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MCAS Chemistry Review

 

Chemistry Vocabulary

element – a substance that cannot be decomposed into simpler substances by chemical means; the simplest substance (hydrogen, oxygen, carbon, nitrogen, gold, titanium, etc)

atom – the smallest unit of matter

compound – two or more atoms bonded chemically
example:  NaCl (salt), H20 (water), NH3 (ammonia)

molecule – the smallest unit of a compund

mixture – two or more substances blended together (example: sugar water, Chex mix)

chemical change – a change in which one or more new substances are formed
example:  iron (Fe) and water (H20) react to form rust (Fe2O3)

physical change – a change in appearance of the same substance.
example:  water changing from solid (ice) to liquid

Reading Chemical Formulas

Number of molecules: The coefficient (large number to the left of the formula) tells the number of molecules represented by the formula. If no number is present, there is one molecule.

Elements in the molecule: The letters represent the elements present in the formula. Remember: Elements are represented by one capital letter or one capital letter followed by one or more lower case letters. These abbreviations are called Element Symbols. Use the periodic table in your agenda (p. R-7) if needed.

How many atoms of each element in the molecule: The subscripts (small numbers to the right of the element symbol) tell how many atoms of each element there are. Subscripts go with the element symbol that comes before them. If no subscript is present, then there is one atom of that element. If there is a coefficient, then the subscript needs to be multiplied by the coefficient to determine the number of atoms.

Example: H2O

Question

Answer

How many molecules are represented in this formula?

1

How many elements are represented in this molecule? Name them.

2

(Hydrogen and Oxygen)

How many atoms of each element are there?

Hydrogen = 2 atoms

Oxygen = 1 atom

Example: 5H2O

Question

Answer

How many molecules are represented in this formula?

5

How many elements are represented in this molecule? Name them.

2

(Hydrogen and Oxygen)

How many atoms of each element are there?

Hydrogen = 10 atoms

Oxygen = 5 atom

Ball-and-Stick Models

Ball-and-Stick Models are used to represent molecules. The balls represent atoms and the sticks represent the bonds between the atoms. Balls representing the same element will usually be color-coded. Also, atoms of different elements will usually be represented as different sizes if the atoms of those elements are different sizes.

Screen shot 2013-10-03 at 9.14.53 PM

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