The Chemical Context Of Life

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ammonia

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A small, very toxic molecule (NH₃) produced by nitrogen fixation or as a metabolic waste product of protein and nucleic acid metabolism.

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anion

Back 2

A negatively charged ion.

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atom

Back 3

The smallest unit of matter that retains the properties of an element.

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atomic mass

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The total mass of an atom, which is the mass in grams of 1 mole of the atom.

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atomic nucleus

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An atom’s dense central core, containing protons and neutrons.

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atomic number

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The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.

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cation

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A positively charged ion.

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chemical bond

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An attraction between two atoms, resulting from a sharing of outer-shell electrons or the presence of opposite charges on the atoms. The bonded atoms gain complete outer electron shells.

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chemical equilibrium

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In a chemical reaction, the state in which the rate of the forward reaction equals the rate of the reverse reaction, so that the relative concentrations of the reactants and products do not change with time.

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chemical reaction

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The making and breaking of chemical bonds, leading to changes in the composition of matter.

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compound

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

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covalent bond

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A type of strong chemical bond in which two atoms share one or more pairs of valence electrons.

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dalton (amu)

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A measure of mass for atoms and subatomic particles; the same as the atomic mass unit, or amu.

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double bond

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A double covalent bond; the sharing of two pairs of valence electrons by two atoms.

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electron

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A subatomic particle with a single negative electrical charge and a mass about 1/2,000 that of a neutron or proton. One or more move around the nucleus of an atom.

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electron shell

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An energy level of electrons at a characteristic average distance from the nucleus of an atom.

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electronegativity

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The attraction of a given atom for the electrons of a covalent bond.

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element

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Any substance that cannot be broken down to any other substance by chemical reactions.

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energy

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The capacity to cause change, especially to do work (to move matter against an opposing force).

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hydrogen bond

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A type of weak chemical bond that is formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.

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ion

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An atom or group of atoms that has gained or lost one or more electrons, thus acquiring a charge.

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ionic bond

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A chemical bond resulting from the attraction between oppositely charged ions.

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ionic compound (salt)

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A compound resulting from the formation of an ionic bond; also called a salt.

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isotope

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One of several atomic forms of an element, each with the same number of protons but a different number of neutrons, thus differing in atomic mass.

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mass number

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The sum of the number of protons and neutrons in an atom’s nucleus.

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matter

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Anything that takes up space and has mass.

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molecular formula

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A type of molecular notation representing the quantity of constituent atoms, but not the nature of the bonds that join them.

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molecule

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Two or more atoms held together by covalent bonds.

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neutron

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A subatomic particle having no electrical charge (electrically neutral), with a mass of about 1.7 × 10^-24 g, found in the nucleus of an atom.

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nonpolar covalent bond

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A type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity.

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nucleus

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An atom’s central core, containing protons and neutrons.

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orbital

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The three-dimensional space where an electron is found 90% of the time.

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polar covalent bond

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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.

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potential energy

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The energy that matter possesses as a result of its location or spatial arrangement (structure).

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product

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A material resulting from a chemical reaction.

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proton

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A subatomic particle with a single positive electrical charge, with a mass of about 1.7 × 10^-24 g, found in the nucleus of an atom.

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radioactive isotope

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An isotope (an atomic form of a chemical element) that is unstable; the nucleus decays spontaneously, giving off detectable particles and energy.

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reactant

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A starting material in a chemical reaction.

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salt

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A compound resulting from the formation of an ionic bond; also called an ionic compound.

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single bond

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A single covalent bond; the sharing of a pair of valence electrons by two atoms.

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structural formula

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A type of molecular notation in which the constituent atoms are joined by lines representing covalent bonds.

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theory

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An explanation that is broad in scope, generates new hypotheses, and is supported by a large body of evidence.

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trace element

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An element indispensable for life but required in extremely minute amounts.

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valence

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The bonding capacity of a given atom; usually equals the number of unpaired electrons required to complete the atom’s outermost shell.

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valence electron

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An electron in the outermost electron shell.

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valence shell

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The outermost energy shell of an atom, containing the valence electrons involved in the chemical reactions of that atom.

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van der Waals interactions

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Weak attractions between molecules or parts of molecules that result from localized charge fluctuations.