Chapter 2: Life's Chemical Basis

atom

Particle that is a fundamental building block of matter; consists of varying numbers of electrons, protons, and neutrons

proton

Positively charged subatomic particle in the nucleus of all atoms. The number of protons (the atomic number) defines the element

neutron

Uncharged subatomic particle in the atomic nucleus

electron

Negatively charged subatomic particle that occupies orbitals around the atomic nucleus

nucleus

The positively charged dense center of an atom

atomic number

The number of protons in the nucleus of atoms of a given element

elements

A substance that consists only of atoms with the same number of protons

isotopes

Forms of an element that differ in the number of neutrons their atoms carry

mass number

Total number of protons and neutrons in the nucleus of an element's atoms

radioisotope

Isotope with an unstable nucleus; decays into predictable daughter elements at a predictable rate

radioactive decay

Process by which atoms of a radioisotope spontaneously emit energy and subatomic particles when their nucleus disintegrates

tracer

A molecule with a detectable label attached; researchers can track it after delivering it into a cell or other system

PET scan

positron emission tomography scan; a visual display of brain activity that detects where a radioactive form of glucose goes while the brain performs a given task

orbital

A three-dimensional region around the nucleus that indicates the probable location of an electron

shell model

Model of electron distribution in an atom; orbitals are shown as nested circles, electrons as dots; 2-8-8-18

ion

Atom that carries a charge because of an unequal number of protons and electrons

free radical

An atom or group of atoms with at least one unpaired electron

electronegativity

A measure of an atom's ability to pull electrons away from other atoms

chemical bond

An attractive force that arises between two atoms when their electrons interact

molecule

Group of two or more atoms joined by chemical bonds

compounds

Type of molecule that has atoms of more than one element

mixture

Two or more types of molecules intermingled in proportions that vary

ionic bond

Type of chemical bond; strong mutual attraction between ions of opposite charge

covalent bond

Chemical bond in which two atoms share a pair of electrons

double covalent bond

A type of covalent bond in which two atoms share two pairs of electrons; symbolized by a pair of lines between the bonded atoms

triple covalent bond

A type of covalent bond in which three pairs of electrons are shared by two atoms

nonpolar covalent bond

A covalent bond in which the bonding electrons are shared equally by the bonded atoms, resulting in a balanced distribution of electrical charge

polar covalent bond

A covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive

polarity

Any separation of charge into distinct positive and negative regions

hydrogen bond

Attraction that forms between a covalently bonded hydrogen atom and an electronegative atom taking part in a separate covalent bond

solvent

Substance, typically a liquid, that can dissolve other substances; e.g., water

solute

A dissolved substance

hydrophilic

Describes a substance that dissolves easily in water; e.g., a salt
Having a strong affinity for water

hydrophobic

Describes a substance that resists dissolving in water; e.g., an oil
Lacking affinity for water

temperature

Measure of molecular motion

evaporation

Transition of a liquid to a gas; requires energy input

cohesion

Tendency of molecules to stick together under tension; a property of liquid water

hydrogen ions (H+)

an atom of hydrogen that has lost its electron. they are the key to the reactions of acids

hydroxide ions (-OH)

released from bases when added to water

pH

A measure of the number of hydrogen ions in a solution

acids, acidic

Any substance that releases hydrogen ions in water

bases, basic

A substance that accepts hydrogen ions as it dissolves in water

salt

Compound that dissolves easily in water and releases ions other than H+ and OH-

buffer system

Set of chemicals that can keep the pH of a solution stable by alternately donating and accepting ions that contribute to pH