Download Atoms, Molecules and Ions

Atoms, Molecules and Ions
Chapter 2
The Modern Atomic Theory
Modern Atomic theory has four assumptions:
1. Atoms make up all matter.
2. The atoms of one element are different from the
atoms of another element.
3. Atoms combine in definite ratios to make
compounds.
4 Combinations of atoms in compounds can change
4.
only when a chemical reaction happens. This
means reactions alter atom combinations, but the
identity of the atoms themselves remain the same.
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Structure of Atoms
• Atoms are made up of three main particles,
neutron, electron, and proton.
Particle
Symbol
Charge
Mass
electron
e-
-1
0.0005486 amu
proton
p+
+1
1.007276 amu
neutron
no
0
1.008665 amu
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Atomic number, Z:
The identity of an element is controlled by the number
of protons in the nucleus.
In the neutral atom: number of protons inside the
nucleus is the same number of electrons around the
nucleus.
Atomic number = # of Protons = # of Electrons
Every element has its own unique atomic number.
Example What is the atomic number for nitrogen, N?
Nitrogen is in the seventh position in the periodic table.
This means nitrogen atoms have 7 protons in the nucleus, 7 electrons
around the nucleus, and they have an atomic number of 7.
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• Mass number, A:
It is equal to the sum of neutrons and protons inside the
nucleus, because the "massive" particles in the atom are
protons and neutrons.
M
Mass
number
b = # off Protons
P t
+ # off neutrons
t
# of neutrons = Mass number - # of Protons
# of neutrons = A – Z
23
How many neutrons, electrons and protons are in an atom of Na?
Sodium, Na, has atomic number 11.
# of Protons =11 # of Electrons = 11
N b off neutrons
Number
t
=A-Z
Number of neutrons = 23 - 11 = 12
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An atom with a mass number of 39 contains 20 neutrons. What is
the atomic number and identity of the element?
The atomic number is Z = 39- 20 =19.
The identity is potassium because K is element 19
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Isotopes
All the atoms of the same element have the same atomic
number, but they can have different:
numbers of neutrons and mass numbers
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3
Atomic Mass
The atomic mass of an element is the mass average of the atomic masses of the
different isotopes of an element.
For example, naturally occurring carbon, for example, is a mixture of two
isotopes, 12C (98.89%) and 13C (1.11 %).
Individual carbon atoms therefore have a mass of either 12.000 or 13.03354 amu.
But the average mass of the different isotopes of carbon is 12.011 amu.
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Isotope abundances
The isotopes of an element do not occur with equal frequency.
The relative abundance depends on the relative stability of the isotope.
The isotopes contribute to the average atomic mass based on their
abundance.
The atomic weights in the periodic table are weighted averages.
This means the tabulated value doesn't
doesn t match any actual atom
atom, but is closer to
the most common isotope.
Average weight = % First isotope abundance x its mass +
% Second isotope abundance x its mass
What is the average atomic mass for thallium, Tl, if there are two isotopes with the
following masses and abundances? (Tl-203 (203Tl) has a mass of 203.059 amu
with
ith an abundance
b d
off 29.52
29 52 %,
% Tl
Tl-205
205 (205Tl) h
has a mass off 205.059
205 059 amu with
ith an
abundance of 70.48 %)
Step 1: Convert percents to decimals 29.52 % to 0.2952 and 70.48 % to 0.7048
Step 2: Average weight = 0.2952 x ( 203.059 amu) + 0.7048 x ( 205.059 amu)
204.466 amu rounded off to 204.5 amu with 4 significant.
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The Periodic Table
• The modern periodic table (PT) represents the
arrangements of elements according to the building up
(Aufbau principle), where each element has one
electron more than the previous element.
element
• The periodic table can be classified in many different
ways:
 Metallic character:
metals, nonmetals, and metalloids.
 Position in the periodic table:
horizontal rows (Periods)
vertical columns (Groups)
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The periodic table contains the following
important groups:
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5
A molecule is an aggregate of two or more atoms in a
definite arrangement held together by chemical forces
H2
H2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl,
HCl CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
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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
i b
it
becomes a cation.
i
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative
g
charge
g
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
12
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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-
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Do You Understand Ions?
H
How
many protons
t
and
d electrons
l t
are iin
27 3+
13 Al ?
13 protons, 10 (13 – 3) electrons
How many protons and electrons are in
78
234 Se ?
34 protons, 36 (34 + 2) electrons
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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
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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
g on the cation(s)
( ) and anion(s)
( ) in each
formula unit must equal zero
The ionic compound NaCl
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Formula 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
CO3218
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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
19
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