Atomic
Structure
© 2013 Marshall Cavendish International (Singapore) Private Limited
Atomic Structure
1
Inside Atoms
2
The Proton Number and Nucleon Number
3
Isotopes
4
Arrangement of Electrons in Atoms
2
1
Inside Atoms
Learning Outcomes
At the end you should be able to:
• state the relative charges and masses of a proton,
a neutron and an electron;
• describe the structure of an atom.
3
1
Inside Atoms
What are Atoms Made up of?
Atoms are made up of 3 sub-atomic particles:
• protons;
• neutrons;
• electrons.
4
1
Inside Atoms
Where are these Sub-Atomic Particles Found?
Nucleus
• protons (+) Positive Charge
• neutrons (neutral) No Charge
Shells around the nucleus
• electrons (–) Negative Charge
5
1
Inside Atoms
Proton
• Represented by the letter, p
• Found in the nucleus
• Relative mass = 1
• Relative charge = +1
p
6
5.1
Inside Atoms
Neutron
• Represented by the letter, n
• Found in the nucleus
• Relative mass = 1
• Relative charge = 0
n
7
1
Inside Atoms
Electron
• Represented by the letter, e
• Found in shells surrounding
nucleus
1
• Relative mass =
1840
• Relative charge = –1
e
8
1
Inside Atoms
Summary of Sub-Atomic Particles
Particle
Symbol
proton
p
1
+1
neutron
n
1
0
e
1
1840
–1
electron
Relative mass
Relative charge
9
Atomic Structure
1
Inside Atoms
2
The Atomic Number and Nucleon
Number
3
Isotopes
4
Arrangement of Electrons in Atoms
10
2
The Atomic Number and Nucleon
Number
Learning Outcomes
At the end of this section, you should be able to:
• Define atomic number and nucleon (mass) number;
• deduce the number of protons, neutrons and
electrons in an atom;
• interpret and use symbols that represent an
element’s nucleon and proton numbers (ZA X ).
11
2
The Proton Number and Nucleon
Number
Atomic Number (Z)
The number of protons in an atom is called its atomic
number.
Since an atom is electrically neutral (i.e. has no overall
charge),
Atomic number = number of electrons
12
2
The Atomic Number and Nucleon
Number
Atomic Numbers
Each element has a unique atomic number.
Atoms of different elements have different atomic
numbers.
Example
Atomic number of carbon = 6
Any atom with 6 protons must be
a carbon atom.
nucleus of a carbon
atom
13
5.2 The Atomic Number and Nucleon
Number
Where is the atomic number of an element
found in the Periodic Table?
11
Na
Sodium
23
Mass
Number
(Average
atomic mass)
14
2
The Atomic Number and Nucleon
Number
Exercise
1. Find the atomic number of the atoms of the
following elements:
(a) Nitrogen
7
(b) Helium
2
(c) Sulfur
16
2. Which element has an atomic number of
(a) 17?
Chlorine
(b) 3?
Lithium
(c) 20?
Calcium
15
2
The Atomic Number and Mass
Number
Mass Number(A)
The total number of protons and number of neutrons
in an atom is called the mass number.
The mass of an atom depends on the number of
protons and neutrons.
Mass number (A) = number of protons + number of neutrons
16
Find the number of neutrons
To find the number of neutrons we only need to use our
periodic tables.
Mass number – Atomic number = Number of neutrons
17
Atomic Structure
1
Inside Atoms
2
The Proton Number and Nucleon Number
3
Isotopes
4
Arrangement of Electrons in Atoms
18
5.3
Isotopes
Learning Outcome
At the end of this section, you should be able to:
• define isotopes.
19
5.3
Isotopes
Isotopes of Hydrogen
These are 3 atoms of hydrogen.
Hydrogen-1
Hydrogen-2
Hydrogen-3
What are the similarities and differences of these 3 atoms?
20
5.3
Isotopes
What are Isotopes?
Isotopes are atoms of the same element with the
same number of protons but different numbers of
neutrons.
Example 1
35
17
Cl
37
17
Cl
Chlorine gas consists of
75% chlorine-35, 25%
chlorine-37.
21
5.3
Isotopes
What are Isotopes?
Example 2
14
6
C
13
6
C
12
6
C
Carbon consists of 98.93%
carbon-12, 1.07% carbon-13,
and trace amounts of carbon14.
22
5.3
Isotopes
Properties of Isotopes
Isotopes have the same chemical properties but
slightly different physical properties.
Same chemical properties are due to
•
the same number of electrons;
•
only electrons involved in chemical reactions.
23
5.3
Isotopes
Properties of Isotopes
Different physical properties are due to
• different relative atomic masses;
• result in different densities, melting and boiling
points.
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5.3
Isotopes
Uses of Isotopes
Isotopes that emit high-energy radiation are called
radioisotopes.
They are radioactive substances. The radiation
emitted is dangerous because it can damage living
cells and cause cancer.
25
5.3
Isotopes
Uses of Isotopes
However, radioisotopes can have important applications
and can be safely used if they are handled properly.
For example, smoke detectors use a radioisotope.
Smoke entering the smoke detector absorbs the
radiation. This sets off an alarm in the smoke detector.
26
5.3
Isotopes
Protons, Neutrons, Electrons and Isotopes
Review
Uses of Isotopes
27
Chapter 5 Atomic Structure
5.1 Inside Atoms
5.2 The Proton Number and Nucleon Number
5.3 Isotopes
5.4 Arrangement of Electrons in Atoms
28
5.4 Arrangement of Electrons in Atoms
Learning Outcome
At the end of this section, you should be able to:
• use diagrams to describe atoms as containing:
– protons and neutrons in nucleus;
– electrons arranged in the electron shells
(energy levels).
29
5.4 Arrangement of Electrons in Atoms
Electronic Structure
Electrons move around the nucleus in regions
known as electron shells.
The 1st shell
• is closest to the nucleus;
• holds a maximum of 2 electrons;
• is always filled first;
• has the lowest energy level.
30
5.4 Arrangement of Electrons in Atoms
Electronic Structure
The 2nd shell
• can hold up to 8 electrons;
• has higher energy than 1st shell.
The 3rd shell
• can usually hold up to 8
electrons;
• is filled up after the 2nd shell.
31
5.4 Arrangement of Electrons in Atoms
Electronic Configuration/ Electronic Structure
Electronic configuration/structure is the arrangement of
electrons in an atom.
Magnesium atom (Z = 12)
1st shell: 2 electrons
2nd shell: 8 electrons
Nucleus
12p, 12n
3rd shell: 2 electrons
Magnesium atom
Electronic configuration = 2, 8, 2
32
5.4 Arrangement of Electrons in Atoms
Valence Shell and Valence Electrons
The valence shell or outer shell of an atom refers to the
shell that is furthest away from the nucleus of the atom.
Valence electrons are the electrons found in the valence
shell.
Valence shell/
outer shell
Magnesium has
2 valence electrons
Magnesium atom
33
5.4 Arrangement of Electrons in Atoms
Valence Electrons
The chemical properties of an element depend on the
number of valence electrons.
Example 1
Sodium (2, 8, 1) and Potassium (2, 8, 8, 1)
• have similar chemical properties;
• each has 1 valence electron.
Example 2
Fluorine (2, 7) and Chlorine (2, 8, 7)
• have similar chemical properties;
• each has 7 valence electrons.
34
5.4 Arrangement of Electrons in Atoms
The Periodic Table
Elements are arranged in order of increasing proton number.
35
5.4 Arrangement of Electrons in Atoms
The Periodic Table
Horizontal rows of elements
are called periods.
Vertical columns of elements are called groups.
36
5.4 Arrangement of Electrons in Atoms
Valence Electrons and the Periodic Table
Elements with the same number of valence electrons
belong to the same group in the Periodic Table.
Sodium (2, 8, 1) and potassium (2, 8, 8, 1) belong to Group I.
Fluorine (2, 7) and chlorine (2, 8, 7) belong to Group VII.
Hence, elements in the same group of the Periodic Table
have similar chemical properties.
37
Chapter 5 Atomic Structure
Concept Map
38