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Chapter 2
In 1808 an English scientist John Dalton, formulated a precise
definition of the of atoms:
2.2 The Structure of the Atom
After Dalton a series of investigations led to the discovery of three smaller
particles: electrons, protons and neutrons.
Atomic components:
All atoms can be identified by the number of protons and neutrons they
contain. The atomic number (Z) is the number of protons in the nucleus of each
atom of an element. In a neutral atom the number of protons is equal to the
number of electrons, so the atomic number also indicates the number of
electrons present in the atom.
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
Z
Atomic Number
1
1H
7
235
92
2
1
U
X
Element Symbol
H (D)
238
92
3
1
U
H (T)
The Isotopes of Hydrogen
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How many protons, neutrons, and electrons are in 24Na
11
?
11 protons, 13 (24 - 11) neutrons, 11 electrons
23
How many protons, neutrons, and electrons are in Na ?
11
11 protons, 12 (23-11) neutrons, 11 electrons
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Ions
When a neutral atom accept or lose one electron or more it is
named an ion. If it lose electrons it is called Cation ( ion with a –
ve charge. And named Anion if it accept electrons.
Examples page 36 – 40
Arabic book
2.4 The Periodic Table
The periodic table, a chart in which elements having similar chemical and
physical properties are grouped together. Figure 2.10 shows the modern
periodic table in which the elements are arranged by atomic number (shown
above the element symbol) in horizontal rows called periods and in vertical
columns known as groups or families, according to similarities in their
chemical properties.
The elements can be divided into three categories—metals, nonmetals, and
metalloids. A metal is a good conductor of heat and electricity
while a nonmetal is usually a poor conductor of heat and electricity.
A metalloid has properties that are intermediate between those
of metals and nonmetals. Figure 2.10 shows that the majority of known
elements are metals; only 17 elements are nonmetals, and 8 elements are
metalloids. From left to right across any period, the physical and chemical
properties of the elements
change gradually from metallic to nonmetallic.
The periodic table correlates the properties of the elements in a systematic way and
helps us to make predictions about chemical behavior. We will take a closer look at this
keystone of chemistry in Chapter 8.
The Modern Periodic Table
Noble Gas
Halogen
Group
Alkali Earth Metal
Alkali Metal
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Period
2.5 Molecules and Ions
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, Cl2, HCl, CO
diatomic elements
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
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
15
17 protons
17 electrons
Cl-
17 protons
18 electrons
A monatomic ion contains only one atom
K+, F-, Mg2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
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How many protons and electrons are in
14
7
3-
N?
7 protons, 10 (17+3) electrons
56 2+
How many protons and electrons are in
Fe ?
26
26 protons, 24 (26-2) electrons
17
2.6 Chemical Formulas
Chemists use chemical formulas to express the composition of molecules and
ionic compounds in terms of chemical symbols. By composition we mean not
only the elements present but also the ratios in which the atoms are combined.
Here we are concerned with two types of formulas: 1- molecular formulas and
2- empirical formulas.
Molecular Formulas
A molecular formula shows the exact number of atoms of each element in the
smallest unit of a substance.
Thus, H2 is the molecular formula for hydrogen,
O2 is oxygen, O3 is ozone, and H2O is water.
Molecular Models:
ball-and-stick models and space-filling models ( Figure 2.12)
The structural formula:
which shows how atoms are bonded to one another in a molecule.
Formulas and Models
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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
H2O
C6H12O6
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empirical
H2O
CH2O
O3
O
N2H4
NH2
ionic compounds consist of a combination of cations and an anions
The formula is usually 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
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The most reactive metals (green) and the most reactive nonmetals (blue) combine to form ionic
compounds.
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Formula of Ionic Compounds
2 x 3+ = 6+
3 x 2- = 6Al2O3
Al3+
O2-
1 x 2+ = 2+
2 x 1- = 2MgCl2
Ca2+
Br-
1 x 2+ = 2+
1 x 2- = 2Na2CO3
Na+
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CO32-
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
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An oxoacid is an acid that contains hydrogen, oxygen,
and another element.
HNO3
H2CO3
H3PO4
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nitric acid
carbonic acid
phosphoric acid
A base can be defined as a substance that yields
hydroxide ions (OH-) when dissolved in water.
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NaOH
sodium hydroxide
KOH
potassium hydroxide
Ba(OH)2
barium hydroxide