CH1 Section 1 - Basic Ideas about atoms

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  • Created on: 18-04-16 11:32

Radioactive decay

Isotopes are atoms with the same number of protons but a different number of neutrons.

If the ratio of protons: neutrons rises above 2:3, the isotope will become unstable.

Unstable nuclei give off three types of radiation:

  • Alpha particles
  • Beta particles
  • Gamma radiation
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Alpha Particles

  • Alpha particles consist of two protons and two neutrons (helium nucleus).
  • They are the most ionising as they are large, carry two charges and are slow moving.
  • They are the least penetrating: they are stopped by a few cm of air or a sheet of paper.
  • Alpha particles decrease the mass number by four and the atomic number. 
  • Symbol: 

                        

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Beta Particles

  • Beta radiation consists of a high energy, fast moving electron.
  • Beta particles are moderately penetrating: they are stopped by 5mm of Aluminium.
  • Beta particles increase the atomic number by one. The mass number remains the same.
  • Symbol:

                   (http://ths.talawanda.org/~bramblen/classroom/Pictures/betaparticle.JPG)

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Gamma Radiation

  • Gamma radiation consists of a high energy electromagnetic wave.
  • It is the most penetrating: it is stopped by at least a metre of concrete or several cm of lead.
  • It has no effect on the atomic number or mass number.
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Electron capture and Positron emission

Electron Capture - This occurs when a nucleus captures an electron, turning a proton into a neutron. This decreases the atomic number by one.

Positron Emission - This occurs when a proton splits into a neutron and an electron. This decreases the atomic number by one. 

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The effect of an electric field

Electric Field

(http://a.files.bbci.co.uk/bam/live/content/z4sk2hv/small)

The alpha particles are deflected slightly towards the negative plate, due to their positive charge. They are only deflected slightly as they are heavy and slow moving.

The beta particles are deflected towards the positive plate due to their positive charge. They are deflected greatly as they are light and fast moving. 

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The effect of a magnetic field

Magnetic Field

The direction of the particles can be worked out using Fleming's Left-hand rule.

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Half Life

Half-life - The time taken for the radioactivity of an isotope to fall to half its original value. 

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Consequences of radiation on living cells

  • Cancer cells: If DNA is exposed to ionising radiation it can become damaged. This can change the function of the cell, which can lead to mutations and the formation of a tumour. If a tumour spreads, cancerous cells can form.

Alpha radiation is most dangerous when ingested, as it can be absorbed by living cells very easily.

Gamma radiation is the most dangerous outside the body, as it can easily penetrate the skin and damage living cells. 

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Benefits of using radioactivity

Medicine

  • High energy gamma radiation can be used to kill cancerous cells and stop the spreading of a tumour. 
  • A radioactive tracer can be injected into the bloodstream and be used to produce a picture of a particular part of the body.

Radio Dating

  • Carbon - 14 is a radioactive isotope found in all organisms. Therefore, it can be used to calculate the age of plant and animal remains.
  • Potassium - 40 is used to calculate the geological age of rocks.

Industry

  • Beta radiation is used to control the thickness of materials. 
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Ionisation Energy

Ionisation energy is the energy required to remove mole of electrons from one mole of atoms in the gaseous state to form one mole of positive ions.

Ionisation energy is affected by three factors:

  • Effective nuclear charge
  • Atomic radius
  • Shielding

Ionisation energy increases across a period. This is because the atomic radius decreases and effective nuclear charge increases, whilst shielding remains the same.

Ionisation energy decreases down a group. This is because the atomic radius increases, the shielding increases and so the effective nuclear charge decreases. 

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Ionisation energy: the two exceptions

Group 3 elements

Q. Why is the IE of Al lower than that of Mg?

The outer electron of aluminium is in a slightly higher energy 3p orbital, and so less energy is required to remove it.

Group 6 elements

Q. Why is the IE of S lower than P?

The outer electrons of sulphur are in a paired 3p orbital: the repulsion between two electrons makes it easier to lose one. 

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Successive Ionisation Energies

An atom has as many ionisation energies as it has electrons.

E.G. Neon has 10 electrons = 10 ionisation energies.

                                          (http://scienceaid.co.uk/chemistry/fundamental/images/ionisation.jpg)

Successive ionisation energies always increase due to a decrease in atomic radius, a decrease in shielding, and so an increase in effective nuclear charge.

A large increase in successive ionisation energy shows that the electron is being removed from a new shell that is closer to the nucleus. This indicates the group to which the element belongs.

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Absorption Spectra

An absorption spectrum is a series of short, sharp, black lines on a coloured background. 

                           (http://www.bbc.co.uk/staticarchive/fc5c0b7b64dc88687bfecf57ec4f494a5b238db2.gif)

These lines arise when white light is passed through sample particles in the gaseous phase. The atoms absorb certain wavelengths and remove them from the light, hence why dark lines occur. 

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Atomic Emission Spectrum of a Hydrogen Atom

The Atomic Emission Spectrum of a hydrogen atom is a series of short, sharp, coloured lines on a black background. These lines arise from an electron absorbing additional energy, promoting itself to a higher energy level. It then releases energy of a fixed frequency as it returns to its original n = 2 energy level. The energy levels are fixed, not continuous, and so only fixed frequencies can be observed.

Q. Why do the lines get closer together at the high-frequency end of the spectrum?

The energy levels furthest from the nucleus converge together as the energy difference decreases, so the transitions are of the highest frequency. 

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Lyman, Balmer and Paschen Series

(http://intranet.micds.org/upper/science/text/Chemistry_Textbook/Ch03/images/balmer.jpg)

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Lyman, Balmer and Paschen Series

Lyman Series - occurs in the ultraviolet region

  • Highest energy
  • All electrons return to the n=1 energy level
  • In the Lyman Series, the convergence limit is the point where the nucleus loses all control over its outer electron. Therefore, the difference in energy between n=1 and the convergence limit represents the ionisation energy of a hydrogen atom.

Balmer Series - occurs in the visible region

  • Moderate energy
  • All electrons return to the n=2 energy level.
  • Red line is always on the left

Paschen SeriesInfrared region

  • Lowest energy
  • Electrons return to n=3
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