Topic 4 - Atomic Structure

Radius of an atom and nucleus

Atom - 1×10^-10m

Nucleus - 1×10^-14m (this is 1/10,000th of an atom)

Basic structure of the atom:

The basic structure of an atom is a positively charged nucleus in the centre of the atom composed of both protons and neutrons surrounded by negatively charged electrons

• Electron - negatively charged

• Protons - positively charged

• Neutrons - neutral (have no charge)

Subatomic particle charge and mass

How may the electron arrangements change with the absorption of electromagnetic radiation or by the emission of electromagnetic radiation?

• Absorption of electromagnetic radiation = move further from the nucleus; a higher energy level

• Emission of electromagnetic radiation = move closer to the nucleus; a lower energy level

Why are all atoms electrically neutral (no overall charge)

Equal number of protons (+) and electrons (-)

What are atomic and mass numbers

Atomic:

• The number of protons in the nucleus of an atom

Mass:

• The total number of protons and neutrons in an atom

What are isotopes

Atoms with the same number of protons but different number of neutrons

How to calculate the number of protons, neutrons, and electrons in an atom using atomic and mass numbers

P = 19

E = 19

N = 21 (40-19)

K has 19 protons, 19 electrons, 21 neutrons

Explain how atoms turn into positive ions

Atoms turn into positive ions if they lose one or more outer electrons

What were atoms once thought of

Solid spheres which was disproved from experiments from JJ Thomson

Early classification

Protons + electrons has not been discovered in the 19th century so there was no knowledge of atomic number (neutrons has also not been discovered)

Plum Pudding Model

Who discovered it?

= JJ Thomson

JJ Thomson discovered the electron, and his new theory was the plum pudding model

Description:

• A ball of positive charges

• Electrons embedded throughout

• No empty space

• Mass is spread throughout

Nuclear Model

Rutherford + Marsden’s experiment: positive alpha particles were fired at gold atoms

Observations and what they suggested:

• Most alpha particles passed straight through the foil - so it is suggested the atom is mostly empty space

• Some alpha particles were deflected, so it is suggested positive charges in the atom must be concentrated in the centre

The results of this experiment disproved the plum pudding model, which made Rutherford’s new theory: the nuclear model

Description:

• Tiny nucleus of positive charges

• Cloud of electrons outside the nucleus surrounded the atom

• Most of the atom is empty space

• The mass of atom is concentrated in the nucleus

Similarities and differences between Plum Pudding model and Nuclear model

Similarities:

• Both have positive charge

• Both have negative charges/electrons

• Both have no neutrons (not yet discovered)

Differences:

• Plum pudding is a ball of positive charge spread throughout, whereas Nuclear has positive charge concentrated at the centre

• Plum pudding has electrons spread throughout, where as Nuclear has electrons outside the nucleus

• Plum pudding has no empty space in the atom, whereas in Nuclear most of the atom is empty space

• Plum pudding has mass spread throughout, whereas Nuclear has mass concentrated at the centre

The Bohr Model

Neil’s Bohr altered Rutherford’s model:

• Electrons orbit nucleus in shells (energy levels)

• Electrons were at specific distances from nucleus

The work of James Chadwick

• After protons were discovered, there was still an issue of atoms of the same element with different mass numbers

• They became known as isotopes, but could not be explained until experiments by James Chadwick:

• He discovered neutrons

• It explained why atoms of the same element had the same atomic number (same number of protons)

• But had different mass numbers (different number of neutrons)

What is the development of the theory of the structure of an atom an example of

• How theories change with new evidence

• Confirming a scientific explanation when predictions are verified by experiment

The order of discovery of subatomic particles

Electrons →Protons → Neutrons

What is radioactive decay

The random process involving unstable nuclei emitting radiation to become more stable

• (its entirely random so you can’t predict exactly which nucleus in a sample will decay next, or when any of on them will decay)

what is Activity

The rate at which a source of unstable nuclei decays

• measured in becquerel (Bq)

What is Count-rate

The number of decays recorded each second by a detector (eg Geiger-Muller tube)

What do radioactive substances emit

Ionising radiation

• Radiation that knocks electrons off atoms, creating positive ions. The ionising power of a radiation source tells you how easily it can do this.

What our the four types of radiation that can be emitted during nuclear decay

• Alpha

• Gamma

• Beta

• Neutron

Alpha decay

What is it - a particle made of two protons and two neutrons

Mass - 4

Charge - +2

Penetrating power (stopped by) - a sheet of paper

Range in air - 5 - 6 cm

Ionising ability - strong

Used for - smoke alarms (because it ionises air particles, causing a current to flow. If there is smoke in the air, the smoke binds to the ions, reducing the number avaliable to carry a current. The current falls and the alarm sounds)

Beta decay

What is it - A fast-moving electron released by a nucleus

Mass - 1/200

Charge - -1

Penetrating power (stopped by) - thin sheet of aluminum

Range in air - 1m

Ionising ability - moderately

Used for - thickness monitoring of thin sheets of metal (because the particles are not immediately absorbed by the material like alpha radiation would be, and do not penetrate as far as gamma rays)

Gamma decay

What is it - very short-wavelength electromagnetic waves released by the nucleus

Mass - 0

Charge - 0

Penetrating power (stopped by) - thick sheet of lead or 1m of concrete

Range in air - unlimited

Ionising ability - weak

Used for - traces (because it can penetrate through a thick material)

What is half life

The time it takes for the number of nuclei in a radioactive isotope in a sample to halve

• (or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.

What is the concept of half life

• The radioactivity of a sample decreases over time. Each time a radioactive nucleus decays to become a stable nucleus, the activity will decrease, so older sources emit less radiation

• How quickly the activity drops off varies. For some isotopes, it takes just a few hours before nearly all of the unstable nuclei have decayed, whilst others last for millions of years

• The problem with trying to measure this is that the activity never reaches zero, which is why we use half-life to measure how quickly the activity drops off

How is half life related to the random nature of radioactive decay

Even though their decays are random, you can find out the time it takes for the amount of radiation emitted by a source to halve which can be used to make predictions about radioactive soures

Short half life and long half life

Short half life:

• activity falls quickly because the nuclei are very unstable and rapidly decay

• sources with a short half life can be very dangerous because of the high amount of radiation they emit at the start, but they quickly become safe

Long half life:

• activity falls more slowly because most of the nuclei don’t decay for a long time, they just sit there, releasing small amounts of radiation over a longer period

• This can be dangerous because nearby areas are exposed to radiation for years

What is irradiation and contamination

Irradiation:

• The process of exposing an object to nuclear radiation

• The irradiated object does not become radioactive

Contamination:

• The unwanted presence of materials containing radioactive atoms on other materials

• The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard

How to prevent irradiation and contamination

Irradiation:

• Keeping sources in lead-lined boxes

• Standing behind barriers or being in a different room

• Using remote-controlled arms to handle sources

Contamination:

• Gloves and tongs should be used when handling sources

• wear protective suits

Explain why the findings of studies into the effects of radiation on humans should be published and shared with other scientists

So that the findings can be checked by peer review

Which type of ionising radiation is most dangerous outside the body and inside the body

Outside:

• beta and gamma sources

• because they can penetrate the body and get to the delicate organs

• alpha is less dangerous because it can’t penetrate the skin and is easily blocked by a small air gap

• so irradiation is the major concern when working with beta and gamma sources

Inside:

• alpha sources

• because they do all their damage in a very localised area

• beta and gamma sources are less dangerous inside the body because they mostly pass straight out without doing much damage, as they have a lower ionising power

• so contamination is the major concern when working with alpha sources

What is radiation dose

A measure of the risk of harm to your body due to exposure to radiation

• it depends on the the type and amount of radiation you’ve been exposed to

• it’s measured in sieverts (Sv)

• 1 Sv = 1000 mSv

What is background radiation and what are its sources

Low-level radiation that is presented at all times, all around us, wherever you go

Sources:

• natural sources such as rocks and cosmic rays from space

• man-made sources such as the fallout from nuclear weapons testing and nuclear accidents

Affect of location on the level of background radiation and radiation dose

• certain underground rocks can cause higher levels of radiation at the surface, especially if they release radioactive radon gas, which gets trapped inside people’s houses

(a radon detector can tell you if your house has a dangerous level of radon and a radon outlet pipe can be used to keep the level down)

• people who live at high altitudes are exposed to more background radiation in the form of cosmic rays than people who live at sea level

Affect of occupation on the level of background radiation and radiation dose

• Nuclear industry worker and uranium miners are exposed to 10 times the normal amount of radiation

(they wear protective clothing and face masks, and monitor their radiation dose with special radiation badges and regular check up)

• Radiographers work in hospitals using ionising radiation and so have a higher risk of radiation exposure

(they wear lead aprons and stand behind lead screen)

• underground (mines) the radiation dose increases because of the rocks all around, posing a risk to miners

Do all radioactive isotopes have the same half life?

No radioactive isotopes have a very wide range of half-life values.

Explain why the hazards associated with radioactive material differ according to the half-life involved

Because radioactive isotopes have different half-lives and therefore their activity changes at different rates

What is the effect of radiation on living cells

Ionising radiation can be very harmful to living cells. Alpha, beta and gamma radiation enters living cells and collide with molecules. These collisions cause ionisation, which damages or destroys the molecules

Low doses vs high doses of radiation effect on living cells

Lower doses:

• cause miner damage without killing the cell

• this can give rise to mutant cells which divide uncontrollably. The cells keep dividing, making more cells and forming a tumour

Higher doses:

• kill cells completely

• which causes radiation sickness if a lot of body cells are killed at once

What does the extent of the harmful effect of radiation depend on

1. How much exposure you have to radiation

2. The energy and penetration of the radiation, since some types are more hazardous than others