CHAPTER 5 Atomic Structure © 2013 Marshall Cavendish International (Singapore) Private Limited

CHAPTER 5Atomic Structure

© 2013 Marshall Cavendish International (Singapore) Private Limited

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

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What connects Everything?

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• In 465 BC, the teacher Democritus of Abdera, Greece; suggested tiny particles that are indivisible and indestructible. He called them atoms.

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• In 1787, an English teacher studied the ratios in which elements combine in chemical reactions. John Dalton came up with Dalton’s atomic theory. – Atoms of same element are identical

– Atoms of different elements can physically mix.

– Atoms can chemically combined.

– Chemical reactions occur due to rearrangement of atoms.

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• In 1880, JJ Thomson discovered electrons with the help of positive rays. He noticed atoms can be divided concluding that atoms have a positive core and electrons within.

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• Ernst Rutherford, a student of Thomson, proved that the atom has a solid atomic nucleus.

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• In 1933, Niehls Bohr identified that the electrons are arranged by it’s energy levels.

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5.1 Inside Atoms

Learning Outcomes

• state the relative charges and masses of a proton, a neutron and an electron;

• describe the structure of an atom.

At the end of this section, you should be able to:

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What are Atoms Made up of?

Atoms are made up of 3 sub-atomic particles:

• protons;

• neutrons;

• electrons.

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5.1 Inside Atoms

Where are these Sub-Atomic Particles Found?

Nucleus

• protons (+ve)

• neutrons (neutral)

Shells around the nucleus

• electrons (–ve)

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5.1 Inside Atoms

• Relative mass = 1

• Relative charge = +1

• Represented by the letter, p

Proton

• Found in the nucleus

p

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5.1 Inside Atoms

• Relative mass = 1

• Relative charge = 0

• Represented by the letter, n

• Found in the nucleus

n

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Neutron

5.1 Inside Atoms

• Relative mass =

• Relative charge = –1

• Represented by the letter, e

• Found in shells surrounding nucleus

e

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Electron

5.1 Inside Atoms

Particle Symbol Relative mass Relative charge

proton p 1 +1

neutron n 1 0

electron e –1

Summary of Sub-Atomic Particles

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5.1 Inside Atoms

5.1 Inside Atoms

5.2 The Proton Number and Nucleon Number

5.3 Isotopes

5.4 Arrangement of Electrons in Atoms

Chapter 5 Atomic Structure

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Learning Outcomes

• define proton (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 ( ).

At the end of this section, you should be able to:

XAZ

5.2 The Proton Number and Nucleon Number

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The number of protons in an atom is called its proton number.

Proton number = number of electrons

Proton Number (Z)

Since an atom is electrically neutral (i.e. has no overall charge),

It is also known as the atomic number.

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5.2 The Proton Number and Nucleon Number

nucleus of a carbon atom

Each element has a unique proton number.

Atoms of different elements have different proton numbers.

Proton Numbers

Example

Proton number of carbon = 6

Any atom with 6 protons must be a carbon atom.

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5.2 The Proton Number and Nucleon Number

23 Na

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Sodium

Proton number

Where is the proton number of an element found in the Periodic Table?

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5.2 The Proton Number and Nucleon Number

Exercise

1. Find the proton number of the atoms of the following elements:(a) Nitrogen

(b) Helium

(c) Sulfur

2. Which element has a proton number of(a) 17?

(b) 3?

(c) 20?

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2

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Chlorine

Lithium

Calcium22

5.2 The Proton Number and Nucleon Number

The total number of protons and number of neutrons in an atom is called the nucleon number.

Nucleon Number (A)

Nucleon number (A) = number of protons + number of neutrons

Nucleon number is also known as mass number.

The mass of an atom depends on the number of protons and neutrons.

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5.2 The Proton Number and Nucleon Number

5.1 Inside Atoms

5.2 The Proton Number and Nucleon Number

5.3 Isotopes

5.4 Arrangement of Electrons in Atoms

Chapter 5 Atomic Structure

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Learning Outcome

• define isotopes.

At the end of this section, you should be able to:

5.3 Isotopes

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These are 3 atoms of hydrogen.

What are the similarities and differences of these 3 atoms?

Isotopes of Hydrogen

Hydrogen-1 Hydrogen-2 Hydrogen-3

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5.3 Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Example 1

Chlorine gas consists of 75% chlorine-35, 25% chlorine-37.

What are Isotopes?

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5.3 Isotopes

Carbon consists of 98.93% carbon-12, 1.07% carbon-13, and trace amounts of carbon-14.

C14

6 C12

6C13

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What are Isotopes?

Example 2

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5.3 Isotopes

Isotopes have the same chemical properties but slightly different physical properties.

Properties of Isotopes

Same chemical properties are due to

• the same number of electrons;

• only electrons involved in chemical reactions.

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5.3 Isotopes

Different physical properties are due to

• different relative atomic masses;

• result in different densities, melting and boiling points.

Properties of Isotopes

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5.3 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.

Uses of Isotopes

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5.3 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.

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5.3 Isotopes

Uses of Isotopes

Protons, Neutrons, Electrons and Isotopes Review

Uses of Isotopes

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5.3 Isotopes

5.1 Inside Atoms

5.2 The Proton Number and Nucleon Number

5.3 Isotopes

5.4 Arrangement of Electrons in Atoms

Chapter 5 Atomic Structure

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Learning Outcome

• use diagrams to describe atoms as containing:

– protons and neutrons in nucleus;

– electrons arranged in the electron shells (energy levels).

At the end of this section, you should be able to:

5.4 Arrangement of Electrons in Atoms

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The 1st shell• is closest to the nucleus;• holds a maximum of 2 electrons;• is always filled first;• has the lowest energy level.

Electrons move around the nucleus in regions known as electron shells.

Electronic Structure

5.4 Arrangement of Electrons in Atoms

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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.

5.4 Arrangement of Electrons in Atoms

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Nucleus12p, 12n

Electronic configuration/structure is the arrangement of electrons in an atom.

Electronic configuration = 2, 8, 2

Electronic Configuration/ Electronic Structure

Magnesium atom

Magnesium atom (Z = 12)

1st shell: 2 electrons

2nd shell: 8 electrons

3rd shell: 2 electrons

5.4 Arrangement of Electrons in Atoms

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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

Valence Shell and Valence Electrons

Magnesium has 2 valence electrons

Magnesium atom

5.4 Arrangement of Electrons in Atoms

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The chemical properties of an element depend on the number of valence electrons.

Example 1Sodium (2, 8, 1) and Potassium (2, 8, 8, 1) • have similar chemical properties; • each has 1 valence electron.

Example 2Fluorine (2, 7) and Chlorine (2, 8, 7)• have similar chemical properties;• each has 7 valence electrons.

Valence Electrons

5.4 Arrangement of Electrons in Atoms

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The Periodic TableElements are arranged in order of increasing proton number.

5.4 Arrangement of Electrons in Atoms

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Horizontal rows of elements are called periods.

Vertical columns of elements are called groups.

5.4 Arrangement of Electrons in Atoms

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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.

Valence Electrons and the Periodic Table

5.4 Arrangement of Electrons in Atoms

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Concept Map

Chapter 5 Atomic Structure

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