A Level Biology CCEA

Molecules

Introduction to Molecules

• Molecules are composed of two or more atoms bonded together. They may be composed of identical atoms, such as O2, or different atoms, like H2O.
• Atomic Structure is the key to understanding molecular formation. Atoms contain a core nucleus of protons and neutrons, with electrons orbiting in shells.
• Chemical Bonds are the forces holding atoms together to form molecules. There are two primary types: covalent bonds and ionic bonds.

Covalent Bonds

• A Covalent Bond forms when two atoms share a pair of electrons. This results in a strong bond that often forms between non-metal atoms.
• Single, Double, and Triple Covalent Bonds occur when atoms share one, two or three pairs of electrons, respectively.
• Polar Covalent Bonds occur when there is unequal sharing of electrons between atoms due to differences in electronegativity.
• Nonpolar Covalent Bonds occur when the electronegativity difference between the atoms is minimal, resulting in an equal sharing of electrons.

Ionic Bonds

• Ionic Bonding is the complete transfer of valence electron(s) between a metal and non-metal. This results in the formation of two oppositely charged ions.
• The electrostatic attraction between these oppositely charged ions forms the ionic bond.
• Compounds created through ionic bonding are known as Ionic Compounds, e.g. salt (NaCl).

Water Molecules

• Water Molecules (H2O) are polar, with oxygen being more electronegative than hydrogen. This results in a partial negative charge on oxygen and a partial positive charge on hydrogen.
• The polarity of water molecules leads to Hydrogen Bonding between water molecules, contributing to its unique properties.

Organic Molecules

• Organic molecules contain Carbon Atoms and are primarily found in living organisms.
• There are four main groups of organic compounds in living organisms: Proteins, Nucleic Acids (DNA and RNA), Carbohydrates, and Lipids.

Biological Molecules

• Proteins are made up of amino acids. They can serve as enzymes, structural proteins, transport proteins, and more.
• Nucleic Acids, which include DNA and RNA, store and transmit genetic information.
• Carbohydrates include sugars and polymers of sugar. They serve as a primary energy source and help in cell structure.
• Lipids are non-polar and include fats, waxes, and steroids. They're important for long-term energy storage, insulation, and cell membrane formation.