BONDING AND REACTIONS

Chemistry

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atoms

The nucleus:

  • in the middle of the atom
  • contains protons and neutrons
  • has a postive charge because of the protons
  • almost whole mass of atom is concentrated in the nucleus
  • size-wise tiny compared to the rest of the atom

The electrons:

  • move around the nucleus
  • negatively charged
  • tiny but cover alot of space
  • volume of their orbit determines how bit the atom is
  • have virtually no mass and occupy shells around the nucleus

Protons are heavy and positively charged, neutrons are heavy and neutral and electrons are tiny and negatively charged. Neutrak atoms have no charge overall. (Top number is the mass number, bottom number is the atomic number on each atom on periodic table)

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elements, compounds and isotopes

Elements consist of one type of atom only:

- Quite alot of everyday substances are elements e.g. copperm aluminium, iron, oxygen and nitrogen

Compounds are chemically bonded:

- They are formed when two or more elements chemically react together. CO2 is a compount formed from a chemical reaction between carbon and oxygen. It is difficult to separate the two original elements out again and the properties of a compound are totally different from the properties of the original elements.

Isotopes are the same except for an extra neutron or two:

- They are different atomic forms of the same element which have the SAME number of PROTONS but a DIFFERENT number of NEUTRONS.

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the periodic table

The periodic table is a table of all known elements:

- There are 100ish elements which all materials are made of (more being discovered). Shows the elements in order of ascending atomic number. Elements with similiar properties form columns.

Elements in a group have the same number of outer electrons:

- The elements in each group all have the same number of electrons in their outer shell thats why they have similiar properties.

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electron shells

ELECTRON SHELL RULES:

  • Electrons always occupy shells (sometimes called energy levels)
  • The lowest energy levels are always filled first - these are the ones closest to the nucleus
  • Only a certain number of electrons are allowed in each shell: 1st shell = 2, 2nd shell = 8, 3rd shell = 8
  • Atoms are much happier when they have full electron shells - like the nocle gases in group 0
  • In most atoms the outer shell is not full and this makes the atom want to react
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ionic bonding

Ioninc bonding - transferring electrons:

- In ionic bonding, atoms lose or gain electrons to form charged particles (called ions) which are then strongly attracted to one another (because of the attraction of opposite charges, + and -)

A shell with just one electron is well keen to get rid...

- All atoms on left hand side (sodium, potassium, calcium) have just one or two electrons in  their outer shell. They tend to leap at the first passing ion with an opposite charge and stick to it like glue

A neaely full shell is well keen to get that extra electron...

- Group 6 and 7 (oxygen and chlorine have outer shells with are nearly full). Keen to gain the extra one or two electrons and when they do they become ions.

Giant ionic structures -> ionic bonds always produce giant ionic structurem, the ions form a closely packed regular lattice arrangement, very strong chemical bonds between all the ions, single crystal of salt is one giant ionic lattice. They have high meling and boiling points, they dissolve to form solutions that conduct electricity and conduct electricity when molten (when it melts, ions free to move and will carry electric current)

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electron shells and ions

Groups 1,2,6 and 7 most likely to form ions:

  •  Remember atoms that have lost or gained an electron(s) are ions!!!

- Group 1 and 2 are metals and lose electrons to form +ve ions or cations and group 6 and 7 elements are non-metals and gain electrions to form -ve ions or anions.  When any of the cations react with anions they form ionic bonds.

Show the electronic structure of simple ions with brackets [ ]:

A useful way or representing ions is by specifying the ions name followed by its electron configuration and the charge of the ion. 

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covalent bonding

Covalent bonds - sharing electrons:

  • sometimes atoms prefer to make covalent bonds by sharing electrons with other atoms
  • this way both atoms feel that they hvae a full outer shell (happy)
  • each covalent bond provides one extra shared electron for each atom
  • each atom involved has to make enough covalent bonds for fill up its outer shell
  • Examples:

Hydrogen (H2) and Chlorine (Cl2) = Hydrogen Chloride (HCl)

Ammonia (NH3) = has 5 outer electrons, forms 2 covalent bonds

Methane (CH4) = 4 outer electrions which is half a full outer shell

Water (H20) and Oxygen (O2)

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covalent substances: two kinds

Simple molecular substances:

  • atoms form very strong covalent bonds to form small molecules for several atoms, forces of attraction between these molecules are very weak, results of these inter-molecular forces is that melting and boiling points are low because molecules are easily parted from each other, most molecular substances are gases or liquids at room tmemp, and dont conduct electricity because there are no ions
  • 

Giant covalent structures:

  • these are similar to giant ionic structures except there are no charged ions, all atoms are bonded to each other by strong covalent bonds, very high melting and boiling points, dont condust electricity (not even when molten) and usally insoluble in water. Main examples are diamand and graphite (both made from carbon atoms)
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metallic structures

Metal properties are all dye to the sea of free electrons:

  • metal also consists of diant structures, metallic bonds involve the all important 'free electrons' which produce all the propertires of metals (these are free electons come fom the outer shell of every metal atom in the structure),these electrons are free to move and so metals are good conductors of heat and electricity
  • 

Identifying the bonding in a substance by its properties:

  • FOUR types: ionic, giant covalent, simple molecular or metallic
  • 
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new materials

Smart materials have some really wierd properties:

  • behave differently depending on the conditions (temperature), e.g. nitionol - a shape memory alloy. Its a metal but when its cool you can bend it and twist it like rubber but bend it too far and it will stay bent, if you heat it above a certain temp it goes back to a remembered shape. It is made from about half nickel and half titanium
  • other examples include dyes that change colour depending on temp or light intensity, liquids that turn solid when you put them in a magnetic field etc
  • 

Nanomaterials are really really really really really REALLY tiny!:

  • include fullerenes, these molecules of carbon shaped like hollow balls or closed tubes and each carton atom forms 3 covalent bonds with its neighbouts leaving free electrions that can conduct electricity. They have useful properties - have a huge surface area for industrial catalysts.
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relative formula mass

Relative atomic mass A r -> way of saying how heavy different atoms are compared with the mass of an atom of carbon-12. The relative atomic mass is usually the same as the mass number of the element

Look at periodic table for atomic masses

Add them together

= the relative formula mass!!

SIMPLE.

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two formula mass calculations

Calculating % mass of an element in a compound

= A r x number of atoms (of that element)/ M r (of whole compound) x 100

Finding the empirical formula (from masses or percentages:

  • list all the elements in the compound (usually about 2 or 3)
  • underneath them, write their experimental masses or percentages
  • divide each mass or percentage by A r for that particular element
  • turn the numbers you get into a nice simple raio by multiplying and/or dividing them by well chosen numbers
  • get the ratio in its simplest form = empirical formula
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calculating masses in reactions

Three important steps

1) write out the balanced equation

2) work out M r - just for the two bits you want

3) apply the rule: divide to get one then multiply to get all

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the mole

A mole = 6.023x10(to the power 23)

One mole of atoms or molecules of any substances will have a mass in grams equal to the relative formula mass (A r or M r) for that substance.

Number of moles = mass in g (of element or compound)/ M r 9of element or compound)

A 1M solution contains one mole per L

Number of moles = volume in litres x moles per litre of solution

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the atom economy

Atom economy = % of reactants changed to useful products:

- alot of reactions make more than one product, some of them were useful and some will just be waste. The atom economy tells you how much of the mass of the reactants ends up as useful products.

atom economy = total M r of useful products / total M r of reactants x 100

High ato economy is better for profits and the environment:

- making lots of waste is a problem, need to be disposed of somehow

- raw materials are expensive to buy

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percentage yield

Percentage yield compares actual and predicted yield:

percentage yield = actual yield (g)/ predicted yield (g) x 100

  • They are always less than 100%
  • For big industrial processes, the sort of reaction is it depends on what appartus is being used. Lots of things can go wrong such as...

1) the reaction is reversible - like the haber process

2) filtration - when you filter a liquid to remove solid particles

3) transferring liquids - always losea bit of liquid when you transfer it from one container to another

4) unexpected reactions - dont always exactly go to plan

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