ATOMS, MOLECULES, AND IONS
Atomic Theory of Matter
Modern atomic theory is based on the experimental work of many scientists over the course of
many years. Experimentation done as early as the 18th century resulted in generalizations which are
summarized in what are called “laws”.
Dalton’s Atomic Theory of Matter —





Matter consists of individual atoms.
All atoms of a given chemical element are identical.
Different chemical elements have differing atoms of different mass.
Atoms are indestructible – they retain their identity in reactions.
Compounds are formed from a combination of elements in small whole number ratios.
The Law of Constant Composition —
for
the relative kinds and numbers of atoms are constant
a given compound. (e.g. water, H2O, always contains
2 hydrogen atoms and 1 oxygen atom)
The Law of Conservation of Mass —
during a chemical reaction, the total mass before
reaction is equal to the total mass after reaction.
N2 + 3 H2  2 NH3
34 g
The Law of Multiple Proportions —
34 g
if two elements A and B combine to form more than one
compound, then the mass of B that combines with the
mass of A is a ratio of small whole numbers. (e.g. CO,
CO2)
Modern View of Atomic Structure
Experiments by Thomson and Millikan confirmed the existence of electrons as the negatively
charged particles within an atom. Electrons have a charge of e = 1.6021773  10–19 C and a mass
of 9.109390  10–31 kg. Later experiments by Rutherford determined that at the center of an atom
is a positively charged, compact, heavy nucleus. The charge on the atomic nucleus is +Ze (Z is the
atomic number of the atom). The fundamental unit of positive charge in the nucleus is the proton.
 Chemical identity of an atom is determined by the number of protons.
 The nucleus also contains neutral particles, neutrons, which are essential for the stability of the
nucleus. They contribute to the mass of the atom but have no charge.
Atomic Mass Units
1
Angstroms (Å)
Isotopes, Atomic Numbers, and Mass Numbers
Z=
N=
A=
Ex. 1
An isotope detected in a mass spectrometer has atomic number 82 and relative mass
205. Write the symbol for this isotope, and list the subatomic particles composing
it.
The Periodic Table
2
Molecules and Molecular Compounds
Chemical Formulas
Empirical and Molecular Formulas
Empirical Formula
Molecular Formula
3
Ex. 2
From the following list, find the groups of compounds that have the same empirical
formula: C2H2, N2O4, C2H4, C6H6, NO2, C3H6, C4H8
Ions and Ionic Compounds
A cation results when an atom or molecule loses electrons (giving it a positive charge).
An anion results when an atom or molecule gains electrons (giving it a negative charge).
Ionic Charges and Ionic Compounds
Ex. 3
Predict formulas for compounds formed from
a. Polonium (Po) and strontium (Sr)
b.
Mg2+ and PO43–
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c.
Gallium (Ga) and fluorine (F)
Naming Inorganic Compounds
Ionic Compounds
Cations
Anions
Ionic Compounds
5
Acids
Binary Molecular Compounds
Ex. 4
Many familiar substances have common, unsystematic names. For each of the
following, give the correct systematic name:
a. saltpeter (KNO3)
b.
c.
d.
soda ash (Na2CO3)
lime (CaO)
muriatic acid (HCl)
e.Epsom salts (MgSO4)
f. milk of magnesia (Mg(OH)2)
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Nomenclature Rules for Polyatomic Ions
Polyatomic Oxoanions with Standard Names
Key Element
Formula
Name of Ion
NO3
Nitrogen
nitrate ion

NO2
SO4
Sulfur
nitrite ion
2–
sulfate ion

HSO4
SO3
Phosphorus
Carbon
Chlorine
hydrogen sulfate ion
2–
sulfite ion

HSO3
hydrogen sulfite ion
PO43–
phosphate ion
HPO42–
hydrogen phosphate ion
H2PO4–
dihydrogen phosphate ion
PO33–
phosphite ion
CO32–
carbonate ion
HCO3
hydrogen carbonate ion

ClO4–
perchlorate ion
ClO3–
chlorate ion
ClO2–
chlorite ion
ClO–
hypochlorite ion
BO33
Boron
borate ion
Polyatomic Ions with Non-Standard Names
Key Element
Formula
Carbon
Name of Ion
C2H3O2 (CH3COO–
acetate ion
CN
cyanide ion
C2O42–
oxalate ion
)
Nitrogen
NH4+
ammonium ion
Oxygen
H3O+
hydronium ion
OH–
hydroxide ion
O22–
peroxide ion
O2–
superoxide ion
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Metals
MnO4–
permanganate ion
CrO42–
chromate ion
Cr2O72–
dichromate ion
Names of Common Acids
Binary Acids
Oxoacids
Organic Acids
HF
hydrofluoric acid
HClO4 perchloric acid
HCOOH
formic acid
HCl
hydrochloric acid
HClO3 chloric acid
CH3COOH
acetic acid
HBr
hydrobromic acid
HClO2 chlorous acid
C6H5COOH
benzoic acid
HI
hydroiodic acid
HClO
HOOC–COOH
oxalic acid
HCN
hydrocyanic acid
H2SO4 sulfuric acid
H2S
hydrosulfuric acid
H2SO3 sulfurous acid
hypochlorous acid
HNO3
nitric acid
HNO2
nitrous acid
H3PO4 phosphoric acid
H2CO3 carbonic acid
Ex. 5
Iodic acid has the molecular formula HIO3. Write the formulas for the following:
a. the iodate anion
b.
the periodate anion
c.
the hypoiodite anion
d.
hypoiodous acid
e.periodic acid
Ex. 6
that
From the following list of elements – Ar, H, Ga, Al, Ca, Br, Ge, K, O – pick the one
best fits each description; use each element only once:
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a. an alkali metal
b.
an alkaline earth metal
c.
a noble gas
d.
a halogen
e.a metalloid
f. a nonmetal listed in group IA
g. a metal that forms a 3+ ion
h.
a nonmetal that forms a 2– ion
i.
an element that resembles aluminum
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