Chemical Basis of Life

Introduction

The chemical basis of life is founded on the properties and interactions of atoms and molecules. Understanding atomic structure, electron configuration, and chemical bonding is essential for studying biological molecules and processes.

Atoms

Atomic Structure

Atoms are the fundamental units of matter, composed of a central nucleus containing protons (positively charged) and neutrons (neutral), surrounded by electrons (negatively charged) in defined shells.

• Nucleus: Contains protons and neutrons.

• Electron Shells: Electrons orbit the nucleus in energy levels or shells.

• Element Example: Hydrogen has 1 proton and 1 electron; Carbon has 6 protons, 6 neutrons, and 6 electrons.

Electron Shells and Valence Electrons

The valence shell is the outermost electron shell of an atom. The chemical reactivity of an atom is largely determined by the number of electrons in its valence shell.

• Full Shells: Atoms are most stable when their valence shell is full.

• Unpaired Electrons: The number of unpaired electrons in the valence shell determines bonding capacity.

• Example: Carbon has 4 unpaired electrons, allowing it to form up to 4 covalent bonds.

Table: Number of Unpaired Electrons in Common Elements

Chemical Bonds

Covalent Bonds

Covalent bonds are formed when two atoms share one or more pairs of electrons to fill their valence shells. This type of bond is fundamental in biological molecules.

• Single Covalent Bond: One pair of electrons is shared (e.g., H2).

• Double Covalent Bond: Two pairs of electrons are shared (e.g., O2, CO2).

• Triple Covalent Bond: Three pairs of electrons are shared (e.g., N2).

• Example: In a water molecule (H2O), each hydrogen shares one electron with oxygen.

Molecular Geometry and Models

Molecules can be represented in various ways to illustrate their structure and geometry.

• Molecular Formula: Shows the number and type of atoms (e.g., CH4 for methane).

• Structural Formula: Shows how atoms are connected (e.g., H–O–H for water).

• Ball-and-Stick Model: Visualizes atoms as balls and bonds as sticks.

• Space-Filling Model: Shows the relative size and position of atoms in a molecule.

Table: Examples of Molecular Representations

Key Points on Covalent Bonds

• Bond Formation: Atoms form covalent bonds to achieve a full valence shell, leading to greater stability.

• Bond Types: Single, double, and triple bonds differ in the number of shared electron pairs.

• Example: Carbon forms four single covalent bonds in methane (CH4).

Additional info:

• Atoms with full valence shells (e.g., noble gases like neon) are chemically inert.

• Unpaired electrons are crucial for chemical reactivity and bond formation.