Section 1.1 Basic Chemistry and Water

Matter

Anything that takes up space and has mass.

Element

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

Compound

A substance made of two or more different elements combined in a fixed ratio.

Essential elements

Elements required by an organism to survive and reproduce.

Trace elements

Elements required by an organism in very small quantities.

Atomic number

The number of protons in an atom.

Atomic mass

The average mass of an element's isotopes, based on the number of protons and neutrons.

Valence electrons

Electrons in an atom's outermost shell that are involved in chemical bonding.

Periodic table groups

Elements in the same vertical column have the same number of valence electrons.

Periodic table periods

Elements in the same horizontal row have the same number of electron shells.

Bohr model

A model that shows electrons arranged in shells around an atom's nucleus.

Lewis dot model

A simplified model that shows only the valence electrons surrounding an element's symbol.

Octet rule

Atoms tend to gain, lose, or share electrons to complete their valence shell and become more stable.

Chemical bond

An attraction between atoms resulting from the sharing or transferring of valence electrons.

Electronegativity

An atom's ability to attract electrons toward itself.

Covalent bond

A chemical bond in which atoms share electrons.

Nonpolar covalent bond

A covalent bond in which electrons are shared equally between atoms.

Polar covalent bond

A covalent bond in which electrons are shared unequally between atoms, producing partial charges.

Ionic bond

An attraction between oppositely charged ions that forms after electrons are transferred between atoms.

Cation

A positively charged ion.

Anion

A negatively charged ion.

Hydrogen bond

A weak attraction between a partially positive hydrogen and an electronegative atom, usually O, N, or F, in another polar molecule.

Isomer

A molecule with the same chemical formula as another molecule but a different arrangement of atoms.

Water polarity

Water is polar because electrons are shared unequally between oxygen and hydrogen, giving oxygen a partial negative charge and hydrogen a partial positive charge.

Why water forms hydrogen bonds

The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another water molecule.

Cohesion

The attraction between molecules of the same substance.

How hydrogen bonding causes cohesion

Hydrogen bonds attract neighboring water molecules, causing water molecules to stick together.

Biological importance of cohesion

Cohesion helps water move through plant xylem and contributes to water's surface tension.

Surface tension

The resistance of a liquid's surface to an external force caused by cohesive forces between molecules.

Adhesion

The attraction between molecules of different substances.

Biological importance of adhesion

Water can cling to polar xylem cell walls, helping resist the downward pull of gravity.

Capillary action

The upward movement of water resulting from cohesion, adhesion, and surface tension.

High specific heat

Water's ability to resist changes in temperature.

Why water has high specific heat

Energy must be absorbed to disrupt hydrogen bonds, so a large amount of energy is required to change water's temperature.

Biological importance of high specific heat

Water helps stabilize temperatures in organisms, oceans, lakes, and the surrounding environment.

Heat of vaporization

The amount of energy required for liquid water to become a gas.

Evaporative cooling

Cooling that occurs when the highest-energy water molecules escape as gas, lowering the average kinetic energy of the remaining water.

Biological importance of evaporative cooling

Evaporation helps prevent organisms and plant leaves from overheating and helps moderate Earth's climate.

Transpiration

The loss of water from plants through evaporation; higher temperatures increase transpiration rates.

Why ice is less dense than liquid water

As water freezes, hydrogen bonds stabilize a crystalline structure that spaces water molecules farther apart.

Biological importance of floating ice

Floating ice forms an insulating layer that reduces heat loss from the water below, allowing liquid water to remain underneath.

Water as a solvent

Water is a versatile solvent because its polar molecules can interact with ions and other polar molecules.

Solution

A homogeneous mixture of two or more substances.

Solvent

The substance that dissolves another substance in a solution.

Solute

The substance that is dissolved in a solution.

"Like dissolves like"

Polar substances tend to dissolve in polar solvents because they can interact with one another.

Why water dissolves ionic compounds

Water's partial charges attract ions; oxygen interacts with positive ions while hydrogen interacts with negative ions.