2.1 Atomic structure and chemical bonding

Atomic Structure and Chemical Bonding

Atoms are the building blocks of all matter in living systems, made up of protons, neutrons, and electrons. How these subatomic particles are arranged determines an atom's properties and how it interacts with other atoms. Understanding atomic structure is the foundation for understanding chemical bonds, which in turn hold together every biological molecule you'll study in this course.

Atomic Structure and Subatomic Particles

Subatomic particles in atoms

Every atom contains three types of subatomic particles, each with a distinct role:

• Protons carry a positive charge and sit in the dense central nucleus. The number of protons defines the atomic number, which uniquely identifies each element (hydrogen has 1 proton, carbon has 6, oxygen has 8).
• Neutrons have no charge and also reside in the nucleus. They contribute to the atom's mass but don't directly influence its chemical behavior.
• Electrons carry a negative charge and move around the nucleus in regions called electron shells (energy levels). The arrangement of electrons, especially the outermost ones, determines how an atom bonds with others.

Valence electrons are the electrons in the outermost shell. These are the ones that actually participate in chemical bonding. An atom like sodium, with just 1 valence electron, is highly reactive because it can easily lose that electron to reach a stable configuration.

Isotopes are variants of the same element that differ in their number of neutrons. Carbon-12 and carbon-14 both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. They behave the same chemically because they have the same number of electrons and protons, but they differ in atomic mass.

Electron configuration and reactivity

Electron configuration describes how electrons are distributed across an atom's shells and subshells. Electrons fill orbitals in a predictable order, starting from the lowest energy level and working up (1s, 2s, 2p, 3s, and so on). Orbitals are grouped into energy levels (shells) and sublevels (s, p, d, f) based on their shape and orientation.

The valence electrons in the outermost shell are what govern reactivity:

• Atoms with a full valence shell are stable and largely unreactive. This typically means 8 electrons in the outer shell (the octet rule), except for the first shell, which is full with just 2. Noble gases like neon and argon already have full valence shells, which is why they rarely form bonds.
• Atoms with incomplete valence shells are reactive. They form bonds to achieve a stable configuration, either by gaining, losing, or sharing electrons.

The number of valence electrons also predicts what kind of ion an atom will form:

• Atoms with 1, 2, or 3 valence electrons tend to lose them, forming positively charged cations (metals like sodium and magnesium).
• Atoms with 5, 6, or 7 valence electrons tend to gain electrons, forming negatively charged anions (nonmetals like oxygen and chlorine).
• Atoms with 4 valence electrons, like carbon, most commonly share electrons through covalent bonding. Carbon's ability to form four covalent bonds is exactly why it's the backbone of organic and biological molecules.

Chemical Bonding in Biological Molecules

Types of chemical bonds

Three types of bonds are especially relevant in biology, and they differ in strength and mechanism:

Ionic bonds form when one atom transfers electrons to another, creating oppositely charged ions that attract each other. Sodium (1 valence electron) donates its electron to chlorine (7 valence electrons), producing $Na^+$ and $Cl^-$, which are held together by electrostatic attraction. In biological systems, ionic bonds help maintain the structure of some proteins and contribute to mineralized tissues like bones and teeth. Ionic compounds also dissociate into ions in water, which is critical for nerve signaling and muscle contraction.

Covalent bonds form when atoms share electron pairs rather than transferring them. These bonds can be:

• Single bonds (one shared pair, e.g., $H-H$)
• Double bonds (two shared pairs, e.g., $O=O$)
• Triple bonds (three shared pairs, e.g., $N \equiv N$)

Covalent bonds are strong and hold atoms together within molecules. They're the primary bonds in biological macromolecules like proteins, nucleic acids, carbohydrates, and lipids.

Hydrogen bonds are weak attractions between a hydrogen atom (already bonded to an electronegative atom like O, N, or F) and a nearby electronegative atom. They form because of the charge imbalance created by polar covalent bonds. Individually, hydrogen bonds are much weaker than ionic or covalent bonds, but collectively they have enormous biological importance:

• They stabilize the secondary structure of proteins (alpha helices and beta sheets)
• They hold together the two strands of DNA through base pairing
• They give water its unique properties, including high specific heat and surface tension

Molecular polarity in biological systems

Polarity refers to the uneven distribution of electrical charge within a molecule. It arises when atoms in a covalent bond have different electronegativities, creating a dipole with a slightly positive end and a slightly negative end. Water is the classic example: the oxygen end is partially negative ($\delta^-$), and the hydrogen ends are partially positive ($\delta^+$).

• Polar molecules are hydrophilic (water-loving). They dissolve readily in water because they can form hydrogen bonds and other electrostatic interactions with water molecules.
• Nonpolar molecules have an even charge distribution and are hydrophobic (water-fearing). They don't form hydrogen bonds with water and tend to cluster together in aqueous environments to minimize contact with water. This clustering is called the hydrophobic effect.

Molecular polarity drives several key biological processes:

• Cell membrane structure: Phospholipids have a polar (hydrophilic) head and two nonpolar (hydrophobic) tails. In water, they spontaneously arrange into a bilayer with the hydrophobic tails facing inward and the hydrophilic heads facing outward toward the water. This is the structural basis of all cell membranes.
• Protein folding: Nonpolar amino acids tend to fold toward the interior of a protein (away from water), while polar amino acids face outward. This arrangement is a major driver of a protein's three-dimensional shape, which determines its function.
• Enzyme-substrate interactions: The specific arrangement of polar and nonpolar regions in an enzyme's active site allows it to bind its substrate with high specificity, enabling efficient catalysis of biochemical reactions.