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GCSE CHEMISTRY UNIT 2
Transcript of GCSE CHEMISTRY UNIT 2
C4 C5 C6
What is the trend of reactivity down Group I?
The metals get more reactive as you go down the group. Want to lose one electron from the outer shell and further down the group they are further away from the nucleus easier to react.
Why are they called alkali metals?
React with water to form alkaline solutions
The properties of Group I metals.
Soft can be cut with a knife
Shiny but tarnish quickly with the air
Good conductors of electricity
Less dense than other metals as they float on water
Melt on gentle heating
The halogens are a group of non- metals in the periodic table
They all have seven electrons in their outer shell this makes them all really reactive; they only have to gain one more electron to fill their outer shell.
Unlike Group One the elements get less reactive as you go down the group
Non-metals typically have low melting and boiling points.
The halogens, like most non-metals, are molecular. They each consist of molecules with the atoms joined in pairs: Cl2, Br2, and I2.
The forces between the molecules are weak, and so it is easy to separate them and turn the halogens into gases.
The chemical industry turns everyday salt (NaCl) into chlorine (and sodium) and then uses the chlorine to make plastics such as
There are hazards too. Some chlorine compounds are
so stable that they persist in the natural environment, where they can be a threat to life or bring about long-term damage. A dramatic example is the impact of chlorofluorocarbons (CFCs) and other halogen compounds on the concentration of ozone in the upper atmosphere.
In the Bohr model for atoms, the electrons orbit the nucleus as the planets orbit the Sun.
Bohr’s idea was that heating atoms gives them energy.
This forces the electrons to move to higher-energy orbits further from the nucleus. These electrons then drop back from outer orbits to inner orbits.
They give out light energy as they do so. Each energy jump corresponds to a particular colour in the spectrum.
The bigger the jump, the nearer the line to the blue end of the spectrum. Only certain energy jumps are possible, so the spectrum consists of a series of lines.
Electrons in shells
The comparison with the Solar System and the use of the term orbit can be misleading. The theory has moved on since Bohr’s time. Scientists still picture the electrons at a particular energy level. However, in the modern theory the electrons do not orbit the nucleus like planets round the Sun. All that theory can tell us is that there are regions around the nucleus
where electrons are most likely to be found. Chemists describe these regions as ‘clouds’ of negative charge.
Think of each electron cloud as a shell around the nucleus. Each shell is
one of the regions in space where there can be electrons.
The shells only exist if there are electrons in them. Electrons in the same shell have the same energy.
When atoms react, it is the electrons in their outer shells which get
involved as chemical bonds break and new chemicals form.
It turns out that elements have similar properties if they have the same number and
arrangement of electrons in the outer shells of their atoms.
In this module you have met the idea that the same element can take different chemical forms with distinct properties. Chemists describe these different forms as chemical species.
The lithosphere is broken into giant plates that fit around the globe like
puzzle pieces. These are the tectonic plates which move a little bit each
year as they slide on top of the upper part of the mantle. The crust and
upper mantle make up the lithosphere. They are about 80 km deep.
The oceans and rivers make up the hydrosphere. This is not a complete
sphere, but the oceans cover two-thirds of the globe, so it almost is.
There are living things in the sea, rivers, and lakes. Large areas of land are covered with living things, and there are billions of living things in soil.
So scientists like to think of a sphere of life encasing the planet; they call this the biosphere.
Atoms and molecules in the air
Most of the chemicals in the atmosphere are made of small molecules.
Only the noble gases exist as single atoms.
All molecules have a slight tendency to stick together. For example, there is an attraction between one O2 and another O2. But these attractive
forces are weak. This is why the chemicals that make up the atmosphere are gases with low melting and boiling points.
The molecule of hydrogen is held together by the electrostatic attraction between the two nuclei and the shared pair of electrons. This is a single bond.
This type of strong bonding is called covalent bonding. ‘Co’ means ‘together’ or ‘joint’, while the Latin word ‘valentia’ means strength.
So we have strength by sharing.
The small charges on opposite sides of the molecules cause slightly stronger attractive forces between the molecules. These small charges also help water dissolve ionic compounds by attracting the ions out of
The attractions between water molecules and their angular shape mean that in ice they line up to create a very open structure.
As a result, ice is less dense than liquid water.
Minerals are naturally occurring chemicals. They may be elements, like
gold and silver, which are found free in rocks. More commonly, they are
compounds, such as silicon dioxide, SiO2; calcium carbonate, CaCO3; rock
salt, NaCl; and iron oxides such as Fe2O3.
The two most common elements in the lithosphere are non-metals:
oxygen (47%) and silicon (28%). These two abundant elements form the major types of minerals in the lithosphere. The simplest example is silicon dioxide, SiO2, which can take various crystalline forms, including quartz.
As a sodium chloride crystal forms, millions of Na+ ions and millions of
Cl– ions pack closely together.
The ions are held together very strongly by the attraction between their opposite charges.
This is called ionic bonding, and the structure is called a giant ionic structure.
Unlike compounds such as water which are made up of individual molecules of H2O, there is not an individual NaCl molecule.
The mineral silica consists of silicon dioxide, SiO2. Its commonest
crystalline form is quartz. Crystalline silica has helped to shape human
history. From the sand used for making glass, to the piezoelectric quartz
crystals used in advanced communication systems,
The skeleton of all biochemicals is made from carbon. Carbon is the
element on which all life is based. The special things about carbon that
make this possible are:
Carbon atoms can form chains by joining to themselves.
Carbon forms four strong covalent bonds, so other atoms can join onto the chains. Very often these are hydrogen atoms.
There are three main ways of fixing nitrogen from the air:
the action of microorganisms (bacteria or algae)
a chemical reaction in the air during lightning flashes
a manufacturing process called the Haber process used to make fertilizers
The manufacture of fertilizers now makes a major
impact on the nitrogen cycle. As much nitrogen is
fixed by industry as is fixed naturally by the natural
processes supplying nitrogen to the soil.
The industry makes bulk chemicals on a scale of thousands or even millions of tonnes per year. Examples are ammonia, sulfuric acid, sodium hydroxide, chlorine, and ethene.
On a much smaller scale, the industry makes fine chemicals such as drugs and pesticides.
Reactions of acids
Acids with indicators
The term pH appears on many cosmetic, shampoo and food labels. It is a measure of acidity. The pH scale is a number scale which shows the acidity or alkalinity of a solution in water. Most laboratory solutions have a pH in the range 1–14.
Indicators change colour to show whether a solution is acidic. Litmus turns red in acid solution. Special mixed indicators, such as universal indicator, show a range of colours and can be used to estimate pH values.
Acids with metals
Acids react with metals to produce salts. The other product is hydrogen gas.
acid + metal --> salt + hydrogen
Acids with metal oxides or hydroxides
An acid reacts with a metal oxide or hydroxide to form a salt with water.
No gas forms.
acid + metal oxide (or hydroxide) --> salt + water
Acids with carbonates
Acids react with carbonates to form a salt, water, and bubbles of carbon
dioxide gas. Geologists can test for carbonates by dripping hydrochloric
acid onto rocks. If they see any fizzing, the rocks contain a carbonate.
This is likely to be calcium carbonate or magnesium carbonate.
2HCl(aq) + CaCO3(aq) --> CaCl2(aq) + H2O(l) + CO2(g)
As well as the proton number shown below each element, another number is shown above it. This is the relative atomic mass of the element. It is a comparative measurement of the mass of one atom of the element. You can use it to see how much heavier an atom of one element is compared with an atom of another element.
For example a magnesium atom has a relative atomic mass of 24. So we know it is twice as heavy as a carbon atom, which has a relative atomic mass of 12.
Reaction with cold water
All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group.
This shows that the reactivity of the alkali metals increases as you go down Group 1
Reactions with metals
The halogens react with metals to make salts called metal halides.
metal + halogen → metal halide
For example, sodium reacts with chlorine to make sodium chloride (common salt).
sodium + chlorine → sodium chloride
2Na(s) + Cl2(g) → 2NaCl(s)
The reaction between sodium and a halogen becomes less vigorous as we move down Group 7. Fluorine reacts violently with sodium at room temperature. Chlorine reacts very vigorously when in contact with hot sodium. Iodine reacts slowly with hot sodium.
Uses of halogens
Halogens are bleaching agents. They will remove the colour of dyes. Chlorine is used to bleach wood pulp to make white paper.
Halogens kill bacteria.
Chlorine is added to drinking water at very low concentrations. This kills any harmful bacteria in the water, making it safe to drink. Chlorine is also added to the water in swimming pools.
Because the halogens are very reactive and poisonous, care must be taken when using them. Chlorine is used only in a fume cupboard. Iodine should not be handled (it will damage the skin). Gloves may be used, and goggles should be worn.
Working out electronic structure from the Periodic Table
Find the element in the Periodic Table. Work out which period it's in, and draw that number of circles around the nucleus.
Work out which group the element is in and draw that number of electrons in the outer circle - with eight for Group 0 elements (except helium).
Fill the other circles with electrons (remember: two in the first, eight in the second and third, 18 in the fourth).
Finally, count your electrons and check that they equal the atomic number.
Catalysts are essential in many industrial processes. They make many processes possible economically. This means that chemical products can be made at a reasonable cost and sold at affordable prices.
Sodium hydroxide and hydrochloric acid react to produce a salt (sodium chloride) and water.
Na+(aq) + OH–(aq) + H+(aq) + Cl–(aq) --> Na+(aq) + Cl–(aq) + H2O(l)
Salts form when a metal oxide, or hydroxide, neutralizes an acid. So every salt can be thought of as having two parents. Salts are related to a parent metal oxide or hydroxide and to a parent acid.
Salts are ionic .
Most salts consist of a positive metal ion combined with a negative non-metal ion.
The metal ion comes from the parent metal oxide or hydroxide. The nonmetal ion comes from the parent acid.