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Atoms, Molecules, and Ions
Chapter 2
1
16 X
+
8Y
8 X2Y
2
Dalton’s Atomic Theory (read pages
47-50)
2
3
The Structure of the Atom
• Dalton’s atomic theory
- atom, the basic unit of element
• However, atom has subatomic particles
-Electrons, neutrons, protons
Electrons
-negative charge
-negligible mass
4
The Structure of the Atom (cont.)
• Atom is neutral
• Nucleus-a dense central core of the atom
Protons
-positive charge
- mass is 1840 times the mass of electron
Neutrons
-electrically neutral
- Mass almost the same as proton
5
Rutherford’s Experiment
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
6
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
“If the atom is the Houston Astrodome, then
the nucleus is a marble on the 50-yard line.”
• Surrounding the nucleus is a diffuse
region of negative charge populated by:
- electrons - negatively charged
particles
- mass p ≈ mass n ≈ 1840 x mass e-
Selected Properties of the Subatomic Particles
Name
Charge
Mass(amu) Mass (grams)
Electrons (e) -1
5.4 x 10-4
9.1095 x 10-28
Protons (p)
+1
1.00
1.6725 X 10-24
Neutrons (n) 0
1.00
1.6750 x 10-24
8
mass p = mass n = 1840 x mass e9
Radioactivity is spontaneous emission of
particles and /or radiation
10
Atomic Number, Mass Number, and
Isotopes
• Atomic number (Z) = number of protons in a
neutral atom
#protons = # of electrons (in a neutral atom)
• Mass number (A) = number of protons (Z) +
number of neutrons
Number of neutrons = A-Z
11
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
Isotopes are atoms of the same element (X) with different
numbers of neutrons in their nuclei
Mass Number
A
ZX
Atomic Number
1
1H
2
1H
Element Symbol
(D)
3
1H
(T),
one proton in all
0, 1, and 2 neutrons in the isotopes
235
92
U
238
92
U
Known as U-235 and U-238
• Isotopes - atoms of the same element
having different masses.
– contain same number of protons
– contain different numbers of neutrons
Isotopes of Hydrogen
Hydrogen
(Hydrogen-1)
Deuterium
(Hydrogen-2)
Tritium
(Hydrogen-3)
13
Problems
1. Write nuclear symbols for three isotopes of oxygen (atomic
no. = 8) in which there are 8, 9, and 10 neutrons,
respectively
2. One of the most harmful species in nuclear fallout is
radioactive isotope of strontium, has a mass number of 90
and atomic number of 38. Write the nuclear symbol for Sr.
How many protons and neutrons in the nucleus? How many
electrons?
3. How many protons, neutrons, and electrons in C-14
4. Write the nuclear symbol for an atom that contains 32
protons and 38 neutrons (Use the Periodic Table to find the
symbol of the element)
14
p76
16
Noble Gas
Halogen
Group
Alkali Metal
Alkali Earth Metal
Period
Table 2-2 p58
Molecules
• A molecule is an aggregate of two or more atoms
in a definite arrangement held together by
chemical forces (also called chemical bonds)
• Atoms of the same element or different elements
in a fixed ratio. When it is of the same element, it
is a pure element, for e.g.., 2 H atoms make one
molecule of H2. Only the six noble gases exist as
monoatomic in nature. Others diatomic,
polyatomic or ionic in nature.
18
19
Ions 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
11 electrons
Na+
11 protons
10 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
20
A monoatomic ion contains only one
atom:
Na+, Cl-, Ca2+, O2-, Al3+, N3↑
Atoms with 1,2, and 3 electrons lost
or gained.
OH-, CN-, NH4+, NO3A polyatomic ion contains more than one atom:
(a) What does S8 and 8S signify?
(b) Protons and electrons in (a) P3- and (b) Ti4+
21
Figure 2-21 p65
Table 2-5 p65
Formulas and Models
24
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.
molecular
empirical
H2O
H2O
C6H12O6
CH2O
O3
O
N2H4
NH2
25
Example
Write the molecular formula of methanol.
There are four H atoms, one C atom, and one O atom.
Therefore, the molecular formula is CH4O.
However, the standard way of writing the molecular formula for
methanol is CH3OH because it shows how the atoms are joined
in the molecule.
26
2.3
Write the empirical formulas for the following molecules:
(a) acetylene (C2H2), which is used in welding torches
(b) glucose (C6H12O6), a substance known as blood sugar
(c) nitrous oxide (N2O), a gas that is used as an anesthetic gas
(“laughing gas”) and as an aerosol propellant for whipped
creams.
27
2.3
Solution
(a) There are two carbon atoms and two hydrogen atoms in
acetylene. Dividing the subscripts by 2, we obtain the empirical
formula CH.
(b) In glucose there are 6 carbon atoms, 12 hydrogen atoms, and 6
oxygen atoms. Dividing the subscripts by 6, we obtain the
empirical formula CH2O. Note that if we had divided the
subscripts by 3, we would have obtained the formula C2H4O2.
Although the ratio of carbon to hydrogen to oxygen atoms in
C2H4O2 is the same as that in C6H12O6 (1:2:1), C2H4O2 is not the
simplest formula because its subscripts are not in the smallest
whole-number ratio.
28
2.3
(c) Because the subscripts in N2O are already the smallest
possible whole numbers, the empirical formula for nitrous
oxide is the same as its molecular formula.
29
ionic compounds consist of a combination of cations
and an anions
• the formula is always the same as the empirical formula
• the sum of the charges on the cation(s) and anion(s) in each
formula unit must equal zero
The ionic compound NaCl
30
31
Formulas of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
2 x +1 = +2
Na+
O22 x -1 = -2
CaBr2
Br1 x -2 = -2
Na2CO3
CO3232
Chemical Substances; Formulas and
Names
• Ionic compounds
Most ionic compounds contain metal and nonmetal atoms; for example, NaCl.
You name an ionic compound by giving the name of the cation followed by the
name of the anion.
A monatomic ion is an ion formed from a single atom.
Chemical Substances; Formulas and
Names
• Rules for predicting charges on monatomic
ions
Most of the main group metals form cations with the charge
equal to their group number.
The charge on a monatomic anion for a nonmetal equals the
group number minus 8.
Most transition elements form more than one ion, each
with a different charge.
Chemical Substances; Formulas and
Names
• Rules for naming monatomic ions
Monatomic cations are named after the element – if there is only
one such ion. For example, Al3+ is called the aluminum ion.
If there is more than one cation of an element, a Roman numeral
in parentheses denoting the charge on the ion is used. This often
occurs with transition elements. i.e. Fe2+ = Iron(II); Fe3+ = Iron(III)
The names of the monatomic anions use the stem name of the
element followed by the suffix – ide. For example, Br- is called
the bromide ion.
Chemical Substances; Formulas and
Names
• Polyatomic ions
A polyatomic ion is an ion consisting of two or more atoms chemically bonded
together and carrying a net electric charge.

NO3 nitrate

NO2 nitrite
2
SO4 sulfate
OH- hydroxide
2
SO 3 sulfite
Chemical Nomenclature
• Ionic Compounds
– Often a metal + nonmetal
– Anion (nonmetal), add “-ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate
37
• Transition metal ionic compounds
– indicate charge on metal with Roman numerals
FeCl2
2 Cl- -2 so Fe is +2
iron(II) chloride
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
38
39
40
• Molecular compounds
• nonmetals or nonmetals + metalloids
• common names
• H2O, NH3, CH4, C60
• 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
Molecular Compounds
HI
hydrogen iodide
NF3
nitrogen trifluoride
SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
42
2.7
Name the following molecular compounds:
(a) SiCl4
(b) P4O10
43
2.7
Strategy
We refer to Table 2.4 for prefixes.
In (a) there is only one Si atom so we do not use the prefix
“mono.”
Solution
(a) Because there are four chlorine atoms present, the
compound is silicon tetrachloride.
(b) There are four phosphorus atoms and ten oxygen atoms
present, so the compound is tetraphosphorus decoxide.
Note that the “a” is omitted in “deca.”
44
An acid can be defined as a substance that yields
hydrogen ions (H+) when dissolved in water.
For example: HCl gas and HCl in water
•Pure substance, hydrogen chloride
•Dissolved in water (H3O+ and Cl−),
hydrochloric acid
45
An oxoacid is an acid that contains hydrogen,
oxygen, and another element.
HNO3
nitric acid
H2CO3
carbonic acid
H3PO4
phosphoric acid
46
Table 2-7 p70
Table 2-8 p70
Figure 2-24 p70
50
51
A base can be defined as a substance that yields
hydroxide ions (OH-) when dissolved in water.
NaOH
sodium hydroxide
KOH
potassium hydroxide
Ba(OH)2
barium hydroxide
52
Hydrates are compounds that have a specific
number of water molecules attached to them.
BaCl2•2H2O
barium chloride dihydrate
LiCl•H2O
lithium chloride monohydrate
MgSO4•7H2O
magnesium sulfate heptahydrate
Sr(NO3)2 •4H2O
strontium nitrate tetrahydrate
CuSO4•5H2O
CuSO4
53
54
Homework Problems, Ch.2
11,12,13,14,15,21, 27, 31,
49,50,51,52,53,55,56,57,58,64,65,71,72,
73,74,82,2.84,85,86
55
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