Chapter 2

Matter

Anything that has mass and takes up space.

Element

A pure substance made of one type of atom.

Atom

The smallest unit of an element.

Molecule

Two or more atoms chemically bonded together.

Compound

A substance made of two or more different elements chemically bonded together.

Four most common elements in living organisms

Oxygen, carbon, hydrogen, and nitrogen.

Atomic number

The number of protons in an atom.

Atomic mass

The total number of protons and neutrons in an atom.

Atomic symbol

The one- or two-letter abbreviation for an element.

Nucleus of an atom

The center of the atom containing protons and neutrons.

Electron shells

Energy levels around the nucleus where electrons are found.

Arrangement of subatomic particles

Protons ($+\text{charge}$) and neutrons ($0\text{ charge}$) reside in the nucleus, while electrons ($-\text{charge}$) move around the nucleus in electron shells.

Electron configuration of Carbon (C)

$2$ electrons in the first shell and $4$ in the second shell.

Electron configuration of Oxygen (O)

$2$ electrons in the first shell and $6$ in the second shell.

Electron configuration of Hydrogen (H)

$1$ electron in the first shell.

Electron configuration of Nitrogen (N)

$2$ electrons in the first shell and $5$ in the second shell.

Reactant vs. product

Reactants are the starting substances in a chemical reaction, while products are the substances formed by the reaction.

Purpose of chemical bonding

Atoms bond to become more stable by sharing or transferring electrons.

Ionic bonds

Relatively strong bonds formed when one atom transfers electrons to another atom.

Covalent bonds

Bonds formed when atoms share electrons; can be nonpolar (shared equally) or polar (shared unequally).

Hydrogen bonds

Weak attractions between a partially positive hydrogen atom and a partially negative atom.

Hydrophilic vs. hydrophobic molecules

Hydrophilic molecules are attracted to water (e.g., salt and sugar), whereas hydrophobic molecules repel water (e.g., oils and fats).

Polarity of water

Oxygen attracts electrons more strongly than hydrogen, resulting in a partial negative charge on oxygen and a partial positive charge on hydrogen.

High heat of vaporization of water

The property where water requires a large amount of energy to evaporate.

Surface tension of water

The property where water's surface resists being broken due to hydrogen bonds.

Cohesion vs. adhesion in water

Cohesion is water molecules sticking to each other, while adhesion is water molecules sticking to other surfaces or substances.

Acid

A substance that increases hydrogen ions ($H^+$) in a solution.

Base

A substance that decreases hydrogen ions ($H^+$) or increases hydroxide ions ($OH^-$) in a solution.

Buffer

A substance that helps resist changes in pH.

pH scale

A scale from $0$ to $14$ measuring acidity or alkalinity: below $7$ is acidic, $7$ is neutral, and above $7$ is basic.

Importance of pH and blood buffers

Organisms need a stable pH for proper enzyme and cellular function; blood buffers keep human blood pH around $7.4$.

Role of carbon as the backbone of organic molecules

Carbon can form $4$ covalent bonds, enabling it to form long chains, rings, and complex molecular structures.

Hydroxyl group

$$-\text{OH}$$

Methyl group

$$-\text{CH}_3$$

Carboxyl group

$$-\text{COOH}$$

Amino group

$$-\text{NH}_2$$

Phosphate group

$$-\text{PO}_4$$