Models of the AtomModels of the Atom

Atomic & Nuclear Physics Atomic & Nuclear Physics Lesson 1Lesson 1

Homework

• Revise for the Skills Test next week.

Quick Exercise

• Draw a diagram of a carbon atom, labelling as many parts as you can.

• Periodic Tables

Which is your diagram closest to?

Learning ObjectivesLearning Objectives

Describe a model of the atom that features a small nucleus surrounded by electrons.

Outline the evidence that supports a nuclear model of the atom.

Outline one limitation of the simple model of the nuclear atom.

Inside the atom…

PropertyProperty ElectronElectron ProtonProton NeutronNeutron

ChargeCharge -1-1ee +1+1ee 00

MassMass 9.11 × 109.11 × 10--

31 31 kgkg1.67 × 101.67 × 10--

2727 kg kg1.67 × 101.67 × 10--

2727 kg kg

Relative Relative MassMass

0.00050.0005 1.00001.0000 1.00041.0004

The electron and the proton have the same value of charge, but the signs are different.  We also use a quantity for the charge called electronic charge unit, e.                     1 e = 1.602 × 10-19 C.

The neutron has almost the same mass as the proton.

Question

• Why don’t the electrons go flying off?

• Let’s now play with magnets…

What happens in each case?

S N

N S

S N

S N

N S S N

Van der Graaf Generator

• Are we still happy with our model?

Challenge - Is it right?

• Examine the six statements, they are all true.

• Which of them, if any, support the nuclear model of the atom and why?

• Do any of the statements present a problem for the nuclear model?

Video

• In Search of Giants (3 of 15) The Discovery of the Atomic Nucleus

Size of the nucleus...

If the atom were the size of the school canteen, the nucleus would be the size of a pea dropped in the middle.

Really small compared to the atom!!

For fun?...(don’t worry about the equations)

221

04

1

r

QQF

221

r

mGmF

Repulsive force between two protons.

Gravitational Attraction.

Q1,Q2=eG is gravitational constant.m are proton massesR is distance between the two protons (10-

15m).

Force Comparison• Coulomb force of repulsion

is about 200N

• Gravitational Force of attraction is only about 1.9×10-34 N.

• So Coulomb force of repulsion is much, much stronger!!

21512

219

)101)(1085.8(4

)106.1(

CF

215

22711

)101(

)1067.1(1067.6

GF

What you need to know…

• Students should be able to describe a simple model involving electrons kept in orbit around the nucleus as a result of the electrostatic attraction between the electrons and the nucleus.

What you need to know…

• A qualitative description of the Geiger–Marsden experiment and an interpretation of the results are all that is required.

Recap

• Describe a model of the atom that features a small nucleus surrounded by electrons. [4 marks]

Recap

• Describe a model of the atom that features a small nucleus surrounded by electrons. [4 marks]

• A small nucleus (√) consisting of protons and neutrons (√) orbited by electrons (√) of about 1/2000th mass and an opposite charge (√).

Recap

• Outline the evidence that supports a nuclear model of the atom.

Recap

• Outline the evidence that supports a nuclear model of the atom. [5 marks]

• Rutherford fired high energy alpha particles (√) at a thin sheet of gold foil (√). The vast majority of particles passed straight through (√) but some of the alpha particles were deflected at large angles (√). This result could only be explained if all the charges of the gold atoms were concentrated at the centre along with mass (√)

Recap

• Outline one limitation of the simple model of the nuclear atom. [2 marks]

Recap

• Outline one limitation of the simple model of the nuclear atom. [2 marks]

• Either:-• The force of repulsion between the positive

charges is stronger than the gravitational attraction (√) so the nucleus should fly apart (√).

• OR:-• A particle in an orbit is accelerating and

accelerating charges emit radiation (√) so the electron should spiral into the nucleus (√).