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Chapter 2
Atoms, Molecules and Ions
The atomic theory
Dalton’s Atomic Theory (1808)
1.
Elements are composed of extremely small particles called atoms.
2.
All atoms of a given element are identical, The atoms of one element are different from the atoms
of all other elements.
3.
Compounds are formed from atoms of more than one element in a fixed ratio.
4.
A chemical reaction involves only the separation, combination, or rearrangement of atoms.
Law of definite proportions
Atoms of any element are alike, atoms of different elements are different.
Law of multiple proportions
CO, carbon monoxide: C/O = 1:1
CO2, carbon dioxide: C/O = 1:2
The ratio of atoms of different elements in a compound is an integer.
Law of conservation of mass
2X+Y
X2Y
Rutherford’s Model of the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
Mass and Charge
Electron cloud outside nucleus
Relative Charge Relative Mass
Proton
1
~ 1 amu
Neutron
0
~ 1 amu
Electron
-1
1/1840 amu
Location
nucleus
nucleus
outside nucleus
Atomic number, Mass number and Isotopes
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons
number of protons = number of electrons
Mass Number
A
Atomic Number
Z
X
Element Symbol
Isotopes
Isotopes: same element, different number of neutrons
The Isotopes of Hydrogen
Hydrogen deuterium tritium
1H 2H 3H
1
1
1
How many protons, neutrons, and electrons are in 14C ?
6 protons, 6 electrons, 14 – 6 = 8 neutrons,
6
The periodic table:
Column: group
1A: alkali metal
Row: period
2A: alkaline earth metal
7A: halogen
8A: noble gas
Different colors: metals, metalloids, nonmetals
Molecules and ions
compounds are of two types:
- Molecular: involves shared electrons and consists of electrically neutral, discrete particles (molecules) H2O, O2
- Ionic compounds: involves electron transfer and charged particles (ions) NaCl
2.2
Molecular Compound
2.3
As a general rule, molecular compounds are formed when nonmetallic elements combine
Period
2
3
4
5
IVA
CH4
VA
NH3
VIA
H2O
H2S
VIIA
HF
HCl
HBr
HI
Noble Gas
Ne
Ar
Kr
Xe
A diatomic molecule contains only two atoms:H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms: O3, H2O, NH3, CH4
An ion is an atom, or group of atoms, that has a net positive or negative charge.
cation – ion with a positive charge.
If a neutral atom loses one or more electrons, it becomes a cation.
Na+:11 protons, 10 electrons
Na:11 protons, 11 electrons
anion – ion with a negative charge. If a neutral atom gains one or more electrons, it becomes an anion.
Cl:17 protons, 17 electrons
Cl-:17 protons, 18 electrons
A monatomic ion contains only one atom: Na+, Cl-, Ca2+, O2-, Al3+
A polyatomic ion contains more than one atom: OH-, CN-, NH4+, NO327
3+
How many protons and electrons are in 13 Al ?
How many protons and electrons are in78
34Se
2- ?
13 protons, 13 – 3 = 10 electrons
34 protons, 34 + 2 = 36 electrons
Common ions:
1A: Li+, Na+, K+
2A: Mg2+, Ca2+, Ba2+
3A: Al3+
6A: O2- S2-
7A: F-, Cl-, Br-, I-
Other: Ag+, Fe2+, Fe3+, Co2+, Zn2+, Mn2+
Simple ionic compounds: formed from metals + nonmetals
Ionic compounds consist of a combination of cations and an anions.
The most reactive metals and the most reactive nonmetals combine to form ionic compounds.
NaCl, ZnO, Al2O3
Molecular formula shows the exact number of atoms of each element in the smallest unit of a substance
Empirical formula shows the simplest whole-number ratio of the atoms in a substance
Molecular H2O C6H12O6 O3 N2H4
Empirical H2O CH2O
O NH2
Naming Molecular compounds
•
•
•
common names H2O, NH3, CH4,
element further left in periodic table is 1st
if more than one compound can be formed from the same elements, use prefixes to indicate number
of each kind of atom (The prefix of “mono” may be omitted for the first element.)
• Name first element first and last element ends in ide
Prefix: mono- di- tri- tetra- penta- hexa- hepta- octa- nona- decaMeaning: 1
2 3
4
5
6
7
8
9
10
Examples:
PF5 phosphorus pentafluoride
SO2 sulfur dioxide
HCl hydrogen chloride
NF3 nitrogen trifluoride
N2O4 dinitrogen tetraoxide
N2O dinitrogen oxide
Formula of Ionic Compounds
The formula is always the same as the empirical formula because molecules don’t exist in ionic compounds
Rules for writing formulas of ionic compounds:
1.
The positive ion is given first in the formula
2.
The subscripts in the formula must produce an electrically neutral formula unit
3.
The subscripts should ne the set of smallest whole numbers possible
4.
The charges on the ions are not included in the finished formula of the substance
Cations: name of element
Anions: change ending of element spelling to -ide
2.4
Formula of Ionic Compounds
Al3+
Al2O3
+3 x 2 = +6, 3 x (-2) = -6
Aluminum oxide
O2The sum of charges on the cations and anions in each formula unit must be zero.
The subscript of cation is numerically equal to the charge on the anion;
The subscript of anion is numerically equal to the charge on the cation;
CaBr2
+2 x 1 = +2, 2 x (-1) = -2
Na2CO3
Mg(OH)2
+1 x 2 = +2, 1 x (-2) = -2
Sodium carbonate
magnesium hydroxide
Calcium bromide
Check Table 2.3 for the cations and anions
Transition metal ionic compounds
indicate charge on metal with Roman numerals
FeCl2 +2 for Fe, Fe2+
FeCl3
iron(II) chloride
+3 for Fe, Fe3+
iron(III) chloride
Acids and Bases
An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water.
HCl
•Pure substance, hydrogen chloride
•Dissolved in water (H+ Cl-), hydrochloric acid
Anions whose names end in “ide" form acids with “hydro-” prefix and “-ic” ending.
H2S hydrosulfuric acid
HCN hydrocyanic acid
(CN-: cyanide)
An oxoacid is an acid that contains hydrogen, oxygen, and another element.
HNO3
H2SO4
nitric acid
sulfuric acid
H2CO3 carbonic acid
HClO3 chloric acid
2.5
1.
2.
3.
Addition of one O atom to “-ic” acid”per…ic”acid HClO4 perchloric acid
Removal of one O atom from “-ic” acid”-ous” acid
HNO2 nitrous acid
Removal of two O atoms from “-ic” acid”hypo…ous” acid HClO hypochlorous acid
Acid
Anion
HClO
hypochlorous acid
ClOhypochlorite
HClO2
chlorous acid
ClO2
chlorite
HClO3
chloric acid
ClO3chlorate
HClO4
perchloric acid
ClO4
perchlorate
A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water.
KOH
potassium hydroxide
Ba(OH)2 barium hydroxide
Hydrates: compounds that have a specific number of water molecules attached to them.
BaCl2•2H2O barium chloride dihydrate
CuSO4•5H2O blue
CuSO4 white
MgSO4•7H2Omagnesium sulfate heptahydrate
Name the following compounds:
Cu(NO3)2
Copper(II) nitrate
KH2PO4
potasium dihydrogen phosphate
NH4ClO3
ammonium chlorate
Common names of some compounds:
H2O
Water
NH3
ammonia
CO2
dry ice
solid carbon dioxide
Some more listed in Table 2.7
NaCl
table salt
sodium chloride
NaHCO3
baking soda
sodium hydrogen carbonate
2.6