Year 9 Chemistry

Atomic Theory of Matter
 In

the early 19th century, John Dalton proposed that there was a certain point at which matter could no longer be broken down.

 The

term he used to describe these particles was ‘atom’.

 Available

scientific evidence at the time led Dalton to propose the law of simple multiple proportions, which stated that when elements combine, they do so in simple ratios


Water (H2O) 2:1



Methane (CH4) 1:4

Dalton’s Atomic Theory of Matter
 Elements

are made up of atoms which are extremely small particles.

 Atoms

of each element are different from those of other elements, including in their masses.

 Atoms

of a given element are identical to each other.

 Chemical

compounds are formed when atoms of one element combine with atoms of another element in the same fixed proportions.

 Atoms

cannot be created or destroyed in a chemical reaction.

Evidence for electron shells- The Emission
Spectrum


Emission Spectrum- Pattern of wavelengths (or frequencies) that appear as coloured lines in a spectroscope that is unique to each element.



Many substances give off a coloured light when samples are passed through a flame.



Niels Bohr explained this phenomenon by stating that when atoms of elements were given energy in a flame the electrons jumped from their normal shell to one further out from the nucleus. Because the higher energy state was unstable (not in their normal structure and contained to much heat) the electrons jumped back (to their normal shell) almost instantly releasing that heat gained through colour. For each jump made a certain amount of energy was given out in the form of a light-wave.
Eventually when all the metal is gone, the light will stop being released.

Emission Spectrum
 Each

element produces a slightly different light-wave, and therefore colour, because the possible values of energy for the electrons present are slightly different for each element.

 This

light-wave/emission spectrum is like a fingerprint for the element, and has allowed scientists to determine what elements are present in stars millions of kilometres away.

Prac: Flame Testing

Radioactive Half-Life
 Radioactive

decay is a random process and we cannot predict which radioactive nuclei in a sample will decay at a given moment, and all elements have different decay times.

 The

rate of radioactive decay does follow a pattern though.

 As

a radioactive sample decays, less and less of the original substance is left and the radioactivity drops.

 Radioactive

half-life- The time taken for half of the radioactive nuclei in a sample to decay.

 When

radioactivity reaches one-half of its original level, one half-life is said to have passed.
When it reaches one-quarter of its original level, two half-lives have passed.

A

radioactive decay graph shows this rate of decay over time.

Calculating Half-Life
 If

you know the half-life of a material you can calculate how much of the material is left after a specific time. Likewise, if you know the rate of decay of a material you can calculate its half life.

 For

example:

The half life of technetium-99m is 6 hours. If the initial radioactive count rate is 1088 counts per minute, what will the count rate be after 30 hours?

 Over

30 hours, there are 5 half-lives:

30 hours/6 hours per half-life = 5 half-lives
After each half life, the count rate is halved, therefore:
Half-life 1: 1088/2 = 544
Half-life 2: 544/2 = 272
Half-life 3: 272/2 = 136
Half-life 4: