Chemistry C4

Atoms:

- The nucleus of an atom is made up of protons and neutrons.

- An atom is neutral beacuse it has an equal number of electrons and protons. The positive charges balance out the negative.

Isotopes:

Isotope = An element with the same atomic number but different mass number

Arrangement of electrons in atoms:

Group = The amount of electrons in the outer shell.

Period = The amount of shells the electrons occupy.

An early theory of atoms was developed by John Dalton. His theory was later backed up with better evidence. When J.J. Thompson, Rutherford and Bohr found new evidence, their explanation changed the model of the atom.

- Atoms can be made stable (full outer shell) by transferring electrons. This is called ionic bonding.

- All metals form positive ions (lose electrons)

- All non-metals form negative ions (gaon electrons)

- Positive ions and negative ions are held together by attraction.

The structure of sodium chloride or magnesium oxide is a giant ionic lattice, in which positive ions have strong electrostatic attraction to negative ions. Sodium chloride solution can conduct electricity.

Sodium chloride and magnesium oxide have high melting points as they have strong attractions but they do not conduct electricity when their solids because the ions cannot move.

- Non-metals can share electon pairs between atoms. This is known as covalent bonding.

- 'Dot and cross' show the formation of simple molecules that contain single and double covalent bonds which only show their outer shells:

Predicting chemical properties:

- Weak intermolecular forces mean the molecules are easy to break apart so the substances have low melting points.

- Strong intermolecular forces mean the molecules are harder to break apart therefore have high melting points.

-Substances only conduct electricity if they have free electrons which are able to move.

- The group 1 elements are found on the left hand side of the periodic table. They are called the alkali metals because they form alkaline compounds.

- Their atoms all have one electron in their outer shell which is why they are put in group 1. This gives the group 1 elements similar chemical properties to each other.

Reactivity:

-Lithium, sodium and potassium all react vigorously with water to form a metal hydroxide and hydrogen:

metal + water → metal hydroxide + hydrogen

Oxidation = Loss of electrons 

Reduction = Gain of electrons

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Flame Tests:

A flame test can be used to find out if lithium, sodium or potassium are present in a compound:

lithium= red

sodium= orange

potassium= lilac

- Group 7 elements have similar properties as their atoms have seven electrons in their outer shell. It is possible to predict the properties of halogens by knowing the properties of other halogens. This is because the properties follow trends.

- The nearer the outer shell is to the nucleus, the easier it is for an atom to gain one electron . The easier it is to gain the electron, the more reactive the halogen.

- When halogens react with alkali metals, a metal halide is made. For example, when potassium reacts with iodine, the metal halide made is potassium iodide.

Coloured compounds:

Copper compounds are often blue.

Iron(II) compounds are often pale green.

Iron(III) compounds are often orange/brown.

Transition elements and their compounds are often used as catalysts.

Thermal Decomposition of metal carbonates:

If a transition metal carbonate is heated it undergoes thermal decomposition to form a metal oxide and carbon dioxide. On heating:

- FeCO3 decomposes forming iron oxide and carbon dioxide.

- CuCO3 decomposes forming copper oxide and carbon dioxide.

- MnCO3 decomposes forming manganese oxide and carbon dioxide.

- ZnCO3 decomposes forming zinc oxide and carbon dioxide.

Precipitation reactions:

Sodium hydroxide solution reacts with compounds of each transition metal to make a solid of a particular colour. The addition of sodium hydroxide solution identifies the presence of the transition metal ions in solution:

Cu2+ ions form a blue solid

Fe2+ ions form a grey/green solid

Metals have specific properties that make them suitible for different uses. A property can either be physical or chemical:

- An example of a physical property is the metals level of thermal conductivity (e.g. copper has a high thermal conductivity)

- An example of a chemical property is how much it reacts with oxygen (e.g. gold does not react with oxygen or acids)

- Other physical properties of metals include being lustrous, shiny and malleable.

Metallic bonding:

Metals have high melting and boiling points, due to their strong metallic bonds. The bonds between these atoms are very hard to break. A lot of energy is needed to break the atoms apart.

A metallic bond is a strong electrostatic force of attraction between close-packed positive metal ions and a 'sea' of delocalised electrons.

A metal conducts electricity because delocalised electrons within its structure can move easily.

Superconductors are materials that conduct electricity with little or no resistance.

When a substance goes from its normal state to a superconducting state, it no longer has any magnetic fields inside it:

- If a small magnet is brought near the superconductor, it is repelled.

- If a small permanent magnet is placed above the superconductor , it levitates.

What are the benefits of superconductors?

- loss-free power transmission.

- super-fast electronic circuits.

- powerful electromagnets.

What are the difficulties with superconductors?

- lots of development is still needed to find the true potential of the superconductors.

- they work at very low temperatures so this limits their use.

There are three main stages to water purification:

Sedimetation - chemicals are added to make solid particles and bacteria settle out.

Filtration - a layer of sand or gravel filters out the remaining fine particles; some types of sand also filter out microbes.

Chlorination - chlorine is added to kill microbes.

Some soluble substances such as pesticides and nitrates are not removed with these methods so extra processes are used to get rid of these.

Water tests:

Water can be tested with precipitation reactions using aqueous silver nitrate and barium chloride solutions.

You can test to see if a solution contains sulfate ions by using barium sulfate. If barium chloride solution is added to a sample of water containing sulfate ions, barium sulfate is formed. Barium sulfate is insoluble in water, and will be seen as a white precipitate.