the atom scientists discovered that materials around us are made up of millions of tiny particles....

The Atom

Scientists discovered that materials around us are made up of millions of tiny particles.

They called these particles atoms.

Atom

Word atom comes from the Greek word for “cannot be split up”

e.g. the smallest piece of copper is called an atom of copper.

Atom Definition

An atom is the smallest particle of an element which still retains the properties of that element.

Each element is made up of atoms of that element e.g. the element gold is made up of atoms of gold

Size of an Atom

Atoms are very small

A full stop . Contains about 1016 atoms of carbon

A special microscope called an electron microscope can be used to see atoms

Dalton’s Theory of Atoms

In 1808 John Dalton proposed a theory on atoms after doing many experiments.

He imagined atoms to be like marbles tiny, round and hard only smaller. He though atoms could not be divided into anything simpler.

Particles inside Atoms

In the 1890’s other Scientists began to study atoms. Found that Dalton was incorrect when he said atoms could not be split up into anything simpler.

They found ting particles in side atoms called sub-atomic particles. Sub atomic means “inside atoms”

Chapter 29 – The AtomChapter 29 – The Atom

The atom

An atom is made up of three subatomic particles:

• Protons• Neutrons• Electrons

The Atom

The electrons which is negatively charged was discovered in 1897

The proton was then discovered it was positively charged

The neutron which has no charge (neutral) was discovered in 1932

Subatomic particles• The nucleus of an atom is made up of protons and

neutrons.• The electrons whizz around the nucleus in paths

called orbits or shells.

Chapter 21 – The AtomChapter 21 – The Atom

Differences in sub-atomic particles

Chapter 21 – The AtomChapter 21 – The Atom

Mass

The mass of these particles is very small.

A new unit of mass called the Atomic Mass Unit (a.m.u) is used to measure their mass.

1 proton = 1 a.m.u.

1 neutron = 1 a.m.u.

1 electron = 1/1840 a.m.u.

Charge

An electron has a negative charge (-1)

A proton has a positive charge (+1)

A neutron has no charge (0 charge)

Structure of an Atom

The Planetary Model

Niels Bohr proposed that electrons move around the nucleus in fixed paths called orbits.

Planetary Model

Review

Complete the table:Name of particle

Where situated in atom

Relative Mass

Relative Charge

Proton

Nucleus 0

Outside the nucleus

- 1 unit

Answer

Name of particle

Where situated in atom

Relative Mass

Relative Charge

Proton Nucleus 1 a.m.u. + 1

Neutron Nucleus 1 a.m.u. 0

Electron Outside the nucleus

1/1840units - 1 unit

Quiz

1. ________ was a famous Scientist who put forward an atomic theory in 1808?

2. List two words to describe Daltons theory of an atom? _______ __________

3. The centre of an atom is called the _____

Answers

1. John Dalton

2. Small and hard

3. Nucleus

Electron Cloud

The movement of the electrons around the nucleus gives rise to an electron cloud

Size of the nucleus

The nucleus of an atom could be imagined as a marble at the centre of Croke Park pitch. The electrons would be like specks of dust at various parts of the stands. Everything in between would be just empty space and the electrons are free to move in this space.

Atomic number and mass number

Each element in the periodic table has its own atomic number and mass number.

Chapter 21 – The AtomChapter 21 – The Atom

Atomic number

Atomic number of an atoms is the number of protons in the nucleus of that atom.

Chapter 21 – The AtomChapter 21 – The Atom

Atomic Number

The atomic number of an atom is equal to the number of protons and the number of electrons in an atom.

e.g. atomic number of copper is 29

Number of protons in copper is 29

Number of electrons in copper is 29

Complete the following table

Element Atomic Number

Number of protons

Number of electrons

Hydrogen

Lithium

Oxygen

Sodium

Magnesium

Periodical Table

Mass numberMass number is the number of protons and

neutrons that an atom at that element possesses.

Mass number = number of protons + number of neutrons

Chapter 21 – The AtomChapter 21 – The Atom

Mass number

Find the Mass number of:

For exampleSo an atom of magnesium has Atomic number = 12• 12 protons• 12 electrons

• Mass number = 2424 – 12 = 12

Number of neutrons = 12

Chapter 21 – The AtomChapter 21 – The Atom

Example

Helium:

Atomic number =

Number of protons =

Number of electrons =

Mass number =

Number of neutrons =

Helium

Atomic number = 2

Number of protons = 2

Number of electrons = 2

Mass number = 4

Number of neutrons = 4 - 2 = 2

Example

Chlorine

Atomic number =

Number of protons =

Number of electrons =

Mass number =

Number of neutrons =

Chlorine

Atomic number = 17

Number of protons = 17

Number of electrons = 17

Mass number = 35

Number of neutrons = 35-17 = 18

Complete the table for the first 20 elements p.g. 207 t/b

Element Atomic Number

Number Protons

Number Electrons

Mass No. Number of Neutrons

Hydrogen

Helium

Lithium

Berillium

Boron

Carbon

Nitrogen

Oxygen

flourine

ElementsElement Atomic

NumberNumber Protons

Number Electrons

Mass No. Number of Neutrons

Hydrogen

Helium

Lithium

Berillium

Boron

Carbon

Nitrogen

Oxygen

flourine

Electronic configuration

Electronic configuration refers to the way electrons are arranged in each shell or orbit.

Chapter 21 – The AtomChapter 21 – The Atom

How electrons are arranged

• Electrons whizz around the nucleus in shells or orbits.

• The first shell can hold up 2 electrons and the other shells can hold up to 8 electrons.

• Electrons must fill a shell before starting a new shell.

Magnesium electronic configuration

As magnesium has 12 electrons, they will be arranged:

2 in the first shell8 in the second shell2 in the third shellor, more simply:

(2, 8, 2)

Chapter 21 – The AtomChapter 21 – The Atom

ReactivityElements react with each other to obtain a full outer

shell of electrons.

Chapter 21 – The AtomChapter 21 – The Atom

IsotopesIsotopes are atoms at the same element that have

the same atomic number but different mass numbers.

In other words, the same numbers of protons but different numbers of neutrons.

Chapter 21 – The AtomChapter 21 – The Atom

Chapter 21 – The AtomChapter 21 – The Atom