Atomic structure and Radioactivity (Paper 1)

What is the size of the radius of an atom?

1×10^-10 m

What is the size of the radius of the nucleus?

Less than 1/10 000 of the radius of an atom

Where is most of the mass of an atom concentrated ?

In the nucleus

What does the nucleus contain?

Protons (+) and neutrons (neutral) meaning that the nucleus is overall positive

What is the nucleus surrounded by?

Electrons (-) in energy levels.

How does the distance from the nucleus affect the energy of the energy level?

Energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus.

How does the position of the electron change if the atom absorbs electromagnetic radiation?

It can move from a lower energy level to a higher energy level.

How does the position of an electron change if the atom emits electromagnetic radiation?

It moves from a higher energy level to a lower energy level.

What is the atomic number?

The number of protons in the atom.

Why do atoms have no overall charge?

Because the number of electrons is equal to the number of protons, so the positive and negative charges cancel each other out

What is the mass number?

The number of protons and neutrons in an atom?

How do we calculate the number of neutrons?

Atomic mass-atomic number

What are isotopes?

Atoms of the same element that have different numbers of neutrons.

If an atom loses 1 or more electrons what does it become?

A positive ion

If an atom gains 1 or more electron what does it become?

A negative ion

Development of atomic structure

1. 1804- John Dalton :

   • agreed that atoms were made up of tiny spheres that couldn’t be divided.

2. 1897- Electrons were discovered by JJ Thompson :

   • led to his plum pudding model of the atom.

   • The plum pudding model suggested →atom is a ball of positive charge with negative electrons embedded in it.

3. 1909-Earnest Rutherford

   • carried out alpha particle scattering experiment → firing a beam of alpha particles at thin gold foil→ some particle went straight through , some deflected , some deflected back the way they had come →led to conclusion that mass of an atom was concentrated at the centre→ in a positive nucleus.

   • This nuclear model replaced the plum pudding model

4. 1913-Niels Bohr:

   • adapted → nuclear model→ by suggesting that negative electrons orbit the nucleus at specific distances. → energy levels

   • This was accepted as his suggestion agreed with the results of other scientists.

   • These orbits are called energy levels.

5. Scientists discovered that the positive charge in the nucleus is due to positive particles called protons.

6. 1932-James Chadwick:

   • Discovered that the nucleus also contains neutral neutrons.→ explained missing mass in the nucleus

Describe the alpha scattering experiment

1. The scientist took a piece of gold foil→ as the gold can be hammered down into very thin foil, only a few atoms thick .

2. Then fired positively charged alpha particles at the gold foil.

3. They Found that most of the alpha particles passed straight through the gold foil without changing direction → some alpha particles were deflected & some bounced straight back off the gold foil.

4. The fact that most passed straight through meant that atoms are mainly empty space → so the plum pudding model had to be wrong.

5. Because some of the alpha particles were deflected→meant the centre of the atom must have a positive charge that repelled the alpha particles.

6. Because some alpha particles bounced straight back→ meant that the mass of the atom must be concentrated in the centre→ This central part of the atom is now called the nucleus

Compare the plum pudding model and the nuclear model

• They both have electrons

• The plum pudding model has no nucleus whereas the nuclear model has a positive nucleus.

• The plum pudding model has electrons embedded in a positively charged ‘pudding’ while the nuclear model has electrons around the edge of the atom.

• The plum pudding model assumes the atom is a solid sphere, while the nuclear model shows that the atom is mostly empty space.

What do some isotopes have?

An unstable nucleus

What is radioactive decay?

The process by which an unstable atomic nucleus gives out radiation in order to become stable

What type of process is radioactive decay?

Random-unpredictable

What is activity?

• The rate at which a source of unstable nuclei decays

• Measured in becquerel (Bq)

• 1 Bq= 1 decay per second

What is used to measure the activity of a radioactive source?

A Geiger-Muller tube

What is the count rate?

The number of decays recorded each second by a detector

What is an alpha particle?

A positive particle that consists of 2 protons and 2 neutrons (same nucleus as a helium nucleus)

What is a beta particle?

An electron which is ejected from the nucleus at very high speed

How do beta particles form if the nucleus doesn’t contain electrons.

When inside the nucleus, a neutron changes into a proton and an electron.

The electron is ejected from the nucleus, forming a beta particle.

What are gamma rays?

A type of electromagnetic radiation from the nucleus

What are the 4 ways nuclear radiation may be emitted?

• Alpha particle (α) → 2 protons+2 neutrons

• Beta particle (β) → high-speed electron

• Gamma waves (γ) → EM waves

• Neutron (n) → Neutral nuclear particle

How far can alpha particles travel & what are they stopped by?

• (They’re large)

• They can travel around 5 cm in air before they collide with air particles and stop.

  STOPPED BY:

• A single sheet of paper

  (least penetrating)

• Skin

• Few cm of air

How far can beta particles travel?

• Travel at high-speed

• They can travel around 15cm in air before stopping

• Lighter than alpha particles

  STOPPED BY:

• About 5mm of aluminium

• A few metres of air

How far can gamma radiation travel and stopped by?

• They can travel several metres in the air before stopping.

• weakly ionising

• Very highly penetrating

STOPPED BY:

• Thick lead (several cm)

• Several metres of concrete

How far can neutrons travel and stopped by ?

• Uncharged particles from the nucleus (mainly in nuclear reactors/weapons)

• Can travel very far(depending on energy)

  STOPPED BY:

• Hydrogen-rich material → water / concrete

• Indirectly ionising → cause other atoms to become radioactive

What does ionising power mean?

When radiation collides with atoms, that can cause atoms to lose electrons and form ions.

How ionising are alpha particles?

Very strongly ionising- this means they can produce a lot of ions when they collide with a material as they cause the atoms to lose their electrons.

How ionising are beta particles?

Quite strongly ionising but not as ionising as alpha particles

Beta particle

• 0 → left hand side top

• e

• -1 → left hand side bottom

What happens during alpha decay to the element?

• The mass number decreases by 4

• The atomic number decreases by 2

What happens during beta decay to the element?

• The mass number stays the same

• The atomic number increases by 1

What happens during gamma decay to the element?

• The mass number and atomic number aren’t changed

What is meant by half-life?

The half-life of a radioactive isotope →the time it takes for the number of nuclei in a sample to halve.

OR

The half-life is also the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half it’s initial level.

Why do scientists use half-life?

Because radioactive decay is random

How to work out Half-life of a radioactive isotope Using a graph & using data

Using graph :

1) half-life is found by the graph by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis.

• Read initial activity→ activity time is 0

• Divide the initial activity by 2 → find value of Alf initial activity

• find this vale on the y-axis & read along horizontally to read the curve

• Then read down from the curve → to find half-life

Using data:

1) Find the activity after each half-life ; E.G → after 2 half-lives → 1 half life 640 / 2 =320 → 2 half-lives 320/2=160

2)divide the final activity by the initial activity , then multiply by 100 to make it a % : e.g. → 160/640)*100 →

What is the big risk linked to ionising radiation?

It can increase the risk of cancer in humans so people who work with radioactive isotopes have got to take precautions.

What is irradiation?

When objects are exposed to nuclear radiation (i.e. alpha etc) from an external source

Why do irradiated objects not become radioactive?

Because the object only comes in contact with the radiation but not the radioactive isotope itself.

What Precautions do people take if they work with radioactive isotopes>

• Shielding:

  • GLOVES : used to successfully protect against alpha radiation.

  • LEAD APRONS : beta and gamma radiation are more penetrating, aprons are used to protect

  • LEAD WALLS & LEAD -GLASS SCREENS: for high levels of radiation

• Monitoring:

  • RADIATION MONITOR: to track how much radiation a person has been exposed to → regularly check & log exposure levels → too high then people must stop working

What’s meant by radioactive contamination?

- unwanted radioactive isotopes end up on other materials making them radioactive→they decay and emit nuclear radiation.

• It’s hazardous → now have the radioactive source on you / in you & could get a large dose of radiation.

• level of hazard depends on the type of radiation emitted.

How hazardous the radioactive contamination is based off the type of radiation.

• Alpha radiation:

  • Strongly ionising but low penetration → stopped by dead cells on the skin surface

  • BUT can be very dangerous if inhaled or swallowed (contaminated internally )→as the alpha particles can cause cell damage & damage their DNA.

• Beta radiation:

  • Quite ionising

  • can penetrate into the body where they can damage cells.

  • Can be hazardous both internally & externally

• Gamma radiation:

  • Weakly ionising→high penetration can penetrate into body but are likely to pass straight through

  • Mostly external hazardous → passes through body & can damage cells from outside

  • Less dangerous if contaminated internally then alpha or beta radiation