Chapter 2

The Structure of the Atom

2.1 Matter

• Matter: Anything that occupies space and has mass

• Based on theory: Matter is made up of tiny and discrete particles. There are spaces between these particles

• Types of particle:

Atoms

Molecules

Ions

• Atom: The smallest particle of an element that can participate in a chemical reaction

• Molecule: A group of two or more atoms which are chemically bonded together

• Ion: A positively-charged or negatively-charged particle

• Diffusion: Occurs when particles of a substance move in between the particles of another substance

• Diffusion of matter occurs most rapidly in gases, slower in liquids and slowest in solids → due to the different arrangement and movement of particles

• Observe this video

Kinetic Theory of Matter

• Types of matter:

Solid

Liquid

Gas

The arrangement and movement of particles in solid, liquid and gas are described in the kinetic theory of matter.

The Change in The State of Matter

Sublimation

Sublimation

Condensation

Boiling / Evaporation

Melting

Freezing

When solid is heated;

• The particles in the solid gain kinetic energy and vibrate more vigorously.

• The particles vibrate faster as the temperature increases until the energy they gained is able to overcome the forces that hold them at their fixed positions.

• At this point, the solid becomes a liquid. • This process called melting. • The temperature at which this happens is called

melting point.

When liquid is heated;

• The particles in the liquid gain kinetic energy and move faster.

• The particles move faster as the temperature increases until the energy they gained is able to overcome the forces that hold them.

• At this point, the liquid becomes a gas.

• This process is called boiling.

• The temperature at which this happens is called boiling point.

When liquid is cooled;

• The particles in the liquid lose energy and move slower.

• As the temperature decreases, the particles lose more energy until they did not have enough energy to move freely.

• At this point, the liquid changes into a solid.

• This process is called freezing.

• The temperature at which this happens is called freezing point.

Definition:

• Melting point

The temperature at which a solid changes into a liquid at a particular pressure.

• Freezing point

The temperature at which a liquid changes into a solid at a particular pressure.

The heating curve

Temperature (°C)

Time (min)

Melting point

A

B C

D

solid

begin to melt

liquid

solid-liquid

The temperature remains constant because: The heat energy absorbed is used to overcome the force attraction between particles so that solid can change into liquid

Heat energy is absorbed. Particles in solid gain kinetic energy and vibrate faster

Heat energy is absorbed. Particles in liquid gain kinetic energy and move faster

The cooling curve

Temperature (°C)

Time (min)

Freezing point

E

F G

H

liquid

solid

liquid-solid

The temperature remains constant because: The heat loss to the surrounding is equal to the heat energy formed during particles attraction to form a solid

Heat energy is released. Particles in liquid lose kinetic energy and move slower

Heat energy is released. Particles in solid lose kinetic energy and vibrate slower

2.2 The Atomic Structure

Historical development of atomic models Model Structure Characteristic

Dalton’s atomic model proposed by John Dalton in 1805

The atom was imagined as a small indivisible ball similar to a very tiny ball.

Thomson’s atomic model proposed by J.J. Thomson in 1897

J.J Thomson discovered electron, a negatively-charged particle. The atom was described as a sphere of positive charge embedded with electrons.

Model Structure Characteristic

Rutherford’s atomic model proposed by Ernest Rutherford in 1911

Ernest Rutherford discovered proton, a positively-charged particle in an atom. The central region of atom has a very small positively-charged nucleus, which contains almost all the mass of the atom.

Bohr’s atomic model proposed by Neils Bohr in 1913

The electrons in an atom move in shells around the nucleus which contains protons.

Model Structure Characteristic

Chadwick’s atomic model proposed by James Chadwick in 1932

Chadwick proved the existence of neutrons, the neutral particle in the nucleus. The nucleus of the atom contains protons & neutrons, and the nucleus is surrounded by electrons.

Subatomic particles in atom

Proton

Neutron

Electron

Subatomic particles

Relative electric charge: +1 Relative mass: 1

Relative electric charge: 0 Relative mass: 1

Properties of subatomic particles

Nucleus

Proton, p

Neutron, n

Relative electric charge: -1 Relative mass: ≈ 0.0005

Electron, e

Proton number and nucleon number

Proton number:

The number of proton in its atom

** also shows the number of electrons in the atom

Nucleon number:

The total number of protons and neutrons in its atom

** also known as mass number

What is the relationship??

Nucleon number

Proton number

Number of neutrons = +

Nucleon number

Number of protons

Number of neutrons = +

OR

Reminder!!!

Proton number

Nucleon number

Neutron number

√

√

X Number of neutron

Symbol of elements

X A

Z

Nucleon number

Proton number

Symbol of element

2.3 Isotopes and Their Importance

Isotopes:

• Atoms of the same element with same number of proton but different number of neutrons.

OR

• Atoms of the same element with same proton number but different nucleon number.

Example of elements with isotopes:

• Hydrogen

H 1

1 H 2

1 H 3

1

Hydrogen-1 Hydrogen-3 Hydrogen-2

• Chlorine

Cl 35

17 Cl 37

17

Chlorine-35 Chlorine-37

Uses of isotopes in our daily lives

In the medical field: • Cobalt-60 Used in radiotherapy for the treatment of cancer • Iodine-131 Treatment of thyroid gland In the argiculture field: • Phosphorus-32 Study the metabolism of phosphorus in plants • Gamma ray of cobalt-60 Used to destroy bacteria in food

Uses of isotopes in our daily lives

In industrial field:

• Sodium-24

Used detect leakages in pipes

In archeology field:

• Carbon-14

To estimate the age of fossils and artifacts

2.4 The Electronic Structure of an Atom

The first shell : 2 electrons

The second shell : 8 electrons

The third shell : 8 electrons

Nucleus

Example:

Chlorine atom

• Proton number of 17

• Hence, chlorine atom has 17 electrons

• Electron arrangement:

2.8.7

Valence electrons

• The electrons in the outermost occupied shell

Outermost occupied shell

Valence electron