Atoms, Ions, Molecules, and Water: Foundations of Chemical Evolution in Biology

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Atoms, Ions, and Molecules: The Building Blocks of Chemical Evolution

Atoms and Subatomic Particles

Atoms are the smallest identifiable units of matter and serve as the fundamental building blocks for all substances. In biological systems, just a few elements—hydrogen (H), carbon (C), nitrogen (N), and oxygen (O)—make up the majority of living matter.

Atom: Consists of a nucleus (containing protons and neutrons) surrounded by electrons.
Proton: Positively charged particle found in the nucleus.
Neutron: Neutral particle found in the nucleus.
Electron: Negatively charged particle orbiting the nucleus.

Atomic number (Z): Number of protons in the nucleus; defines the element.

Mass number (A): Sum of protons and neutrons in the nucleus.

Atomic mass: The actual weight of a specific atom, often close to the mass number.

Isotopes: Atoms of the same element with different numbers of neutrons.

Subatomic Particle	Charge	Atomic Mass Unit (amu)	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	~0	Electron cloud

Atomic Structure and Electron Arrangement

The arrangement of electrons around the nucleus determines how atoms interact and form chemical bonds. Electrons occupy energy levels or shells:

First shell holds up to 2 electrons.
Subsequent shells hold up to 8 electrons each.
Valence electrons: Electrons in the outermost shell; determine chemical reactivity.
Octet rule: Atoms are most stable when their outer shell is full (usually 8 electrons).

Chemical Bonds and Molecules

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

Cation: Positively charged ion (loses electrons).
Anion: Negatively charged ion (gains electrons).
Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl- ions, which combine to form NaCl (table salt).

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. These bonds are strong and form the backbone of most biological molecules.

Nonpolar covalent bond: Electrons are shared equally (e.g., H2 molecule).
Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O molecule).

Hydrogen Bonds

Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one molecule and the slightly negative atom (often oxygen or nitrogen) of another molecule. These are crucial in stabilizing the structures of proteins and nucleic acids, and in the properties of water.

Properties of Water

Water Is Polar

Water molecules have a bent shape and polar covalent bonds, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other.

Water Stabilizes Temperature

Water absorbs and releases heat more slowly than many other substances, helping to stabilize temperatures in organisms and environments. This is due to the energy required to break hydrogen bonds before water molecules can move faster (increase temperature).

High specific heat: Water can absorb a lot of heat before its temperature rises.
High heat of vaporization: Large amounts of energy are needed to convert water from liquid to gas.

Water Is an Excellent Solvent

Water's polarity allows it to dissolve many ionic and polar substances, making it a universal solvent in biological systems. Water forms hydration shells around ions and polar molecules, keeping them dispersed in solution.

Hydrophilic: Substances that dissolve in water (e.g., salts, sugars).
Hydrophobic: Substances that do not dissolve in water (e.g., oils, fats).

Water Is Cohesive and Adhesive

Cohesion: Water molecules stick to each other due to hydrogen bonding, contributing to surface tension.

Adhesion: Water molecules stick to other polar or charged surfaces, aiding processes like capillary action in plants.

Carbon and Organic Macromolecules

Importance of Carbon

Carbon atoms form the backbone of organic molecules. With four valence electrons, carbon can form up to four covalent bonds, allowing for a diversity of stable, complex structures.

Carbon can bond with H, O, N, P, S, and other carbon atoms.
Forms chains, rings, and branched structures.
Functional groups attached to carbon skeletons determine the chemical properties and reactivity of organic molecules.

Functional Group	Structure	Properties	Example
Amino	-NH2	Acts as a base; can pick up H+	Glycine (an amino acid)
Carboxyl	-COOH	Acts as an acid; can donate H+	Acetic acid
Hydroxyl	-OH	Polar; forms hydrogen bonds	Ethanol
Phosphate	-PO4	Contributes negative charge; can release energy	ATP
Sulfhydryl	-SH	Forms disulfide bonds in proteins	Cysteine

Example: The amino group (-NH2) in amino acids allows them to act as bases and participate in peptide bond formation, which is essential for building proteins.

Additional info: The study of atoms, ions, molecules, and water is foundational for understanding all biological processes, as these chemical principles underlie the structure and function of cells and macromolecules.