2.1 Atoms, Molecules, and Chemical Bonds

Atoms, molecules, and chemical bonds form the foundation of life's chemistry. These building blocks combine in countless ways, creating the diverse substances that make up living and nonliving matter alike.

Understanding atomic structure and chemical bonding is key to grasping how living things function. From DNA to proteins, these concepts explain how biological molecules form and interact within organisms.

Atomic Structure

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Subatomic Particles

Atoms are the basic units of matter and the defining structure of elements. Every atom consists of three subatomic particles, each with a distinct role:

• Protons carry a positive charge and sit in the nucleus. The number of protons defines the element (hydrogen always has 1, carbon always has 6). This number is the element's atomic number.
• Neutrons carry no charge and also sit in the nucleus. They contribute to the atom's mass and help stabilize the nucleus, but they don't affect the element's identity.
• Electrons carry a negative charge and occupy the space outside the nucleus. These are the particles involved in chemical bonding.

Electrons are arranged in energy levels (shells) around the nucleus, filling in a specific order: 1s, 2s, 2p, 3s, and so on. The outermost electrons, called valence electrons, are the ones that determine how an atom bonds with other atoms. For example, carbon has 4 valence electrons, which is why it can form up to 4 bonds and is so versatile in biological molecules.

Atomic Models

Two models come up most often in this course:

• The Bohr model shows electrons orbiting the nucleus in fixed circular shells, almost like planets around a sun. It's a simplified picture that works well for visualizing energy levels, but it doesn't fully capture how electrons actually behave.
• The quantum mechanical model is more accurate. Instead of fixed orbits, it describes regions of probability (called orbitals) where an electron is most likely to be found. You won't need to solve the math behind it for this course, but you should know that electrons don't travel in neat circles; they exist in cloud-like regions around the nucleus.

Chemical Building Blocks

Elements and Atoms

An element is a pure substance made of only one type of atom. Each element has a unique chemical symbol (H for hydrogen, O for oxygen, C for carbon) and is organized on the periodic table by atomic number and chemical properties.

Atoms of the same element always have the same number of protons, but they can differ in their number of neutrons. These variants are called isotopes. For example, carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Both behave almost identically in chemical reactions, but carbon-14 is radioactive, which is why scientists use it for carbon dating.

Molecules and Compounds

A molecule is two or more atoms held together by chemical bonds. Molecules can be made of the same element ($O_2$, two oxygen atoms) or different elements ($H_2O$, water).

A compound is a specific type of molecule made of two or more different elements in a fixed ratio. Compounds have properties entirely different from their individual elements. Sodium (Na) is a reactive metal and chlorine (Cl) is a toxic gas, but combined as $NaCl$, they form ordinary table salt.

The chemical formula tells you exactly which atoms are present and how many. In $CH_4$ (methane), there's 1 carbon atom bonded to 4 hydrogen atoms.

Chemical Bonds

Covalent Bonds

A covalent bond forms when two atoms share one or more pairs of valence electrons. Both atoms hold onto the shared electrons, which pulls them together. This type of bonding is most common between nonmetal atoms.

Covalent bonds come in different strengths depending on how many electron pairs are shared:

• Single bond: one shared pair (e.g., $H_2$, each hydrogen shares its 1 electron)
• Double bond: two shared pairs (e.g., $O_2$)
• Triple bond: three shared pairs (e.g., $N_2$)

More shared pairs means a stronger, shorter bond. Covalent bonds are the primary bonds within biological molecules like carbohydrates, lipids, proteins, and nucleic acids.

Ionic Bonds

An ionic bond forms when one atom transfers electrons to another, rather than sharing them. This typically happens between a metal and a nonmetal.

Here's the process:

1. A metal atom (like sodium) loses one or more valence electrons, becoming a positively charged ion called a cation ($Na^+$).
2. A nonmetal atom (like chlorine) gains those electrons, becoming a negatively charged ion called an anion ($Cl^-$).
3. The opposite charges attract each other strongly, holding the ions together in an ionic bond.

The resulting ionic compounds, like $NaCl$, tend to form crystalline structures and dissolve easily in water. When dissolved, they separate into individual ions, which is why salt water conducts electricity.

Hydrogen Bonds

A hydrogen bond is not a true bond in the way covalent and ionic bonds are. It's an intermolecular force, meaning it occurs between molecules rather than within them.

Hydrogen bonds form when a hydrogen atom that's covalently bonded to a highly electronegative atom (typically N, O, or F) is attracted to another electronegative atom on a nearby molecule. The unequal sharing of electrons in the covalent bond gives hydrogen a slight positive charge, which is attracted to the slight negative charge on the neighboring electronegative atom.

Individually, hydrogen bonds are much weaker than covalent or ionic bonds. But in large numbers, they have powerful effects:

• Water's unique properties: Hydrogen bonds between water molecules are responsible for water's high boiling point, surface tension, and cohesion. Without them, water would be a gas at room temperature.
• Biological molecule structure: Hydrogen bonds hold together the two strands of DNA's double helix and help proteins fold into their functional 3D shapes.