IB Biology Topic 1 Notes

Water is the most essential molecule needed to sustain life on Earth. Also known as dihydrogen monoxide, it is composed of two hydrogen atoms covalently bonded to an oxygen atom. To understand this, let's cover some essential concepts. First is that of a covalent bond, which is the sharing of a pair of electrons between two nuclei. The number of pairs dictates the degree of the bond:

[INSERT_IMAGE]

1. A single covalent bond involves one pair of shared electrons.

2. A double covalent bond involves two pairs of shared electrons.

3. A triple covalent bond involves three pairs of shared electrons.

Molecules are thus the complexes that form when two or more atoms are held together by covalent bonds. Ultimately, how the electrons are shared determines what type of molecule is formed. Electrons can be shared in two ways: equally or unequally.

[INSERT_IMAGE]

1. The electrons can be shared equally, meaning both nuclei have the same strength and so the electrons lie in the middle of the bond.
2. The electrons can be shared unequally, meaning one nucleus is stronger and so pulls the electrons in the bond closer to itself.

As a result of this electron sharing, two types of molecules are formed: polar or non-polar.

[INSERT_IMAGE]

1. Polar molecules are formed when there is unequal sharing, so one side of the molecule has more electrons than the other. As a result, the molecule has a negatively charged pole and a positively charged pole, called a dipole.
2. Non-polar molecules are formed when there is equal sharing, so all sides of the molecule have the same number of electrons and no poles form.

[INSERT_IMAGE]

1. If one nucleus is stronger than another, it pulls electrons closer to itself, creating a negative side, called a polar bond.

2. If both nuclei are of equal strength, the electrons are equally shared, resulting in no charge differential, called a non-polar bond.