its valence shell. The hydrogen electrons provide the "glue" that holds the atoms together.