Do now 1 Can you write the title

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Do now 1. Can you write the title Radioactivity in your books? 2. Draw

Do now 1. Can you write the title Radioactivity in your books? 2. Draw a diagram of an atom

Radioactivity

Radioactivity

Today’s lesson • describe the structure of an atom in terms of protons, neutrons

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)

The atom orbiting electrons Nucleus (protons and neutrons)

Nuclide notation Atomic mass (mass number) = number of protons and neutrons 7 Li

Nuclide notation Atomic mass (mass number) = number of protons and neutrons 7 Li 3 Atomic number (proton number) = number of protons

Isotopes It is possible for the nuclei of the same element to have different

Isotopes 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) 7 6 3 3 Li Li

Isotopes For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 4 neutrons

Isotopes For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 4 neutrons 3 neutrons 7 6 3 3 Li Li

Relative atomic mass On average, lithium atoms have a mass of 6. 941 (relative

Relative atomic mass On average, lithium atoms have a mass of 6. 941 (relative to Carbon 12) 6. 941 3 Li

Isotopes of Hydrogen The three isotopes of Hydrogen even have their own names! Hola!

Isotopes of Hydrogen The three isotopes of Hydrogen even have their own names! Hola! Mi nombre es tritium y yo soy de Madrid! They call me deuterium Hi! I’m hydrogen 1 2 3 1 1 1 H H H

Questions! Element Chemical symbol Atomic number Hydrogen H 1 Helium He 2 Lithium Li

Questions! Element Chemical symbol Atomic 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

Particles in the modern model 11 of 40 © Boardworks Ltd 2007

Particles in the modern model 11 of 40 © Boardworks Ltd 2007

Atomic structure – key words 12 of 40 © Boardworks Ltd 2007

Atomic structure – key words 12 of 40 © Boardworks Ltd 2007

How do we know the structure of the atom?

How do we know the structure of the atom?

The Plum Pudding Atomic Model Before about 1910 many scientists believed that an 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 Model In 1909 Ernest Rutherford suggested that an atom consists of a

Rutherford’s Atomic Model In 1909 Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons. Lord Rutherford 1871 - 1937

Types of radiation Unstable nucleus New nucleus Alpha particle Beta particle Unstable nucleus New

Types of radiation Unstable nucleus New nucleus Alpha particle Beta particle Unstable nucleus New nucleus Gamma radiation 02/10/2020 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.

Geiger & Marsden’s alpha particle scattering experiment In 1909 Hans Geiger and Ernest Marsden

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 5 4 3 1

The apparatus 2 5 4 3 1

What was observed alpha source thin metal foil 1. Virtually all of the alpha

What was observed alpha source thin 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.

How their results supported Rutherford’s atomic model 1. The relatively small number of deflections

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.

How the results can be explained 1. Deflections occur because there is a force

How the results can be explained 1. Deflections occur because there is a force between the charged nucleus and the positively charged alpha particles. atom 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 (highly enlarged)

Rutherford did the calculations! Rutherford (their supervisor) calculated theoretically the number of alpha particles

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

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

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 (pingpong ball

Choose appropriate words to fill in the gaps below: Rutherford an atom consists of

Choose appropriate words to fill in the gaps below: Rutherford an atom consists of a tiny, According to _____ positively nucleus ______ charged _____ surrounded by a cloud of ____ negative electrons. The nucleus also contains most of the ______ mass of an atom. alpha particle scattering This model was supported by the ______ experiment in 1909. In this experiment most alpha particles straight through a thin metal foil with only about 1 passed ____ backwards in 10000 being deflected _____. WORD SELECTION: Rutherford mass backwards negative straight positively alpha nucleus

Unstable nuclei Some nuclei are unstable, for example Uranium 235 Hi! I’m uranium-235 and

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 Weeeeeee!

Unstable nuclei To become stable, an unstable nuclei emits a particle Weeeeeee!

Unstable nuclei We say the atom has decayed Weeeeeee!

Unstable nuclei We say the atom has decayed Weeeeeee!

Unstable nuclei The decay of an unstable nucleus is random. We know it’s going

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. Weeeeeee!

Becquerels (Bq) • The amount of radioactivity given out by a substance is measured

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

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

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

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

Radiation Safety • Run away! Mr Porter

Radiation Safety • Run away! Mr Porter

Radiation Safety • Run away! • In other words keep the distance between you

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!

Radiation Safety • Don’t waste time! • In other words limit the time you

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!

Radiation Safety • If you can’t run away, hide behind something! • Put a

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.