- Nuclei are made up of protons and neutrons,
Protons = positive charge
Neutrons = neutral charge
Electrons = negative charge
- Isotopes are elements with the same atomic number but different mass numbers
e.g Carbon has six protons and six neutrons, an isotope of carbon has six protons and a mass of 14 so therefore it has eight neutrons!
- The different numbers of neutrons in isotopes can be deduced by taking away the number of protons from the atomic mass number
Arrangement of electrons in atoms/Development of a
- Elements in the periodic table are arranged in order of increading atomic number
- an element with an electronic configuration of (2,8,6) has three electron shells, so is in the third row.
- Elements in the same group (with the same number of electrons in the outer shell) are arranged vertically
- Elements in the same period (in order of how many shells the electrons occupy) are arranged horizontally
Early theory of atoms developed by John Dalton. His explanation was provisional, later confirmed by evidence. J.J. Thompson, Rutherford and Bohr found new evidence which changed this explanation - later their predictions were confirmed. Ideas change rapidly following unexpected results - Geiger and Marsden added to ideas.
- Atoms with full outer shells are stable
- Atoms can be made stable by transferring electrons = ionic bonding
- Metal atoms loose electrons - Non metal atoms gain electrons
- Therefore the metal atom becomes positive and the non metal atom becomes negative - these atoms then attract eachother as they are opposites.
The 'dot and cross' models show ionic bonding :
Covalent bonding and intermolecular force
Non-metals can share electron pairs between atoms. This is known as covalent bonding. By sharing electrons these atoms can have full outer shells and therefore become stable. This can be shown in a dot and cross diagram which only shows the outer shell electrons.
Attraction between certain molecules, such as in water, is called intermolecular force.
The weak intermolecular forces in substances with a simple molecular structure, like water, mean the molecules are easy to break apart so the substances have low melting points.
Because there are no free electrons, water does not conduct electricity.
Development of the periodic table
In 1865 Newlands put 56 elements into groups and saw that every 8th element behaved similarly. This wasn't accepted for 50 years, after which evidence was provided.
In 1869 Mendeleev arranged the elements in order in a table. He noticed periodic changes in properties, and that there were gaps in this pattern - therefore predicting new elements would be found.
In 1891 Mendeleev's table still did not contain the noble gases (group 0). Later investigations confirmed his idea of periodicy and its patterns, also the predictions he had previously made.
Group 1 metals all have similar properties because they have 1 electron in outer shell:
- They react vigorously with water
- Hydrogen gas is given off
- The metal reacts with water to form an alkali- the hydroxide of the metal
e.g lithium + water => lithium hydroxide + hydrogen
The reactivity of the alkali metals with water increases down group 1.
Alkali metals react to loose 1 electron to have a stable electronic structure and to become a positive ion. The more reactive the alkali metal the easier it is to loose the electron.
Flame tests can be used to find out if Lithium, sodium or potassium are in a compound.
- A flame test wire is moistened with dilute hydrochloric acid
- The flame test wire is dipped in the solid chemical
- Then it is held over a blue bunsen burner flame
- Colour of the flame is recorded
Lithium = red flame
Sodium = yellow flame
Potassium = lilac flame
Halogens, trends and predictions
At room temperature chlorine = green gas, bromine = orange liquid, iodine = grey solid.
It possible to predict the properties of halogens knowing the properties of other halogens - this is because they follow trends which can be shown in tables.
They are similar because when they react they all gain one electron to become stable. The nearer the outer shell is to the nucleus the easier it is for the atom to gain one electron. The easier it is to gain, the more reactive the halogen.
When halogens react with alkali metals, a metal halide is made.
- e.g potassium + iodine => potassium iodide
- 2K + l2 => 2Kl
The balanced symbol equation is the same for each alkali metal reacting with a halogen:
- e.g Lithium + bromide => lithium bromide
- 2Li + Br2 => 2LiBr
If halogens are bubbled through solutions of metal halides then they either have no reaction or a displacement reaction.
- e.g Chlorine displaces the bromide to form bromine solution
- chlorine + potassium bromide => potassium chloride + bromine (orange solution)
- Cl2 + 2KBr => 2KCl + Br2 (balanced)
The trend in reactivity is shown by displacement reactions. The reactivity of the halogens increases further up the group. Knowing this trend allows the reactions between halogens and metal halides to be predicted.
Transition metals and thermal decomposition
Compounds that contain a transition metal are often coloured:
- Copper compounds = blue
- Iron (II) compunds = pale green
- Iron (III) compounds = orange/brown
Tranisition elements (and their compounds) can be catalysts
- Iron is used in the haber process to make ammonia
- Nickel is used in the manufacture of margarine
If a transition metal carbonate is heated it undergoes thermal decomposition to form a metal oxide and carbon dioxide.
- e.g copper carbonate => copper oxide + carbon dioxide
- CuCO3 => CuCO + CO2
Properties of metal
- Thermal conductivity of copper is high therefore used as saucepan bases
- Chemcial property of gold is that it is not reactive with oxygen or acids
- Physcial properties also may be lustrous and shiny, malleable or ductile
- Aluminium has a low density therefore important in aircraft industries and modern car making
Metals have high melting points and high boiling points due to strong metallic bonds - which are very hard to break and need a lot of energy to be able to break atoms apart. A metallic bond is a strong electrostatic force of attraction between closely packed positive metal ions and a 'sea' of delocalised electrons.
A metal conducts electricity because the delocalised electrons can move easily in its structure.
Superconductors are metals that can conduct electricity with little/no resistance.
When a substance goes from its normal stare to a superconducting state, it no longer has any magnetic fields inside it.
- If a small magnet goes near a superconductor it is repelled
- If a small permanent magnet is placed above the superconductor, it levitates
Benefits of superconductors are: loss free power transmission, super fast circuits, powerful electromagnets.
Difficulties are that they only work at very low temperatures which limits their use and superconductors which function at 20C need to be developed.
3 main stages:
- Sedimentation - chemicals added to make solid particles and bacteria settle out
- Filtrations of very fine particles - a layer of sand or gravel filters out the remaining fine particles; some types of sand also removes microbes
- Chlorination - kills microbes
Some soluble substances such as pesticides and nitrates are not removed by the standard purification method (as they are dissolved in)- extra processes are needed.Sea water has so many dissolved substances that it is undrinkable and needs techniques like distillation to remove them. Distillation takes huge amounts of energy so it is expensive and only used when there is no fresh water.
Sulfates in water can be tested using barium chloride, halides can be tested using silver nitrate.