Structure Of An Atom
Atoms: Protons and neutrons are in the central nucleus.
Electrons are in shells around the nucleus.
An atom has the same number of protons and electrons.
The + charge of a proton balances the - charge = An atom has no overall charge!
Nucleus is a smaller size compared to the overall size of the atom.
Number of protons determine which element the atom is part of.
Particle Relative Mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 1/1840 -1
The atomic number of an element is the number of protons in the nucleus.
The mass number is the total number of protons and neutrons in the nucleus.
The number of neutrons=mass number-atomic number.
Isotopes are atoms of the same elements with a different number of neutrons.
This means that some relative atomic masses are not whole numbers.
The Periodic Table & Medeleev
The groups contain elements with similar properties.
Metals are on the left-hand side and centre.
Non-metals are on the right-hand side.
There are 7 groups and 7 periods.
1 --------- Relative atomic mass
H--------- Element symbol
1---------- Atomic number
- Made the periodic table.
- Put elements in order of relative atomic mass.
- Checked the properties of the elements and compounds.
- Swapped some elements so that elements with similar properties lined up.
- He left gaps where he thought there were other elements and predicted their properties.
Shell configuration and Electron groups
The electrons are arranged in shells around the nucleus.
The symbol is in the middle of the shell, the first shell holds 2 electrons, the second and third
shell holds 8 electrons and the electrons are drawn as dots or crosses
The group the element belongs to is the same as the number of electrons it has, and the period of the element tells you which period it belongs to.
Ions- Cations & Anions
An atom becomes and ion by losing or gaining 1 or more electron.
An ion does have a charge because it no longer has the same number of protons and electrons.
We show that an ion is an ion by putting either a + or - sign next to the symbol.
Cations are positive Anions are negative
- Metals lose an electron to become - Non metal atoms gain electrons to
a positively charged cations. form a negatively charged anion.
- Metals in group 1 lose 1 electron to form an - Elements in group 6 gain 2 electrons
Ion with a 1+ charge. to form ions with a 2- charge.
-Metals in group 2 lose 2 electrons to form an - Elements in group 7 gain 1 electron to
Ion with a 2+ charge. form an ion with a 1- charge.
E.g- When sodium and chlorine react, soduim loses electrons and chlorine gains an electron.
They now have opposite charges- This makes them attract ( Think of a magnet!) This creates an ionic bond between the ions and a compound is formed.
An ionic compound formed between a metal and a non metal is called a salt.
Compound Ions- Some groups of atoms can form ions, this is called compound ions.
The name of the compiund tells you about the element in it e.g sodium chloride contains sodium and chlorine in the form of ions.
If a compound ends in "ate" oxygen is present in that compound, E.g Calcium carbonate- Calcuim, carbon and oxygen. If a compound ends in "Ide" no extra oxygen is present, e.g copper sulfide contains copper and sulfur.
Formula of a Compound
You must be able to find out the formula of a compound if you are given the ions and their charges!
Easy method to remember:
1. Swap the numbers that give the charge for each ion and write then after the relevant symbol.
2. If the number is 1, do not write it in the formula.
3. If the numbers are the same, do not write them in the formula.
Properties of Ionic Compounds
Melting and boiling points
Ionic substances have high melting points and high boiling points, this is because the ions are held together in the lattice by very strong electrostatic forces.
Ionic substances are solids at room temperature and have to be heated strongly to make them melt.
A substance only conducts electricity if it contains charged particles that are free to move.
Ionic substances do not conduct electricity when they are solid because the ions are held together in a lattice.
They conduct electricity when they are molten or dissolved in water because the particles can move around.
Insoluble in water Soluble in water Soluble in water Insoluble in water
Most carbonates -Sodium carbonate All common salts:
-Potassium carbonate - Sodium
-Ammonium carbonate - Potassium
Most hydroxides -Sodium hydroxide All Nitrates
- Ammonium Hydroxide Most chlorides - SIlver chloride
- Lead chloride
Most sulfates - Lead sulfate
- Barium sulfate
- Clacium sulfate
If you mix solutions of 2 soluble salts, the ions in the mixture can combine to form new salts.
If one of the new combinations produces an insoluble salt, it will appear as a precipitate.
Pure samples of insoluble salts can be prepared using precipitation reactions.
1. Mix solutions of 2 substances that will form the insoluble salt.
2. Filter the mixture, the insoluble salt will be trapped in the filter paper.
3. Wash the salt with pure water.
4. Leave to dry on the filter paper.
Ion tests- Flame Tests
The flame test is used to determine which ion is present.
A damp splint is placed into the solid and then held under a bunsen burner flame. The colour of the flame will identify the ion present.
Metal ion Flame colour
Sodium (Na+) - Yellow
Potassium (K+) - Lilac
Calcium (Ca2+) - Red ( Brick red)
Copper ( Cu2+)- Green/ Blue
Molecules consist of 2 or more atoms chemically joined together.
The atoms are held together by covalent bonds.
A covalent bond is a pair of electrons that is shared between two atoms.
Covalent bonding take places between non metal atoms.
Dot and cross diagrams
We can show the formation of simple molecular covalent substances using dot and cross diagrams.
The electrons in the atoms are all the same, we use dot and crosses to help us see which electron came from which atom.
Substances with covalent bonds can form small molecules or giant structures. The two types of substances have different properties.
Simple molecular covalent bonds
There are only weak forces between the moelcules. These substances have low melting points and low boiling points. These substances are liquids or gases at room temperature or solids with low melting points e.g Wax, sugar or iodine. There are no charged particles so they do not conduct electricity.
Giant molecular covalent substances
Diamond and graphite are giant molecular covalent substances that are joined up by billions of atoms.
Diamond- Is used as a cutting tool because it is very strong, it has high melting and boiling points but it does not conduct electricity.
Graphite- Similar to diamond but it is much softer and it does conduct electricity.
Alkali metals are in group 1.
They are very soft and can be cut easily.
They have low melting points ( they are very easy to melt)
Reactions with water:
The alkali metals are all in the same group of the periodic table so that they have similar reactions. They all have 1 electron in the outer shell so when they react each atom loses 1 electron to become an ion with a 1+ charge.
The reactivity increases as you go down the group, this is their reactions in water.
- Lithium floats on the water and fizzes.
- Sodium melts from the heat and whizzes around on the surface as a molten ball ( sometimes catches fire)
- Potassium reacts even faster and the hydrogen produced burns with a lillac flame.
Halogens are in group 7.
They have similar reactions to each other because if they gain 1 electron they complete their outer shell.
They react with metals to form compounds called halides.
Flourine is the mst reactive halogen and the reactivity decreases as you go down the group.
Properties of Halogens
Element Symbol State at room temp Colour
Flourine F Gas Pale Yellow
Chlorine Cl Gas Yellow-Green
Bromine Br Liquid Red-Brown
Iodine I Solid Grey
The noble gases are the elements in the group 0 of the periodic table.
They all have a full outer shell of electrons. This is the most stable arrangement of electrons.
Other elements ahcieve this by losing/ gaining electrons in covalent bonds, this is why the noble gases are inert compared with other elements. This means it is very difficult for them to react.
Discovery of Noble gases:
Lord Rayleigh noticed that the density of the nitrogen made in the reaction was lower then the density of nitrogen obtained from the air.
William Ramsey thought that the air might also contain a denser gas that was mixed with the nitrogen.
Rayleigh and Ramsey crried out some careful experiments and discovered the gas, argon.
Ramsey also discovered helium, he later discovered neon, krypton and xenon.
Endothemic and Exothermic Reactions
Exothermic reactions- Heat energy is given out ( makes the surrounding hotter)
Endothermic reactions- Heat energy is taken in ( the reaction mixture gets colder)
Bonds and energy: When a reaction takes place, the bonds that hold the atoms together are broken. The atom can then come together in new arrangements to form the products.
Breaking bonds is endothermic ( Energy is needed)
Making bonds is exothermic ( Energy is released)
Rates of Reactions 1
Reactions happen when particles of different substances collide with each other. The particles need to collide with enough energy to make a reaction happen, so not all collisions result in a reaction. The higher the frequency of collisions, and the higher the energy of the collisions, the faster the reaction.
Temperature: Reactions happen faster when the temperature is higher. Particles move faster at higher temperatures so they collide more often and with a lot more energy.
Surface area: Reactions happen faster when solid reactants are broken up into smaller pieces. Smaller pieces have a bigger surface than the same mass of larger pieces, so there is more oppurtunity for collisions between reactions.
Concentration: Reactions happen faster when more concentrated solutions are used. A more concentrated solution has more solute particles in a given volume, the more particles there are, the more likely they are to collide and react
Catalyst: This speeds up a reaction without being used up itself.
These are used in cars to reduce pollution caused by waste gases from the engine.
Carbon monoxide and unburned fuel in the exhaust combine with oxygen to form carbon dioxide and water.
The platnium catalyst in converters is made into a fine mesh to give it a larger surface area. This allows more of the waste gas product to come into contact with the catalsyt and so helps to speed up the reaction.
catalystic converters work best at high temperatures.
Relative Masses and Formulae
The masses of atoms are so small that we do not talk about their mass in kilograms. Instead we use Realtive Atomic Mass (Ar). The Relative Formula Mass (Mr) is the sum of the relative atomic masses of all the atoms and ions in its formula.
Ca(NO3)2= 1 X Ca + 2 X (1 X N + 3 X 0)
= 1 X Ca + 2 X N + 6 X 0
Mr= 40 + (2 X 14) + ( 6 X 16 )
The empirical formula of the substance is the simplest whole number ratio of atoms or ions of each element in the substance. The empirical formulaof soduim chloride is NaCl. This means that for every ion of sodium, there is one chloride ion.
The molecular formula is the actual number of atoms of each element in the molecule.
How to find the empirical formulae
1.Divide each mass by the relative atomic mass of the element.
2. Divide both numbers by the smallest number to find the ratio.
The percentage compostion by mass of a compound id a way of working out how much of an element is present in a compound.
Percentage by mass
of an element = number of atoms X Relative atomic mass ( divided ) Relative formula mass X 100
in a compound
Masses of Reactants and Products
No atoms are created or destroyed in a reaction, so the total mass of the products must be equal to the total mass of the reactants.
You can use balanced equations to work out the masses of other substances involved.
1. Always start with balanced equations, you do not need state symbols.
2. Work out the relative masses and mulitply by the balancing numbers.
3. Divide by the numbers for the element to find the mass produced.
4. Mulitply the mass by the amount given in the question.
The yield of a reaction is the amount of useful product you get at the end.
Theoretical yield: Calculate the mass of the product you expect to get from a reaction using the balanced equation and the relative masses of the compounds.
Actual yield: The amount actually produced in a reaction.
The formula is: Percentage yield = Actual yield divided by theoretical yield X 100%
We dont always get a full yield because:
1. A reaction does not always finish- some reactants could be left over at the end.
2. Some reactants or products can be lost during the process.
3. There may be unwanted reactions taking place- Some reactants may react different to make a different product.
Waste and Profit
Explain 3 ways in which chemist industries try to find the most economically favourable reactions...
1. Reactions with high yields so that a lot of product is made in a reaction.
2. Reactions where all the products are useful as they will make more money.
3. Reactions hat occur failry quickly so that useful products can be made quickly.
Disposing waste products:
- House prices could drop if new plants are built for waste.
- Running out of landfill sites.
- Some waste products are harmful and cost money to treat safely.
- Smoke from incinerators causes pollution.
- Lorries taking waste to sites cause dust and disturbance.
- Many waste products that are not harmful still need disposing of, this costs money.
- People do not like living near landfill sites or incinerators