National Chemistry

 

 

 Every element is made up of very small particles called atoms each with a different number of protons called the Atomic Number.

 Atoms of different elements vary in size and in mass.

Atomic structure lecture

             

                     

             particle                         mass               charge                  position in nucleus  

             proton                              1                    +1                        nucleus at centre

             neutron                            1                neutral                     nucleus at centre

            electron-                           0                     -1                        spinning around the nucleus

 

Equal numbers of positive protons and negative electrons make the atom neutral

 

      mass number = number of +ve protons = number of neutral neutrons

                   

                                        mass number = 27

                                                                        Al   = symbol

                                       atomic number=13

 

thus   number of protons = 13

          number of neutrons= mass number - protons = 27-13 = 14

          number of electrons = 13 same as protos in neutral atom (or look it up in data book where electron arrangement is 2,8,3  which add together to be 13

 

or for Sulphur

                                       mass number = 32

                                                                       S   = symbol for sulphur

                                       atomic number=16

 

thus   number of protons = 16

          number of neutrons= mass number - protons = 32-16 = 16

          number of electrons = 16 same as protons in neutral atom (or look it up in data book where electron arrangement is 2,8,6  which add together to be 16)

 

Atomic and mass number demo

 

 Isotopes are atoms of the same "element/ atomic number/ number of protons" but

different "mass number/ number of neutrons"

 

isotopes of chlorine

 

 

                                         mass number = 35                     37

                                                                        Cl        and        Cl

                                        atomic number=17                      17

 

atomic number ie number of protons the same (or they would be different elements).

 

mass number different - subtraction shows 18 neutron and 20 neutrons so neutrons different.

 

 

 

Isotopes of an atom have different abundances which results in the calculation of  an average mass for each element called the relative atomic mass - rarely a whole  number as it is an average.

 

 

Timeline of atomic theory

History of atomic theory

 

Radiation is produced from unstable isotopes of atoms becoming more stable. There are three types of radiation released – alpha, beta and gamma.

 

For example     14             12

                             C               C    are examples of isotopes of carbon

                           6              6

 

 

Carbon-14 is unstable and will produce radiation to become a stable isotope.

 

Beta particles are bent a long way and to the positive electrode – thus they are very light and negative

 

Alpha particles are bent a little and to the negative electrode – thus they are slightly heavier and positive

 

Gamma particles do not have a charge as they are not deflected.

                   

 

 

Alpha (α)particles have a mass of 4 and an atomic number of 2  ie 4

   He

  2

ie a helium nucleus

 

Beta (β) particles have a mass of 0 and an atomic number of -1  ie 0

   β

 -1

 

 

Different radiations have different penetrating powers. Alpha can be stopped by a hand or a sheet of paper. Beta needs a layer of aluminium to stop it and gamma needs several cm of lead to stop it passing through.

 

Nuclear equations.

                                         

 

Note that the mass numbers and the atomic numbers must add so that they are the same on both sides of the equation.

 

Loss of an alpha particle reduces the mass by 4 and the atomic number by 2.

Loss of a beta particle does not affect the mass and increases the atomic number by 1.

 

Half-life.

The half life of an isotope is the time taken for half of it to decay. For example one half life could be represented by:

 

1000 counts --------- 500 counts

       or                   20 g          --------- 10g

 or   100 %         --------- 50%

     or simply                       1       --------- 0.5

 

The half life of an isotope could be in years, minutes, seconds etc

The half life of an isotope is not affected by the starting mass ie 1g has the same half life as 10g

The half life of an isotope is not affected by the state, (s),(l) or(g),that it is in.

The half life of an isotope is not affected by whether it is a compound or an element eg

The half life of an isotope is not affected by temperature or pressure.

 

 

The half life of Titanium-44 is 63 years. If you start with 100g of titanium-44:

 

          63 years          63 years            63 years

  100g   ----------  50g ------------ 25g  ----------- 12.25g

 

Then in 3 half lives ie 189 years there will be 12.25g left. ie 87.75g would have turned into another isotope (not just disappeared)

Uses of radioisotopes.

Americium-241 is used in smoke detectors

Cobalt-60 is used to irradiate food to make it last longer

Iodine-131 is used to treat thyroid cancer

Using radioisotopes near humans requires alpha or beta radiation and a short half life (so it gets used up)

 

Use of isotopes to date materials.

Living things have a constant proportion of radioactive carbon-14.As it decays it is replaced.

When they die they can no longer take in carbon-14.

The carbon-14 continues to decay with a half life of 5700 years.

Thus if a piece of wood has a quarter of the carbon-14 it would have as a living tree:

 

     5700 years          5700 years          

1 ----------  0.5 ------------ 0.25

 

The tree died 11400 years ago

 

 

National 4 and 5

Nuclear Chemistry

 

These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.

 

The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.

 

 The notes are arranged under the following topics

 

National 3        Unit 1

                          Unit 2

                          Unit 3

 

National 4 and 5  Unit 1        Rates of Reaction

                                                  Atomic Structure

                                                  Bonding and Properties

                                                  Acids and bases

 

National 4 and 5  Unit 2       Fuels and Homologous Series

                                                Consumer Products

                                                Carbohydrates

 

National 4 and 5  Unit 3         Metals

                                                   Plastics

                                                   Fertilisers

                                                   Nuclear Chemistry

                                                  Chemical analysis

 

 

 

 

 

 

 

 

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