Physics: Atomic Structure

How big is the atom?

1×10-10nm

Do energy levels further from the nucleus have higher or lower energy?

Higher energy

What happens when an electron absorbs EM radiation?

Can move from lower to higher (outer) energy level

What happens when an electron releases EM radiation?

Electron returns back down to original energy level

What is in the nucleus?

Protons - Positive

Neutrons - Neutral

What is the atomic number?

Number of protons (equal to no. of electrons)

Bottom number

What is the atomic mass number?

Top number

No. of protons + Neutrons

What is an isotope?

Atoms of the same element with different number of neutrons.

What is an ion?

An atom with an overall charge

Atom loses one electron = 1+ charge

Gains one electron = 1- charge

What is the alpha scattering experiment?

To investigate the plum pudding model

Firing alpha particles (positively charged) at gold foil

Most particles went through - atom is mostly empty space

Some particles were deflected - Centre of atom must have positive charge which repelled alpha particles

Sometimes particle went straight back - Mass of atom is concentrated in centre (nucleus)

What was the first (Ancient Greek) belief of atoms?

Small solids spheres which made everything

What was the first scientific theory about the atom?

Plum pudding model

Ball of positive charge

With negative electrons in

What atom model did the alpha scattering experiment prove?

Nuclear model

What did Niels Bohr decide?

Electrons orbit the nucleus at specific distances

(Energy levels / shells)

Who discovered Neutrons? When?

James Chadwick

20 years after nuclear model

What is radio active decay?

Some isotopes have unstable nucleus

Nucleus gives out radiation to become stable

How is radioactive decay like popcorn?

Can’t predict which atom will decay next (random process)

What is the activity?

The rate a source of unstable nuclei decay

Measured in Becquerel (Bq)

1 Bq = 1 decay per second

How to measure activity of radioactive source?

What is the count rate?

Geiger-muller tube

Count rate no. of decays recorded per second by detector

Alpha particles

a

Alpha particle is same as a helium nucleus

2 protons

2 Neutrons

Beta particle

β

Electron ejected from nucleus at high speed

Formed inside nucleus when a neutron changes into a proton and electron

Gamma ray

Electromagnetic radiation from nucleus

Alpha properties

• Travel 5 cm in air

• Stopped by paper

• Strongly ionising

Beta Properties

• Travel 15cm in air

• Stopped by few mm of aluminium

• Strongly ionising not as powerful as alpha

Gamma Properties

• Travel metres in air

• `stopped by several cm of lead

• Weakly ionising

What is the Half Life?

Time taken for number of nuclei in sample to halve (for half to decay)

Time taken for count rate / activity to reach half

radioactive isotopes

Radioactive isotopes decay and emit radiation from their nuclei

The radiation can form charged atoms (ionising Radiation)

This can increase cancer risk in humans

What is irradiation?

Why is it used?

Why is the object not made radioactive?

Exposing an object to nuclear radiation

• alpha

• beta

• gamma

Gamma radiation kills bacteria to sterilise needles etc

The object isn’t radioactive because it is in contact with the nuclear radiation, not the radioactive isotope.

Precautions for radiation

Alpha - Gloves

Beta & gamma - lead apron / lead walls

Radiation monitor - stop working in too dangerous radiation levels

Radioactive contamination

unwanted radioactive isotopes end up on other materials,

This is hazardous as the radioactive atoms decay and emit

ionising radiation.

Alpha radiation contamination

• Strongly ionising

• Stopped by dead skin cells

• Dangerous in inhaled /swallowed

Beta radiation contamination

• Quite ionising

• Penetrate skin into body

• Can get out again

Gamma radiation contamination

• Weakly ionising

• Penetrate body

• Pass through

What is peer review?

Scientists share their findings about the effects of radiation on humans. The findings can be checked

Background radiation

Natural

• Rocks (granite)

• Cosmic rays from space (exploding stars)

Man-made

• Nuclear weapon testing

• Nuclear accidents

Radiation dose

Sv Sieverts

How is iodine used to explore internal organs?

• Drink a solution of radioactive iodine

• Emits gamma radiation which passes through the body

• Scan shows amount of iodine absorbed by thyroid (too high/low) to diagnose condition

Using tracers in exploring internal organs 4 rules

1. The tracer must emit radiation that can pass out of the body and be detected (gamma or beta radiation).

2. The tracer must not be strongly ionising to minimise damage to body tissue.

3. The tracer must not decay into another radioactive isotope.

4. The tracer must have a short half-life so it is not present in the body for a long period.

Controlling or destroying unwanted tissue

• Gamma rays pass into body and destroy tumour

• Healthy tissue may be damaged as radiation passes through

• Use radioactive rod to target tumour procisesly

Nuclear Fission

• In some elements, nucleus is large and unstable

• Nucleus splits when absorbing a neutron

• Daughter nuclei formed when split

• Emit 2/3 neutrons and gamma radiation

• Chain reaction

Controlled: in nuclear reactor

Uncontrolled: in nuclear explosions in weapons

Nuclear fusion

• Two lighter nuclei join to form heavier nuclei

• Some of the mass is converted to energy

• Not chain reaction

Name a nuclear fuel

Uranium

What happens when an element releases alpha radiation (radioactive decay)

Mass (top) number decreases by 4

Atomic (bottom) number decreases by 2

How does a smoke detector work?