AP Chemistry

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Notes

2. Atoms, molecules, and ions

Laws

Law of Conversation of Mass: mass cannot be created or destroyed

Law of Definite Proportions: when two or more atoms combine to make a compound, the ratio is always the same

Law of Multiple Proportions: atoms can combine in different ratios to make different compounds

Atom models

first model of an atom: plum pudding

modern atom:

nucleus

protons (+)

neutrons (x)

electrons (-) surround nucleus

Elements

protons give identity of element

periodic table organizes elements

atomic number: protons

element symbol

average atomic mass (protons + neutrons)

protons = neutrons with neutral charge

ion: atom with charge

charge = protons - electrons

neutrons = atomic mass - atomic number

electrons = protons - charge

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Compounds

Element types

metals (majority of elements): conductive, malleable

nonmetals (mostly on right side): poor conductor, usually gaseous

semi-metals/metalloids: properties of metals and nonmetals

Naming

Ionic compounds

metal + nonmetal

metal: full name

transition metals: use roman numerals to indicate charge

nonmetal

single element: name - suffix + -ide

polyatomic: special names

add elements to neutralize charge

examples

Na + Cl → NaCl → sodium chloride

Ca + F → CaF₂ → calcium fluoride

FeI₃ → ion(III) iodide

Mg(OH)₂ → magnesium hydroxide

Covalent compounds

semi/nonmetal + semi/nonmetal

use prefixes

1: mono-

don’t add this prefix in first element

2: di-

3: tri-

4: tetra-

5: penta-

6: hexa-

7: hepta-

8: octa-

9: nona-

10: deca-

examples

CO → carbon monoxide

CO₂ → carbon dioxide

N₂F₅ → dinitrogen pentafluoride

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