Introductory Chemistry, 2nd Edition
Nivaldo Tro
Chapter 4
Atoms and
Elements
Experiencing Atoms
Atoms are very small; they are building
blocks of matter.
Atom = smallest particle of an element that
retains its characteristics
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Dalton’s Atomic Theory
Elements are composed of tiny
indivisible particles called
atoms
2. All atoms of an element are
identical; atoms of different
elements are different
1.
–
every carbon atom is identical to
every other carbon atom
they have the same chemical and
physical properties
–
but carbon atoms are different
from sulfur atoms
John Dalton
(1766-1844)
they have different chemical and
physical properties
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Dalton’s Atomic Theory
3. Atoms combine in simple, whole-
number ratios to form molecules of
compounds
– each molecule of a compound contains
the exact same types and numbers of
atoms
Law of Constant Composition or Definite
Proportions
Chemical Formulas
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Dalton’s Atomic Theory
4. In chemical reactions, atoms are not
broken or changed into other atoms.
– atoms are not created or destroyed, just
rearranged
total mass will remain the same
Law of Conservation of Mass
– atoms of one element do not change into
atoms of another element in a chemical
reaction
cannot turn Lead into Gold by a chemical
reaction
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Dalton’s Atomic Theory
Law of multiple proportions : if two elements A
and B combine to form more than one
compound, masses of B which combine with
mass of A, is a ratio of small whole numbers.
Ex: CO, (1.33 g O/1g C); CO2 (2.67 g O/g)
The mass of O in CO2 is twice that in CO: 2.67g
O in CO2/1.33 g O in CO2 = 2.00
Thus, when 2 elements combine to form 2
different compounds, they combine in different
ratios or proportions:
CO, CO2 ; SO2, SO3
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Modern Evidence for Atoms
Atoms can be seen through a special instrument
called the scanning tunneling microscope (STM)
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Mass of Atoms
Using compositions of compounds
and assumed formulas, Dalton
determined relative masses of atoms
Dalton based his scale on H = 1 amu
we now base it on C-12 = 12 amu
exactly
unit = atomic mass unit, amu
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The Atom is Divisible!
Work done by J.J. Thomson and others
proved that the atom had particles called
electrons
Thomson found that electrons are much
smaller than atoms and carry a negative
charge
The mass of the electron is 1/1836th the mass
of a hydrogen atom
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Rutherford’s Experiment
Alpha Particles
Striking Screen
Radioactive
Sample
Lead Box
Gold
Foil
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Fluorescent
Screen
10
Conclusions from Rutherford’s
Experiment
Atom mostly empty space
– because most particles went through
Atom contains a dense particle that was small
in volume compared to the atom but large in
mass
– because of the few particles that bounced back
This dense particle was positively charged
– because of the large deflections of some of the
particles
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Rutherford’s Interpretation –
the Nuclear Model
1) Atom contains a tiny dense center called the
nucleus
2) The nucleus has essentially the entire mass
of the atom
– the electrons weigh so little they give practically
no mass to the atom
3) The nucleus is positively charged
– the amount of positive charge balances the
negative charge of the electrons
4) The electrons move around in the empty
space of the atom surrounding the nucleus
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The Modern Atom
Atoms are composed
of three particles protons, neutrons and
electrons
The nucleus contains
protons and neutrons
The electrons move
outside the nucleus
Neutral atom has same
# protons and
electrons.
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The Nuclear Atom:
protons
Nucleus
neutrons
+
-
+
-
electrons
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Subatomic Particles:
Charge? Location?
neutrons (n)
protons (p)
electrons (e)
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Subatomic Particles:
Charge?
Location?
neutron
0
nucleus
proton
+1
nucleus
electron
-1
outside
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Subatomic Particles:
Mass (amu)
neutron
1
proton
1
electron
0
(1/1837)
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Elements
Each element has a unique number of
protons in its nucleus
Number of protons in the nucleus of an atom
is called the atomic number
– the elements are arranged on the Periodic
Table in order of their atomic numbers
Each element has a unique name and
symbol
– symbol either one or two letters
one capital letter or one capital letter + one
lower case
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How many?
protons = depends on element
neutrons = variable for each
element
electrons = same as protons
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atomic number The number of protons in one atom;
each element has a different atomic
number
The number on top of the element
symbol in the periodic table
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The Periodic Table of Elements
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Review
What is the atomic number of boron, B?
What is the atomic mass of silicon, Si?
How many protons does a chlorine atom have?
How many electrons does a neutral neon atom
have?
Will an atom with 6 protons, 6 neutrons and 6
electrons be electrically neutral?
Will an atom with 27 protons, 32 neutrons and
27 electrons be electrically neutral?
Will a Na atom with 10 electrons be electrically
neutral?
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Review
What is the atomic number of boron, B? 5
What is the atomic mass of silicon, Si? 28.09
amu
How many protons does a chlorine atom have?
17
How many electrons does a neutral neon atom
have? 10
Will an atom with 6 protons, 6 neutrons and 6
electrons be electrically neutral? Yes
Will an atom with 27 protons, 32 neutrons and
27 electrons be electrically neutral? Yes
Will a Na atom with 10 electrons be electrically
neutral? No
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Three Types of Elements
= Metal
= Metalloid
= Nonmetal
Metals
Solids at room temperature,
except Hg
Shiny
Conduct heat, electricity
Malleable: can be shaped
Ductile = drawn or pulled into
wires
Lose electrons and form cations
(+ charge) in reactions
About 75% of the elements are
metals
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Nonmetals
Found in all 3 states
Poor conductors of heat &
electricity
Solids are brittle
Gain electrons in reactions
to become anions (- charge)
Only a few on the right of
periodic table
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Metalloids
Show some
properties of
metals and some
of nonmetals
Also known as
semiconductors
Properties of Silicon
shiny
conducts electricity
does not conduct heat well
brittle
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The Modern Periodic Table
Elements with similar chemical and
physical properties are in same column
Columns are called Groups or Families
– designated by a number and letter at top
Rows are called Periods
Each period shows the pattern of
properties repeated in the next period
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The Modern Periodic Table
Main Group = Representative Elements
= ‘A’ groups
Transition Elements = ‘B’ groups
– all metals
Bottom rows = Inner Transition
Elements = Rare Earth Elements
– metals
– really belong in Period 6 & 7
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= Alkali Metals
= Halogens
= Alkali Earth Metals
= Lanthanides
= Noble Gases
= Actinides
= Transition Metals
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Important Groups - Hydrogen
nonmetal
colorless, diatomic gas
– very low melting point & density
reacts with nonmetals to form molecular
compounds
– HCl is acidic gas
– H2O is a liquid
reacts with metals to form hydrides
– metal hydrides react with water to form H2
HX dissolves in water to form acids
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Important Groups – IA, Alkali Metals
hydrogen usually placed
here, though it doesn’t
belong
lithium
soft, low melting points,low
density
sodium
very reactive, never find
uncombined in nature
potassium
tend to form water soluble
compounds
rubidium
react with water to form
basic (alkaline) solutions
cesium
and H2 releasing a lot of
heat
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Important Groups – IIA, Alkali Earth Metals
harder, higher melting,
and denser than alkali
metals
reactive, but less than
corresponding alkali
metal
form stable, insoluble
oxides from which they
are normally extracted
oxides are basic =
alkaline earth
beryllium
magnesium
calcium
strontium
barium
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Important Groups – VIIA, Halogens
nonmetals
F2 & Cl2 gases; Br2
liquid; I2 solid
fluorine
all diatomic
chlorine
very reactive
react with metals to form bromine
ionic compounds
iodine
HX all strong acids
except HF
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Important Groups – VIIIA, Noble Gases
all gases at room
temperature,
– very low melting and
boiling points
very unreactive,
practically inert
very hard to remove
electron from or give
an electron to
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Charged Atoms = Ions
Number of protons (atomic #) identifies the
element!
– all sodium atoms have 11 protons in the
nucleus
In chemical change, number of protons in
nucleus of atom doesn’t change!
Atoms can lose or gain electrons becoming
electrically charged, these are called ions
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Ions
Atoms acquire a charge by gaining or
losing electrons
– not protons!!
Ion Charge = # protons – # electrons
ions with a + charge are called cations
– more protons than electrons
– form by losing electrons
ions with a – charge are called anions
– more electrons than protons
– form by gaining electrons
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Atomic Structures of Ions
Metals form cations
For each positive charge the ion has 1 less
electron than the neutral atom
– Na atom = 11 p+ and 11 e-, Na+ ion = 11 p+ and 10 e– Ca atom = 20 p+ and 20 e-, Ca2+ ion = 20 p+ and 18 e-
Cations are named the same as the metal
sodium
Na  Na+ + 1esodium ion
calcium Ca  Ca2+ + 2ecalcium ion
The charge on a cation can be determined from
the Group number on the Periodic Table
– Group 1A  +1, Group 2A  +2, (Al, Ga, In)  +3
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Atomic Structures of Ions
Nonmetals form anions
For each negative charge the ion has 1 more
electron than the neutral atom
– F = 9 e-, F- = 10 e– P = 15 e-, P3- = 18 e-
Anions are named by changing the ending of
the name to -ide
fluorine
oxygen
F + 1e-  F- fluoride ion
O + 2e-  O-2 oxide ion
Charge on an anion can be determined by
subtracting 8 from the Group number on the
Periodic Table
– Group 7A  7- 8 = -1; Group 6A  6 – 8 = -2
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Example 4.5:
Determining the Number of
Protons and Electrons in an Ion
Example:
Find the number of protons and electrons in the Ca2+
ion.
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Write down the given quantity and its units.
 Given: Ca2+
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Information
Given:
Ca2+
Write down the quantity to find and/or its units.
 Find:
#p and #e
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Design a Solution Map.
ion symbol
Information
Given:
Ca2+
Find: #p and #e
#p and #e
#e
Ion charge = #p - #e
element
atomic number
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Information
Given: Ca2+
Find: #p and #e
Solution Map: ion symbol 
element  atomic number  #p
 #e
Apply the Solution Map.
– Determine the Name of the Element
Ca = calcium
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Information
Given: Ca2+ = calcium
Find: #p and #e
Solution Map: ion symbol 
element  atomic number  #p
 #e
Apply the Solution Map.
– Determine the Atomic Number of the Element
Ca = calcium
Atomic Number = 20
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Information
Given: Ca2+ = calcium, Z = 20
Find: #p and #e
Solution Map: ion symbol 
element  atomic number  #p
 #e
Apply the Solution Map.
– Determine the Number of Protons in an Atom of the
Element
Ca = calcium
Atomic Number = 20
#p = atomic number = 20
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Example:
Find the number of protons
and electrons in the Ca2+ ion
Information
Given: Ca2+ = calcium, Z = 20
Find: #p = 20 and #e
Solution Map: ion symbol 
element  atomic number  #p
 #e
Apply the Solution Map.
– Determine the Number of Electrons in an Ion of the
Element
Ca = calcium
Atomic Number = 20
#p = atomic number = 20
Ion Charge = #p - #e
+2 = 20 - #e
-18 = - #e
18 = #e
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Atomic Structures of Ions
Ion
+
p
-
e
-1
Cl
+1
K
S
-2
Sr
+2
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Atomic Structures of Ions
Ion
-1
Cl
K
+1
-2
S
Sr
+2
+
-
p
e
17
18
19
18
16
18
38
36
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Ion Charge & the Periodic Table
Charge on an ion can often be
determined from an element’s position
on the Periodic Table
Metals are always positive ions,
nonmetals are negative ions
For many main group metals, the
charge = the group number
For nonmetals, the charge = the group
number - 8
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Charges of Main Group Ions
IA
IIA
IIIA
Li+1 Be+2
VIA
VA
VIIA
N-3 O-2
F-1
Cl-1
Na+1 Mg+2
Al+3
P-3 S-2
K+1 Ca+2
Ga+3
As-3 Se-2 Br-1
Rb+1 Sr+2
In+3
Te-2 I-1
Cs+1 Ba+2
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Isotopes
Isotopes = atoms of an element that have
same # of protons but different # of neutrons
Isotopes of an element have different masses
isotopes are identified by their
– mass numbers = protons + neutrons
All isotopes of an element are chemically
identical
– undergo the exact same chemical
reactions
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Isotopes
• Atomic Number
 Number of protons
 Z (symbol)
• Mass Number
 Protons + Neutrons
 Whole number
 A (symbol)
 Abundance = relative
amount found in a sample
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Neon
Number
Number
of
of Protons Neutrons
Symbol
Percent
A, Mass Natural
Number Abundance
Ne-20
20
10
Ne
10
10
20
90.48%
Ne-21
21
10
Ne
10
11
21
0.27%
Ne-22
22
10
Ne
10
12
22
9.25%
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Isotope Symbols
• Cl-35 makes up about 75% of chlorine
atoms in nature, and Cl-37 makes up the
remaining 25%
• Average atomic mass of Cl = 35.45 amu
• Cl-35 has a mass number = 35, 17 protons
and 18 neutrons (35 - 17)
35
17
Cl
X = Atomic Symbol
A = mass number
Z = atomic number
or Cl-35
AX
Z
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Example 4.8:
Determining the Number of
Protons and Neutrons from
Isotope Symbols
Example:
52
How many protons and neutrons in the chromium 24 Cr
isotope
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Write down the given quantity and its units.
 Given: 52
24 Cr
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Example:
How many protons and
neutrons in the chromium
isotope 52
Information
Given: 52
24 Cr
24 Cr
Write down the quantity to find and/or its units.
 Find:
#p and #n
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Design a Solution Map.
ion symbol
Information
Given: 52
24 Cr
Find: #p and #n
#p
#n
Mass # = #p + #n
element
atomic number
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Information
Given: 52 Cr
24
Find: #p and #n
Solution Map: ion symbol 
element  atomic number  #p
 #n
Apply the Solution Map.
– Determine the Name of the Element
Cr = chromium
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Information
Given: 52
24 Cr = calcium
Find: #p and #n
Solution Map: ion symbol 
element  atomic number  #p
 #n
Apply the Solution Map.
– Determine the Atomic Number of the Element
Cr = chromium
Atomic Number = 24
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Information
Given: 52
24 Cr = calcium
Find: #p and #n
Solution Map: ion symbol 
element  atomic number  #p
 #n
Apply the Solution Map.
– Determine the Number of Protons in an Atom of the
Element
Cr = chromium
Atomic Number = 24
#p = atomic number = 24
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Example:
How many protons and
neutrons in the chromium
isotope 52
24 Cr
Information
Given: 52
24 Cr = calcium
Find: #p and #n
Solution Map: ion symbol 
element  atomic number  #p
 #n
Apply the Solution Map.
– Determine the Number of Neutrons in an Atom of the
Element
Cr = chromium
Atomic Number = 24
#p = atomic number = 24
Mass Number = #p + #n
52 = 24 + #n
28 = #n
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Practice - Complete the following table
Atomic
Mass Number Number
Number Number
of
of
Protons Electrons
Number
of
Neutrons
Calcium-40
Carbon-13
Aluminum-27
+3
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Practice - Complete the following table
Atomic
Mass Number Number Number
Number Number
of
of
of
Protons Electrons Neutrons
20
40
20
20
20
Calcium-40
Carbon-13
Aluminum-27
+3
6
13
6
6
7
13
27
13
10
14
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Mass Number is Not the Same
as Atomic Mass
The atomic mass is an experimental
number determined from all naturally
occurring isotopes
The mass number refers to the number
of protons + neutrons in one isotope
– natural or man-made
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