Valence electrons are the ones that are found in the last energy level of an atoms. It is stated that atoms have different energy levels, and in each level they have certain number of atoms. Valence electrons are the ones that are in the last level of each atoms. For example, oxygen has an atomic number of 8, its electronic configuration is 1s2 2s2 2p4. Its last energy level is 2, and it has 6 electrons in this level (2s2 and 2p4, 2+4=6). These electrons are the ones that form bonds with other atoms.
Covalent bonds are the ones in which a pair of electrons is shared for all the atoms to fulfill octet rule. For example, O2:
In this case there are two covalent bonds between the oxygen atoms, sharing two pairs of electrons.
Atomic structure refers to the way an atom is configured, with a nucleus compound of protons and neutrons and electrons orbiting around.
Lewis dot structure is used to represent the number of valence electrons that each atom has. For example, oxygen has 6 valence electrons, its lewis structure is:
Each dot represents a valence electron.
Ionic bonds are the ones in which the atoms don't share a pair of electrons, but one of the atoms gives one of its valence electrons to the other atoms, for example NaCl has a ionic bond in which Na gives its only valence electrons to Cl to make Cl fulfill the octet rule:
It is called ionic bond, because ions are formed when one of the atoms gives loses an electron to give it to the other atom, that is winning an electron.
