Atoms, Molecules and Ions

1
Atoms, Molecules and Ions
2
Daltons Atomic Theory (1808)

• Elements are composed of extremely small
  particles called atoms. All atoms of a given
  element are identical, having the same size, mass
  and chemical properties. The atoms of one
  element are different from the atoms of all other
  elements.

• Compounds are composed of atoms of more than one
  element. The relative number of atoms of each
  element in a given compound is always the same.

• Chemical reactions only involve the
  rearrangement of atoms. Atoms are not created or
  destroyed in chemical reactions.

2.1
3

• Dalton developed the atomic theory to explain
• The Law of Conservation of Mass
• The Law of Definite Proportions
• The Law of Multiple Proportions

Laws are summaries of experimental
observations. Theories are models devised to
explain laws.
4
(No Transcript)
5
Laws of Mass Conservation Definite
Composition
Law of Mass conservation The total mass of
substances does not
change during a chemical reaction. Law of
Definite ( or constant ) composition No matter
what its source, a
particular chemical
compound is composed of the same elements
in the same proportions by
mass.
6
8 X2Y
Law of Conservation of Mass
2.1
7
LAW OF MULTIPLE PROPORTIONS If two elements can
combine to form more than one compound, the
masses of one element that combine with with a
fixed mass of the other element are in the ratio
of small whole numbers. Carbon monoxide 1.00 g
C to 1.33 g O Carbon dioxide 1.00 g C to
2.66 g O
The mass of an element in a compound is
proportional to the number of atoms of the
element present?
8
2
Law of Multiple Proportions
2.1
9
Atoms
10
THE STRUCTURE OF THE ATOM Dalton thought that
the atom was indivisible, but by the end of the
19th century there was evidence that the atom was
composed of smaller particles. The electron was
discovered in the 1890s by J. J. Thomson using a
Crooks tube (cathode ray tube). He was able to
measure the ratio of the electric charge to mass
of the electron to be -1.76 x 108 C/g. Later an
American physicist, Millikan, measured the charge
to be -1.60 x 10-19 C.
11
Rutherford Experiment

• Bombarded a thin gold foil with high-energy alpha
  particles from radium.
• Most alpha particles went through unaffected.
• About 1 in 10,000 deflected through a large
  angle.
• Lead to nuclear model of atom.

12
(1908 Nobel Prize in Chemistry)

• particle velocity 1.4 x 107 m/s
• (5 speed of light)

• atoms positive charge and most of its mass
• are concentrated in the nucleus
• 2. light electrons are in the outer part of the
  atom.

2.2
13
Rutherfords 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
If the atom is the Houston Astrodome Then the
nucleus is a marble on the 50 yard line
2.2
14
By the 1930s it was known that the nucleus
contained two subatomic particles - the proton
and the neutron. The nucleus has a volume which
is only a tiny fraction of the atom. The
electrons occupy the outer part of the atom.
15
Subatomic Particles (Table 2.1)
mass p mass n 1840 x mass e-
2.2
16
Atomic number (Z) number of protons in nucleus
Mass number (A) number of protons number of
neutrons
atomic number (Z) number of neutrons Isotopes
are atoms of the same element (X) with different
numbers of neutrons in their nuclei
2.3
17
2.3
18
Do You Understand Isotopes?
6 protons, 8 (14 - 6) neutrons, 6 electrons
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3
19
THE PERIODIC TABLE Elements with similar
chemical and physical properties are grouped
together in vertical columns called GROUPS or
FAMILIES. Rows are called PERIODS. Group 1 (1A)
- alkali metals Group 2 (2A) - alkaline earth
metals Group 17 (7A) - halogens Group 18 (8A) -
noble gases Transition elements (3-12) metals,
nonmetals, metalloids
20
2.4
21
Molecules and Ions
22
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.
anion ion with a negative charge If a neutral
atom gains one or more electrons it becomes an
anion.
2.5
23
How do we know how many electrons are lost or
gained? For elements near either side of the
periodic table the atoms tend to form ions with
the same number of electrons as the nearest noble
gas (octet rule). What kind of ions do these
elements form? I, Ba, Al, S, Rb, Fe
24
Do You Understand Ions?
13 protons, 10 (13 3) electrons
34 protons, 36 (34 2) electrons
2.5
25
A monatomic ion contains only one atom
Na, Cl-, Ca2, O2-, Al3, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4, NO3-
2.5
26
All metals form cations.
2.5
27
A Polyatomic Ion
Fig. 2.22
28
A molecule is an aggregate of two or more atoms
in a definite arrangement held together by
chemical bonds
A diatomic molecule contains only two atoms
H2, N2, O2, F2, Cl2, Br2, I2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
2.5
29
2.6
30
A molecular formula shows the exact number of
atoms of each element in the smallest unit of a
substance
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
H2O
CH2O
C6H12O6
O3
O
NH2
N2H4
2.6
31

• ionic compounds consist of a combination of
  cations and an anions
• they only have empirical formulas, no molecular
  formula
• the sum of the charges on the cation(s) and
  anion(s) in each formula unit must equal zero

The ionic compound NaCl
2.6
32
Formula of Ionic Compounds
Al2O3
Al3
O2-
CaBr2
Ca2
Br-
Na2CO3
Na
CO32-
2.6
33
Some Polyatomic Ions (Table 2.3)
2.7
34
This is an ionic compound.
This is the ammonium cation. How can you tell if
a compound is molecular or ionic?
35
Which of these are ionic and which are
molecular? SiCl4, LiF, BaCl2, B2H6, KCl, C2H4,
NH4NO3
36
Chemical Nomenclature
37
Chemical Nomenclature

• Ionic Compounds (salts)
• often a metal nonmetal (or polyatomic ion)
• anion (nonmetal), add ide to element name

BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate
2.7
38

• Transition metal ionic compounds
• indicate charge on metal with Roman numerals

iron(II) chloride
FeCl2
2 Cl- -2 so Fe is 2
FeCl3
3 Cl- -3 so Fe is 3
iron(III) chloride
Cr2S3
3 S-2 -6 so Cr is 3 (6/2)
chromium(III) sulfide
2.7
39
TA p45
40
Name these LiF, MgCl2, NaOH, Al2(SO4)3,
NH4NO3, FeO
Write formulas for potassium carbonate,
ammonium sulfide cobalt(II) bromide, aluminum
phosphate
41

• Molecular compounds (binary)
• nonmetals or nonmetals metalloids
• common names
• H2O, NH3, CH4
• element further left in periodic table is 1st
• element closest to bottom of group is 1st
• if more than one compound can be formed from the
  same elements, use prefixes to indicate number of
  each kind of atom
• last element ends in ide

2.7
42
Molecular Compounds
HI
hydrogen iodide
NF3
nitrogen trifluoride
SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
2.7
43
(No Transcript)
44
Practice exercises 2.6 and 2.7Name NBr3 and
Cl2O7Write formulas forsulfur
tetrafluoridedinitrogen pentoxide
45
2.7