AP Biology Sections 2.1 & 2.2 Vocabulary

A complete list of key vocabulary terms and definitions from AP Biology study guide sections 2.1 and 2.2.

Matter

Anything that occupies space and has mass.

Element

A substance that cannot be broken down into simpler substances by ordinary chemical reactions.

Atom

The smallest unit of an element that retains that element's chemical properties.

Proton

A subatomic particle located in the nucleus of an atom with a charge of $+1$ and a mass of $\text{atomic mass unit} \thicksim 1$.

Neutron

A subatomic particle located in the nucleus of an atom with a charge of $0$ and a mass of $\text{atomic mass unit} \thicksim 1$.

Electron

A subatomic particle located in orbitals surrounding the nucleus with a charge of $-1$ and a mass of $\text{atomic mass unit} \thicksim 0$.

Atomic Number

The number of protons in an atom, which determines which element the atom is.

Mass Number

The total number of protons plus neutrons in an atom.

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons.

Half-Life

The time required for half of a radioactive isotope to decay; for example, Carbon-14 has a half-life of about $5\text{,}730\text{ years}$.

Valence Electrons

Electrons located in the outermost electron shell that determine an atom's chemical reactivity and bonding.

Octet Rule

The principle that atoms commonly become stable when they have $8$ electrons in their valence shell.

Ion

An atom or molecule with a net electric charge because it has gained or lost electrons.

Cation

A positively charged ion that forms when an atom loses electrons.

Anion

A negatively charged ion that forms when an atom gains electrons.

Ionic Bond

A chemical bond resulting from the transfer of electrons between atoms, followed by electrostatic attraction between oppositely charged ions.

Covalent Bond

A chemical bond formed when two nonmetal atoms share electrons.

Single Bond

A covalent bond formed by sharing $1$ pair of electrons.

Double Bond

A covalent bond formed by sharing $2$ pairs of electrons.

Triple Bond

An especially strong covalent bond formed by sharing $3$ pairs of electrons, such as in $N_2$.

Nonpolar Covalent Bond

A covalent bond in which electrons are shared equally or approximately equally between atoms with similar electronegativity.

Polar Covalent Bond

A covalent bond in which electrons are shared unequally because one atom is more electronegative, resulting in partial charges.

Electronegativity

The tendency of an atom to attract electrons toward itself.

Hydrogen Bond

A weak attraction between partial positive charges (such as on hydrogen) and partial negative charges (such as on oxygen) of different molecules.

van der Waals Interactions

Weak attractions caused by temporary fluctuations in electron density when atoms or molecules are very close together.

Molecule

Two or more atoms chemically bonded together.

Compound

A substance containing chemically bonded atoms of more than one element.

Hydrophilic

'Water-loving' substances, typically polar molecules or ions, that interact readily with or dissolve in water.

Hydrophobic

'Water-fearing' substances, typically nonpolar molecules such as oils and fats, that do not interact well with water.

High Specific Heat Capacity

The property of water requiring $1\text{ calorie}/g/^\text{o}C$ of heat to change its temperature by $1^\text{o}C$, causing it to heat and cool slowly due to hydrogen bonding.

High Heat of Vaporization

The high amount of energy required to change $1\text{ gram}$ of liquid water into a gas, which accounts for evaporative cooling.

Sphere of Hydration (Hydration Shell)

The surrounding layer of water molecules that forms around a dissolved ion due to attraction between the ion and water's partial charges.

Dissociation

The process where atoms or groups of atoms break off from molecules and form ions in solution.

Cohesion

The attraction between water molecules sticking to other water molecules via hydrogen bonding.

Adhesion

The attraction between water molecules and other distinct molecules or surfaces.

Surface Tension

A property resulting from cohesion that allows liquid water to resist external forces, enabling water droplets to form or insects to stand on water.

Capillary Action

The movement of water within spaces aided by adhesion to surrounding surfaces and cohesion between water molecules.

Transpiration

The evaporation of water from plant leaves that pulls water upward from roots through stems via cohesion and adhesion.

pH Scale

A logarithmic scale ranging from $0$ to $14$ indicating the concentration of $H^+$ ions and the acidity or basicity of a solution.

Acid

A substance that increases the concentration of $H^+$ in solution, lowering the solution's pH below $7$.

Base

A substance that decreases the concentration of $H^+$ in solution (often by providing $OH^-$), raising the solution's pH above $7$.

Buffer

A substance or chemical system that resists changes in pH by absorbing excess $H^+$ or $OH^-$ ions.