AQA GCSE Physics: Combined Science, Atomic Structure and Rutherford Scattering

AQA GCSE Physics: Combined Science

4.1 Atoms & Isotopes

• 4.1.1 Atomic Structure
• 4.1.2 The Absorption & Emission of EM Radiation
• 4.1.3 Atomic & Mass Number
• 4.1.4 Isotopes
• 4.1.5 Positive lons
• 4.1.6 The Plum Pudding Model
• 4.1.7 Rutherford Scattering
• 4.1.8 Bohr's Model of the Atom
• 4.1.9 Discovery of the Proton & Neutron

4.1.10 Changes in the Atomic Model

Your notes

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Your notes

Atomic Structure

Atomic Structure Overview

Atoms are the building blocks of all matter

• They are incredibly small, with a radius of only 1 x 10-10 m

This means that about one hundred million atoms could fit side by side across your thumbnail

Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus

The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom

O = PROTON

O = NEUTRON

O

= ELECTRON

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Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!

Parts of the Atom

• The nucleus contains:

Protons - positively charged particles with a relative atomic mass of one unit Neutrons - no charge, and also with a relative atomic mass of one unit

Almost all of the atom is empty space, but moving around the nucleus there are:

• Electrons - negative charge with almost no mass (1/2000 the mass of a proton or neutron)
• The properties of each of the particles are shown in the table below:

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Particle Properties

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Particle	Location	Relative Charge	Relative Mass
Proton	In the nucleus	+1	1
Neutron	In the nucleus	0	1
Electron	Orbiting the nucleus	-1	1/2000 (Negligible)

Examiner Tip

There are many different models of the atom. As you progress through the topic you will discover that the atom can be described in many different ways, such as the Plum Pudding Model that is covered later, but for your exam, make sure to only use the model and descriptions described here!

Be careful with your terminology:

Atom = nucleus (proton and neutron) and electrons Nucleus = protons and neutrons at the centre of the atom

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Electron Structure

Electron Energy Levels

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Electrons in an atom orbit around the nucleus at particular distances, known as energy levels

• A certain number of electrons can occupy each energy level

For example, only two electrons can orbit in the first energy level

• Only eight electrons can fit in the second energy level, and eight in the third as well
• The higher the energy level, the further the distance of the electron from the nucleus

3rd ENERGY LEVEL

2nd ENERGY LEVEL

·1st ENERGY LEVEL

NUCLEUS

INCREASING ENERGY

DUE TO GREATER

DISTANCE FROM

NUCLEUS

In this diagram the first two energy levels are full. Electrons further from the nucleus have more energy

• Like moving up a ladder, electrons in higher energy levels have greater potential energy because they have more distance between them and the nucleus

Examiner Tip

If you are studying for your Chemistry GCSE then you will need to know the number of electrons that fit into the different energy levels. They may also be called electron shells.

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Electrons & Protons

Atomic Charge

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Although atoms contain particles of different charge, the total charge within an atom is zero

This is because the number of electrons is equal to the number of protons

O = PROTON

O = NEUTRON

O = ELECTRON

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A Lithium atom has three protons, four neutrons and three electrons

The following table sets out the calculation of the total charge in the Lithium atom:

Total Charge Calculation Table

	Relative Charge	Number of Particles in the Lithium atom	Number x charge	Total Charge
Protons	+1	3	+3	(+3) + 0 + (-3) = 0
Neutrons	O	4	0
Electrons	-1	3	-3

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If an atom loses electrons, then it is said to be ionised

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Worked Example: Carbon-12 Electrons

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A nucleus of carbon-12 is shown below.

N

P

N

P

N

P

P

N

N

P

N

P

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How many electrons are there in an atom of carbon-12?

Step 1: Count the number of protons in the carbon nucleus

• There are 6 protons in the carbon atom

Step 2: Determine the number of electrons

Remember, the number of electrons in an atom is equal to the number of protons

Therefore there must be 6 electrons in the carbon atom

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4.1.2 The Absorption & Emission of EM Radiation

Electron Energy Absorption

The Absorption & Emission of EM Radiation

Electrons in an atom are arranged in energy levels

Lower energy levels are closer to the nucleus, whilst higher energy levels are further away from the nucleus of the atom

• These electrons can absorb energy

This happens when waves of electromagnetic radiation (such as light and heat) hit them

When electrons absorb electromagnetic radiation, they move to a higher energy level

0 2nd ENERGY LEVEL ·

-O

ELECTRON

MOVES UP

ENERGY

LEVEL

o 1St ENERGY LEVEL

NUCLEUS

WAVE OF

ELECTROMAGNETIC

RADIATION

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The electron absorbs electromagnetic radiation which causes it to move up an energy level

Dark coloured objects are good absorbers of radiation

They appear dark because they do not reflect the energy that hits them

Emission of Electromagnetic Radiation

When an electron has moved up an energy level, it will be unstable

Eventually, it will move back down to its original energy level, which will be closer to the nucleus

• As it moves back down, It emits a wave of electromagnetic radiation

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Electron Emission Diagram

2nd ENERGY LEVEL

1st

ENERGY LEVEL

ELECTRON MOVES DOWN

AN ENERGY LEVEL AND

EMITS A WAVE OF

ELECTROMAGNETIC RADIATION

V

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The electron emits a wave of electromagnetic radiation and moves down an energy level

All of the colours in the visible spectrum are produced in this way

• The light waves come from electrons moving down energy levels and emitting electromagnetic radiation

Examiner Tip

Whilst you are expected to know about energy levels and the absorption and emission of electromagnetic waves, you are not expected to know about the number of electrons in each energy level or the way in which they fill up - that's all part of Chemistry.

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4.1.3 Atomic & Mass Number

Atomic Number

Atomic Number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)

Elements in the periodic table are ordered by their atomic number

• Therefore, the number of protons determines which element an atom is

The atomic number of a particular element is always the same

• For example:

Hydrogen has an atomic number of 1. It always has just one proton

• Sodium has an atomic number of 11. It has 11 protons

Uranium has an atomic number of 92. It has 92 protons

The atomic number is also equal to the number of electrons in an atom

This is because atoms have the same number of electrons and protons in order to have no overall charge

Mass Number

The total number of particles in the nucleus of an atom is called its mass number

The mass number is the number of protons and neutrons in the atom

The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutron = Mass Number - Atomic Number

For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 - 11 = 12

Examiner Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

Atomic and mass numbers in GCSE Physics are just the numbers that can be found on the periodic table for every element. You have probably used the periodic table in your Key Stage 3 Chemistry lessons, and if you take GCSE Chemistry or Combined Science. You are already familiar with these numbers, just not necessarily using them in Physics! You will not be given a periodic table in your Physics exam, but don't worry, you will be given the atomic numbers and mass numbers you need to perform any calculations.

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Nuclear Notation

The mass number and atomic number of an atom are shown by writing them with the atomic symbol

This is called nuclear notation

• Here are three examples:

MASS NUMBER

(PROTONS + NEUTRONS)

2

H

1

V

23

Na

235

92

U

11

ATOMIC NUMBER

(NUMBER OF PROTONS)

HYDROGEN

SODIUM

URANIUM

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Your notes

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

The top number is the mass number

This is equal to the total number of particles (protons and neutrons) in the nucleus

• The lower number is the atomic number

This is equal to the total number of protons in the nucleus

The atomic and mass number of each type of atom in the examples above is shown in this table:

Atomic and Mass Number Table

Atom	Number of protons (Atomic Number)	Number on Neutrons (Mass number - Atomic number)	Number of electrons (same as the number of protons)
Hydrogen	1	1	1
Sodium	11	12	11
Uranium	92	143	92

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