TOPIC 1

vocabulary

ATOMSmallest part of an element that can enter into a chemical reaction

ATOMIC MASSAverage mass of all the isotopes of a sample of an element

ATOMIC MASS UNIT1/12 the mass of a carbon-12 atom

ATOMIC NUMBERNumber of protons in the nucleus of an atom

COMPOUNDTwo or more elements chemically combined in definite proportion by mass

ELECTRONFundamental particle of matter having a negative charge

ELEMENTSubstance that cannot be broken down or decomposed into a simpler substance by chemical means

EXCITED STATECondition that exists when electrons of an atom occupy higher energy levels, leaving lower energy level(s) vacant

GROUND STATECondition of an atom or ion in which electrons occupy the lowest possible energy levels

HETEROGENEOUSMixture in which substances are not uniformly distributed

HOMOGENEOUSMixture in which substances are uniformly distributed

ISOTOPEAtom of an element with a specific (sometimes unusual) number of neutrons with the protons

MASS NUMBERprotons + neutrons

MIXTURETwo or more substances not chemically combined

NEUTRONParticle in the nucleus of an atom with no charge (neutral)

NUCLEUSDense, positively charged, central core of an atom

ORBITALRegion in an atom (outside the nucleus) where an electron with a specific amount of energy is most likely to be located

PROTONPositively charged particle in the nucleus of an atom

PURE SUBSTANCEMaterial with the same composition throughout; an element or compound

VALENCEOutermost energy level around an atom

WAVE-MECHANICAL MODELCurrent model of the atom in which electrons exist in orbitals of various shapes and pathways, called the electron cloud