Do now

1. Can you write the title

Radioactivityin your books?

2. Draw a diagram of an atom

Radioactivity

Today’s lesson

• describe the structure of an atom in terms of protons, neutrons and electrons and use symbols to describe particular nuclei

• understand the terms atomic (proton) number, mass (nucleon) number and isotope

• What is the evidence?

The atom

orbiting electrons

Nucleus (protons and neutrons)

Nuclide notation

Li3

7

Atomic number (proton number) = number of protons

Atomic mass (mass number) = number of protons and neutrons

Isotopes

Li3

7

It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons)

Li3

6

Isotopes

Li3

7

For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7

Li3

6

4 neutrons3 neutrons

Relative atomic mass

On average, lithium atoms have a mass of 6.941 (relative to Carbon 12)

Li3

6.941

Isotopes of Hydrogen

H1

1

The three isotopes of Hydrogen even have their own names!

H1

2

H1

3

Hi! I’m hydrogen

They call me

deuterium

Hola! Mi nombre es tritium y yo

soy de Madrid!

Questions!Element Chemical

symbolAtomic number

Hydrogen H 1

Helium He 2

Lithium Li 3

Beryllium Be 4

Boron B 5

Carbon C 6

Nitrogen N 7

Oxygen O 8

Radium Ra 88

Thorium Th 90

Uranium U 92

Plutonium Pu 94

11 of 40 © Boardworks Ltd 2007

Particles in the modern model

12 of 40 © Boardworks Ltd 2007

Atomic structure – key words

How do we know the structure of the atom?

The Plum Pudding Atomic Model

Before about 1910 many scientists believed that an atom consisted of:Positively charged matter spread out like a pudding embedded by negatively charged electrons (like plums in a pudding).

The ‘Plum Pudding’ Model

Rutherford’s Atomic ModelIn 1909 Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons.

Lord Rutherford 1871 - 1937

20/04/23Types of radiationTypes of radiation

1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a helium atom)

2) Beta () – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle.

3) Gamma – after or decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed.

Unstable nucleus

Unstable nucleus

Unstable nucleus

New nucleus

New nucleus

New nucleus

Alpha particle

Beta particle

Gamma radiation

Geiger & Marsden’s alpha particle scattering experiment

In 1909 Hans Geiger and Ernest Marsden performed an experiment using alpha particles to determine which of the two models was the better in describing the structure of an atom.

Geiger and Marsden

The apparatus 2

1

5

3

4

What was observedthin metal foil

1. Virtually all of the alpha particles went straight through the metal foil.

2. A few alpha particles were deflected through a small angle.

3. About 1 in 10 000 were deflected backwards.

alpha source

How their results supported Rutherford’s atomic model

1. The relatively small number of deflections indicates that most of the atom is empty space with only a very small nucleus.

2. The backward deflections can only occur if the nucleus is positively charged and contains most of the atom’s mass.

3. The ‘plum pudding’ model would not produce backward deflections.

nucleus (highly enlarged)

atom

How the results can be explained

1. Deflections occur because there is a force between the charged nucleus and the positively charged alpha particles.

2. Most of the alpha particles do not go near enough to the nucleus to be deflected.

3. Backwards deflections occur when the alpha particles make near head on collisions with the positively charged nucleus.

Rutherford did the calculations!

Rutherford (their supervisor) calculated theoretically the number of alpha particles that should be scattered at different angles. He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about 10-15 metres.

Rutherford did the calculations!

That’s 100 000 times smaller than the size of an atom (about 10-10 metres).

Stadium as atom

If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)!

Nucleus (ping-pong ball

Choose appropriate words to fill in the gaps below:

According to __________ an atom consists of a tiny, ___________ charged __________ surrounded by a cloud of ________ electrons. The nucleus also contains most of the ______ of an atom.

This model was supported by the ______ particle scattering experiment in 1909. In this experiment most alpha particles passed ________ through a thin metal foil with only about 1 in 10000 being deflected _________.

positively nucleus

mass

alpha

Rutherford

straight

backwards

WORD SELECTION:

negative

positively nucleus

mass

alpha

Rutherford

straight

backwards

negative

Unstable nuclei

Some nuclei are unstable, for example Uranium 235

Hi! I’m uranium-235 and I’m unstable. I really need to

lose some particles from my nucleus to become more

stable.

Unstable nuclei

To become stable, an unstable nuclei emits a particle

Weeeeeeeeeeeeee!

Unstable nuclei

We say the atom has decayed

Weeeeeeeeeeeeee!

Unstable nuclei

The decay of an unstable nucleus is random. We know it’s going to happen, but we can’t say when! It cannot be affected by temperature/pressure etc.

Weeeeeeeeeeeeee!

Becquerels (Bq)

• The amount of radioactivity given out by a substance is measured in Becquerels. One becquerel is one particle emitted per second.

Detection

• Particles can be detected by photographic film

• Particles can also be detected (and counted) by a Geiger-Müller tube (GM tube) connected to a counter

Background radiation

There are small amounts radioactive particles around us all the time. This is called background radioactivity. The amount varies depending on location.

Background radiation

Background radiation comes from

• Cosmic rays from space

• Radioactive rocks in the ground

• Nuclear tests• Nuclear bombs• Nuclear accidents

Radiation Safety

Radiation Safety

• Run away!

Mr Porter

Radiation Safety

• Run away!• In other words keep the distance between

you and a radioactive source as big as possible!

Mr Porter

Radiation Safety

• Don’t waste time!

Radiation Safety

• Don’t waste time!• In other words limit the time you are exposed

to radiation.

Radiation Safety

• If you can’t run away, hide behind something!

Radiation Safety

• If you can’t run away, hide behind something!• Put a barrier between you and the radiation

source that can absorb the radioactive particles

Let’s try some questions.

Let’s try some questions.