Chemistry of Life

Elements and Compounds

The study of biology begins with understanding the chemical basis of life, which involves elements and compounds essential for living organisms.

• Element: A pure substance that cannot be broken down into other substances by chemical reactions. Examples include oxygen (O), carbon (C), and hydrogen (H).

• Compound: A substance consisting of two or more different elements combined in a fixed ratio. For example, sodium chloride (NaCl) is a compound formed from sodium and chlorine.

• Chemical Reaction: A process that transforms one set of chemical substances to another.

Example: Water (H2O) is a compound formed from hydrogen and oxygen.

Atoms and Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles:

• Proton: Positively charged particle (+)

• Neutron: Neutral particle (no charge)

• Electron: Negatively charged particle (-)

Example: A helium atom contains 2 protons, 2 neutrons, and 2 electrons.

Atomic Number and Mass Number

Atoms are characterized by their atomic number and mass number:

• Atomic Number: The number of protons in an atom, which determines the element.

• Mass Number: The sum of protons and neutrons in the nucleus.

• Atomic Mass: The average mass number of all isotopes of an element.

• Isotope: Atoms of the same element with different numbers of neutrons.

Example: Helium (He) has an atomic number of 2 and a mass number of approximately 4.

Essential Elements of Life

Living organisms require certain elements to survive and reproduce. These are classified as:

• Essential Elements: Required in large amounts (e.g., O, C, H, N, Ca, P, K, S, Na, Cl, Mg).

• Trace Elements: Required only in minute quantities (<0.01% of body mass), such as boron, copper, iron, iodine, manganese, selenium, silicon, tin, vanadium, zinc.

Atomic Structure and Electron Distribution

Electron Shells and Valence Electrons

Electrons are arranged in shells around the nucleus. The outermost shell is called the valence shell, and the electrons in this shell are called valence electrons. These determine the chemical properties and reactivity of the atom.

• First shell: Holds up to 2 electrons

• Second shell: Holds up to 8 electrons

• Third shell: Holds up to 8 electrons (for main group elements)

• Valence: The number of unpaired electrons in the outer shell

Example: Nitrogen (N) has 5 electrons in its outer shell and a valence of 3.

Energy Levels

Electrons can move between energy levels by gaining or losing energy. The energy levels correspond to the electron shells.

• Electrons in higher shells have more energy.

• Electrons can absorb energy to move to a higher shell or release energy to move to a lower shell.

Example: The third shell is the highest energy level in the atomic model shown.

Chemical Bonds and Interactions

Intramolecular and Intermolecular Forces

Chemical bonds hold atoms together within molecules (intramolecular forces) and also allow molecules to interact with each other (intermolecular forces).

• Intramolecular Forces: Forces within a molecule, such as covalent and ionic bonds.

• Intermolecular Forces: Forces between molecules, such as hydrogen bonds and van der Waals interactions.

Covalent, Ionic, and Hydrogen Bonds

Atoms form bonds to achieve stable electron configurations. The main types of chemical bonds are:

• Covalent Bond: Atoms share pairs of valence electrons. Can be single, double, or triple bonds.

• Ionic Bond: Electrons are transferred from one atom to another, resulting in charged ions (cation is positive, anion is negative).

• Hydrogen Bond: A weak bond between a hydrogen atom covalently bonded to a highly electronegative atom (like O or N) and another electronegative atom.

Example: Water molecules are held together by hydrogen bonds.

Polar and Nonpolar Covalent Bonds

Covalent bonds can be classified based on the equality of electron sharing:

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., in water).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., in oxygen gas, O2).

Properties of Water

Unique Properties Due to Hydrogen Bonding

Water exhibits several unique properties due to hydrogen bonding:

• Universal Solvent: Water dissolves more substances than any other liquid due to its polarity.

• Cohesion: Water molecules stick to each other via hydrogen bonds.

• Adhesion: Water molecules stick to other substances.

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change.

Example: Water's high specific heat helps regulate Earth's climate and body temperature.

Hydrophilic and Hydrophobic Substances

Substances interact with water differently based on their polarity:

• Hydrophilic: Substances that dissolve in water (ionic or polar).

• Hydrophobic: Substances that repel water (nonionic and nonpolar).

Solutions, Moles, and Concentration

Solution Terminology

Solutions are mixtures where a solute is dissolved in a solvent (often water).

• Solute: Substance being dissolved.

• Solvent: Substance doing the dissolving (water is the universal solvent).

• Hydration Shell: Sphere of water molecules surrounding a dissolved ion.

Moles and Molecular Mass

Quantifying substances in chemistry involves the concept of the mole:

• Mole: molecules (Avogadro's number).

• Molecular Mass: Sum of the masses of all atoms in a molecule.

• Molarity: Number of moles of solute per liter of solution.

Acids, Bases, and pH

Water Dissociation and pH Scale

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-):

• At equilibrium in pure water:

• pH: A measure of hydrogen ion concentration;

• pH scale ranges from 0 (acidic) to 14 (basic), with 7 being neutral.

Example: A solution with pH less than 7 is acidic; greater than 7 is basic.

Acids, Bases, and Buffers

Acids increase the concentration of H+ ions, while bases decrease it. Buffers help maintain stable pH in biological systems.

• Acid: Substance that donates H+ ions.

• Base: Substance that accepts H+ ions or donates OH- ions.

• Buffer: Substance that minimizes changes in pH.

Example: Blood contains buffers to maintain pH around 7.4.

Biological Relevance of pH

Changes in pH can affect biological processes. For example, increased CO2 concentration in blood can lead to acidosis, a dangerous drop in pH.

• A drop in pH by 1 unit means the concentration of H+ ions has increased tenfold.

*Additional info: The relationship between pH and H+ concentration is logarithmic; each unit change in pH represents a tenfold change in H+ concentration.*