Atomic Structure - Radioactivity

Objectives: 4.6 Describe what radioactivity is. 4.8 Analyze Rutherford’s atomic model.

436

4.6 Radioactivity This was discovered by Becquerel in the 1890’s. A radioactive element is not stable. Unstable elements will decay over time. When an element decays, particles in the

nucleus escape into their surroundings.

These particles make up nuclear radiation

Radioactivity There are different radioactive

particles. Each particle has it’s own charge. Each particle has different

penetration powers.Alpha (α), Charge of +2, stopped by cardboard.Gamma (γ), charge of 0, stopped by concrete.Beta (β), charge of -1, stopped by thin metal.

Alpha (α) Beta (β) Gamma (Y)

Radioactivity If radioactive particles

are allowed to pass through charged plates, some particles will be deflected.

Beta particles go toward the

positive plate

Gamma particles are not deflected

either way

Alpha particles go towards the

negative plate

Rutherford used the ideas of radioactivity to carry out experiments to help him better define the atomic model.

His experiment involved: Exploring matter by shooting alpha

particles against thin sheets of metal.

4.8 Rutherford’s Atomic Model

Radiumin Lead Block

AlphaParticle beam

Fluorescent Screen

Thin sheet ofGold foil

Fluorescent Screen

A FEW particles are sharply deflected

MOST particles passed through the gold foil & were not deflected

SEVERAL particles pass through the gold foil & were slightly deflected

Conclusions from Rutherford’s Experiment:

3 Main Observations

OBSERVATIONS CONCLUSIONS1. MOST (almost all) went

through the gold foil & were not deflected

Atom contains a significant amount of empty space

2. FEW (not many) bounced directly back

The atom’s dense core (nucleus) must be very small

3. SEVERAL (some) passed through the gold foil & were deflected

The nucleus must be positive (law of repulsion)

Alpha Particles

MOST went through empty space

FEW approach nucleus & bounced

back SEVERAL were deflected by the nucleus

Gold Atom

Rutherford Model

Rutherford proposed a new

model based on his findings

and Thomson's Model.

Rutherford’s Model placed electrons orbiting,

like a cloud, around a dense,

positively charged nucleus.

Electrons

Nucleus

What type of radiation corresponds to each number in the diagram and state what type of charge each carries

,sdcmnv

Negative Plate

The diagram below shows radiation passing through a wooden screen and then through an electric field. This electric field has a positive end and a negative end. By analyzing the radiations behavior in this electric field, which type of radiation was used?

Wood

Electric Field

+

Radioactive source -

Rutherford’s famous gold foil experiment is shown below. Which of the four statements stated are true and which are false? (there can be more than one true!)

Lead

Gold

Florescent Screen

Lead Box

(Radio active substance) 1. Since most of the alpha particles went through the gold foil,

the atom consists of mostly of empty space.2. All of the alpha particles are deflected because they repel

the electrons 3. Most of the alpha particles bounce straight back showing

that the nucleus is very large.4. Several of the alpha particles are deflected showing that the

nucleus must have a positive charge

Three types of radiation and their charges: Alpha +2 (weakest) Beta -1 (intermediate) Gamma Ø neutral (strongest)

Rutherford’s Model Be familiar with diagram of experiment Know experiment results:

Particles go through - mostly empty space Particles slightly deflected - same charge as

nucleus so repelled Particles bounced back- direct hit on nucleus

Activities

436 Module 1 Worksheet #1

References Science Quest – Grenier, Daigle, Rheaume – 1998,

Cheneliere Student Study Guide: Physical Science 416-436(MEQ) Doc Brown’s Chemistry Clinic (

http://www.wpbschoolhouse.btinternet.co.uk/page04/4_71atom.htm)

Atom: The Incredible World (http://library.thinkquest.org/19662/low/eng/exp-rutherford.html)

cwx.prenhall.com/.../ medialib/blb0202.html www.sci.tamucc.edu/.../ genchem/atomic/page6.htm library.thinkquest.org/ 3471/physics_genesis_b.. Animation Factory Google Images