Chemistry Unit 1

Atom
The smallest particle of an element that can enter into a chemical reaction.
Element
Substances that cannot be broken down or decomposed into simpler substances by chemical means.
Compound
A substance composed of 2 or more elements that are chemically combined in definite proportions by mass.
Pure Substance
A compound or an element; material in which the composition is the same throughout.
Atomic Mass
The average mass of all the isotopes in a sample of an element.
Isotope
Atom of an element that has a specific number of protons and neutrons.
Mass Number
The total number of protons and neutrons in the nucleus of an atom.
Excited State
When the electrons of an atom occupy higher energy levels while lower energy levels are vacant.
Ground State
An atom or ion in which the electrons occupy the lowest available energy levels.
Mixture
Combinations of 2 or more substances that can be separated by physical means.
Heterogeneous
A mixture in which the substances are not uniformly mixed.
Homogeneous
A substance in which the particles are uniformly mixed.
Wave-Mechanical Model
The current model of the atom that deals with the wave particle duality of nature.
Orbital
A region in an atom which an electron of a particular amount of energy is most likely to be located.
Valence Electrons
The electrons in the outermost energy level of an atom.
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