Eighth Grade Physical Science
Matter Unit II Vocabulary
(Chapters 20, 21)
1. Compound – substance made up of two or more elements chemically combined in
definite proportions
2. Chemical formula – shorthand representation for a molecule of a compound,
shows the kinds and numbers of elements in the compound
3. Molecule – the smallest particle of a compound
4. Ion – charged particle of a substance that has lost or gained electrons
5. Ionic bond – a strong electrical bond between ions, positive and negative, metal
and nonmetal, left and right side of Periodic Table (ionic compounds have high
melting points, crystalline structure, dissolve in water to form ions, solutions
conduct electricity)
6. Covalent bond – weaker bond formed when electrons are shared, two negative
oxidation numbers, two nonmetals, two elements on right side of table (covalent
compounds have low melting points, usually do not have crystalline structure, do
not ionize in water, do not conduct electricity)
7. Metallic bond – a strong bond that holds atoms of a metal together, formed by a
common cloud of electrons surrounding the positive nuclei of the metals, account
for the properties of metals
8. Oxidation number – shows the combining capacity of an element (or polyatomic
ion), shows the number of electrons gained, lost, or shared (group one +1, group
two +2, group three +3, group four 4, group five -3, group six -2, group seven -1,
group eight none (full))
9. Chemical reaction – when one or more substances changes into one or more new
and different substances
10. Reactant – a substance that enters into a chemical reaction, on the left side of the
equation
11. Product – substances that are formed by a chemical reaction, on the right side of
the equation
12. Conservation of mass – matter cannot be created or destroyed only changed from
one form to another, the reason equations must be balanced
13. Chemical equation – a shorthand representation of what happens in a reaction,
shows the elements and compounds involved in a reaction and their numbers
14. Coefficient – a whole number multiplier used to show the number of elements or
compounds in an equation
15. Exothermic reaction – a reaction in which heat is released
16. Endothermic reaction – a reaction in which heat is absorbed
17. Synthesis – a reaction in which two small particles combine to form one larger
compound ( A + B → AB )
18. Decomposition – a reaction in which a larger compound breaks down into smaller
pieces ( AB → A + B )
19. Single Replacement – a reaction in which a single element replaces an element in
a compound ( A + XY → X + AY )
20. Double Replacement – a reaction in which the two metal (A and B) ions switch
places in two reacting compounds ( AX + BY → BX + AY )
21. Catalyst – a substance that speeds up a reaction without being used up by the
reaction, therefore not a reactant. EX. Enzymes in cellular reactions.