Summary
Highlights
This lecture focuses on electrons and bonding. Electrons are energetic particles orbiting the nucleus in shells, and they are the ultimate source of energy for living things. Each shell has a maximum capacity for electrons; the first shell holds up to two, while subsequent shells (like the second and third) can hold up to eight.
The outermost electron shell is called the valence shell, and the electrons within it are valence electrons. The number of valence electrons, or the valence number, determines an atom's reactivity. Atoms prefer full valence shells, making those with incomplete shells, like lithium, highly reactive, while atoms with full shells, like neon, are less reactive.
Chemical bonds form when electrons of two or more atoms interact. These bonds are crucial for life, holding atoms together to form complex structures and storing energy that living organisms access by breaking and making bonds. The lecture will cover covalent, ionic, hydrogen, and Van der Waals bonds.
Covalent bonds involve two atoms sharing electrons to achieve a full outer shell. These are the strongest types of bonds and form molecules. Examples include single covalent bonds (sharing one pair of electrons) and double covalent bonds (sharing two pairs).
Electron sharing in covalent bonds can be even (nonpolar covalent bond) or uneven (polar covalent bond). Uneven sharing occurs when one atom, due to higher electronegativity, pulls electrons closer, creating positive and negative poles. Water is a prime example of a polar covalent molecule.
Ionic bonds involve one atom transferring an electron to another. This creates ions (cations and anions) that are attracted to each other, forming an ionic compound, like sodium chloride. These bonds are of medium strength and can break relatively easily.
Hydrogen bonds are weak interactions between hydrogen atoms and highly electronegative atoms (like oxygen or nitrogen) in different molecules. They are vital for water's properties and for holding DNA strands together.
Van der Waals interactions are the weakest type, involving temporary attractions between the negative region of one atom or molecule and the positive region of another. These forces allow structures like a gecko's feet to adhere to surfaces by providing collective, albeit weak, adhesion.
Chemical reactions rearrange atoms or molecules. They are written with reactants on the left (what you start with) and products on the right (what you end with), separated by an arrow indicating progression. Types include synthesis (combining), decomposition (breaking apart), and exchange/replacement reactions.
In biological systems, synthesis reactions are often called dehydration reactions, where water is removed to form a larger molecule. Decomposition reactions are called hydrolysis reactions, where water is added to break a larger molecule into smaller components.