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A covalent bond forms when two atoms share electrons in a bond There are three types of covalent bonds Non polar covalent, which is when the atoms share the electrons evenly. this happens between two atoms that are generally the same electronegativity, that is each atom has the same level of affinity for the electrons. The diatomic molecules are examples of non-polar covalent molecule as each fluorine that has 7 valence electrons shares one electron in that bond resulting in each atom having 8 valence electrons. the atoms here are shared equally because the electronegativity is the same. |dw:1442003976263:dw| The next type of bond is called polar covalent; This happens when the two atoms don't have the same electronegativity values, and one atom because it has a greater tendency to want the electrons, it results in the electrons not being shared equally or better yet spending more time near one atom than the other. A great example of this is water; oxygen is much more electronegative than hydrogen, why? because oxygen has 6 valence electrons and so, wants to gain the other two so badly that when it does form a bond with hydrogen, the electrons spend more time at the oxygen. What happens is what we call polar covalent, polar means that there is a net negative charge in the molecule and that's where the oxygen is, or dipole moment, that's shown by the arrow drawn below. |dw:1442004199976:dw| The third and final type of covalent bond is called a coordinate covalent bond. What happens here is that in a typical bond between two atoms, usually each atom contributes one electron in a bond shown below. we can see in a typical covalent bond that each chlorine atom contributes one electron and shares it with the other, and the result is the following: a bond formed between two atoms, where the electrons are shared equally. |dw:1442004425045:dw| Coordinate covalent bonding, just means that instead of one atom contributing an electron to form a bond and the other, in the new bond that's formed both the electrons come from one atom. usually this involves a lewis acid which is a molecule that acts as an electron acceptor. Take AlCl3 aluminum trichloride a lewis acid, and a chlorine ion Cl- A chlorine ion for instance donates it's electrons into the empty orbital of the aluminum tri chloride and forms a new bond with aluminum, to give AlCl4- This new bond would be coordinate covalent because the electrons both came from Chlorine, instead of one coming from the aluminum and the other from chlorine. |dw:1442004644297:dw|