Valence Electrons

     The two main types of bonds formed between atoms are ionic bonds and covalent bonds. An ionic bond is formed when one atom accepts or donates one or more of its valence electrons to another atom. A covalent bond is formed when atoms share valence electrons. The atoms do not always share the electrons equally, so a polar covalent bond may be the result. 

Here you can watch for a further explanation of valence electrons and how they relate to different bonds:

Ionic Bonding

        A molecule or compound is made when two or more atoms form a chemical bond, linking them together. In an ionic bond, the atoms are bound together by the attraction between oppositely charged ions. For example, sodium and chloride form an ionic bond, to make NaCl, or table salt. It is a reaction between a metal and a non metal. Ionic compounds dissolve easily in water and other polar solvents. In ionic bonding, valence electrons are taken from the less electronegative atom and the more electronegative atom receives it.

Here is a helpful video to show case ionic bonding:

A metal proposing to a non-metal

Covalent Bonding

     Covalent bonding is between two non metals. In covalent bonding, electrons are shared. For example, there is a covalent bond between oxygen and hydrogen in a water molecule (H2O). Each of the covalent bonds contains two electrons- one from a hydrogen atom and one from the oxygen atom. Both atoms share the electron. In covalent bonding, the valence electrons are shared between atoms; hence, a bond is created.



Here is an example and thorough explanation of covalent bonding:

Covalent bonds sharing electrons

Periodic Trends

      Periodic trends such as polarity and electronegativity affect ionic and covalent bonding. Periodic trends are patterns that are present in the periodic table, which illustrate different aspects of a certain element, including its size and properties. One of the trends is electronegativity. It measures an atoms strength to attract and form bonds with electrons. As you move to the right and across the periodic table, electronegativity increases. As you move down a group, it decreases. The noble gases have a complete valence shell and do not attract electrons.The polarity of the bond depends on the difference in electronegativity  of the elements on each end. For example, the halogens are more electronegative than H so the electrons will be more attracted to the halogen than the hydrogen so the bond will be polar. The dipole will be pointing towards the halogen. 

A cool experiment displaying electronegativity and polarity:

Electronegativity and Polarity Science Expirement