Chemistry

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Atomic Structure and The Periodic Table

Atoms

Atoms are made up of protons, neutrons and electrons.

The nucleus is in the middle and it is made up of protons and neutrons. It is positively charged because of the protons. The whole mass of the atom is concentrated into the nucleus. It has an approximate radius of 0.000000000000001m.

The electrons move around the nucleus in electron shells. They are negatively charged and are tiny but cover up a lot of space in the atom. The volume of their orbit shows the size of the atom. They have a very small mass which is often taken as zero.

Atoms are neutral which means that they overall have no charge - the same number of protons (positive) and electrons (negative) so they cancel each other out. In an ion the number of protons and the number of electrons are different so ions are not neutral.

The nuclear symbol of an atom tells you its atomic number and mass number. The atomic number shows how many protons there are. The mass number shows the total number of protons and neutrons in the atom.

Elements

An element is a substance made up of atoms that will have the same number of protons in their nucleus.

Atoms can have different numbers of protons, neutrons and electrons. It is the amount of protons in the nucleus that decide what type of atom it is. For example: an atom with one proton is hydrogen and two protons is helium. If a substance only contains atoms with the same number of protons it is an element - there are around 100 different elements. Same elements = same number of protons, different elements = different amount of protons.

Atoms can be represented by symbols. There are simple ones and complicated ones. For example: C = carbon, O = oxygen, Mg = magnesium (simple), Na = sodium, Fe = iron, Pb = lead (complicated). These are on the periodic table.

Isotopes are different forms of the same element - same number of protons, different number of neutrons. They have the same atomic number but different mass number.

Relative atomic mass is used instead of mass number when referring to the element as a whole. Relative atomic mass = average mass taking different masses and abundances (amounts) of all the isotopes that make the element into account.

Formula to work out the relative atomic mass :

Compounds

Elements can mix and match to form new substances called compounds.

Elements react - atoms combine with other atoms to form compounds.

Compounds: substances formed from two or more elements, the atoms of each are in fixed proportions and are held together by chemical bonds.

Making bonds can be atoms giving away, sharing or taking electrons. The nuclei of the atoms are not involved, only the electrons are affected.

Chemical reactions are needed to separate the elements out from a compound and it is usually very difficult.

A compound formed from metals and non-metals are made up of ions. Metal atoms lose electrons to form positive ions. Non-metal atoms gain electrons to form negative ions. The opposite charges of the ions mean that they are strongly attracted to each other - ionic bonding.

A compound formed from non-metals are made up of molecules. Each atom shares an electron with another - covalent bonding.

The properties of compounds are different to the original elements.

A formula show what atoms are in a compound. For example: Carbon Dioxide - CO2, Sodium Chloride - NaCl, Calcium Chloride - CaCl2.

Chemical Equations

Chemical changes are shown using chemical equations.

Word equations. For example: You are told that methane burns in oxygen giving carbon dioxide and water: methane + oxygen -- carbon dioxide + water. The molecules on the left hand side are reactants, the molecules on the right hand side are products.

Symbol equations. For example: (magnesium + oxygen -- magnesium oxide) = (2Mg + O2 -- 2MgO). This is the shorthand way of word equations.

Symbol equations need to be balanced. Same number of atoms on both sides - they can't just disappear. You balance equations by putting numbers before the formulas.

Mixtures and Chromatography

Mixtures are easily separated unlike compounds. It is a lot of substances mixed together.

There is no chemical bond between the different parts of a mixture. The parts can be either elements or compounds and can be separated using physical separation methods such as filtration, crystallisation, simple distillation, fractional distillation and chromatography.

Air is a mixture of gases such as nitrogen, oxygen carbon dioxide and argon. Crude oil is a mixture of different length hydrocarbon molecules.

The properties of a mixture are the same as the properties of the substances mixed together, the chemical properties are not affected by the substance being in a mixture.

Chromatography can be used to separate different dyes in an ink.

Draw a line near the bottom of a sheet of filter paper. (Use a pencil because pencil marks are insoluble and will not dissolve in the solvent.)

Add a spot of the ink to the line and place the sheet in a beaker of solvent, e.g. water.

The solvent used depends on what is being tested. Some compounds dissolve well in water whereas other times other solvents such as ethanol are needed.

Make sure the ink is not touching the solvent.

Place the lid on the container to the solvent from evaporating.

The solvent seeps up the paper, carries ink up with it.

Different dye in the ink will travel up the paper at different rates so the dyes will separate out. Each dye will form a spot in a different place - one spot per dye.

If any of the dyes are insoluble they will stay at the baseline.

When the solvent has nearly reached the top of the paper, take the paper out and leave to dry.

The end result is a patter on spots called a chromatogram.

More Separation Techniques (Practical)

Chemists use separation techniques such as filtration and crystallisation to separate solids from liquids.

Filtration can be used if your product is an insoluble solid that needs to be separated from a liquid. It can be used in purification as well. For example, solid impurities in the reaction mixture can be separated from filtration.

You can use evaporation to separate a soluble salt from a solution.

Pour the solution into an evaporating dish.

Slowly heat the solution. The solvent will evaporate and the solution will become more concentrated. Eventually crystals will form.

Keep heating the evaporating dish until all you have are dry crystals.

Evaporation is a quick way of separating soluble salt from a solution but only works if the salt does not decompose or breakdown when it is heated. Otherwise, you will have to use crystallisation.

Pour the solution into an evaporating dish and gently heat the solution. Some of the solvent will evaporate and the solution will get more concentrated.

When some of the solvent has evaporated or crystals start to form, remove the dish from the heat and leave it to cool.

The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution.

Filter the crystals out of the solution and leave them in a warm place to dry.

Filtration and crystallisation can be used to separate rock salt.

Rock salt - mixture of salt and sand. Salt and sand are both compounds but salt dissolves in water, sand doesn't.

Distillation

Distillation is used to separate mixtures which contain liquids. Simple distillation is used to separate out solutions.

The solution is heated. The part of the solution that has a low boiling point evaporates first.

The vapour is cooled, condensed and then is collected.

The rest of the solution is left in the flask.

You can use simple distillation to separate pure water from sea water. The water evaporates and is condensed and collected. You will end up with salt in the flask.

Problem with simple distillation - only use it to separate things with very different boiling points.

Fractional distillation is used to separate a mixture of liquids.

Put your mixture in a flask and stick a fractionating column on top. Heat it.

The different liquids will all have different boiling points - evaporate at different temperatures.

Liquid with the lowest boiling point will evaporate first. When the temperature on the thermometer matches the boiling point of the liquid, it will reach the top of the column.

Liquids with higher boiling points might also start to evaporate. But the column is cooler towards the top so they will only get part of the way up before condensing and come back to the flask.

When the first liquid has been collected, you raise the temperature until the next liquid reaches the top.

The History of the Atom

The theory of the atomic structure has changed over time.

Start of 19th Century - John Dalton described atoms as solid spheres and said that different spheres made up different elements.

1897 - J J Thomson concluded from his experiments that atoms were not solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles - electrons. The new theory was known as the 'Plum Pudding Model'. The 'Plum Pudding Model' showed the atom as a ball of positive charge with electrons in it.

1909 - Ernest Rutherford and his student Ernest Marsden conducted the 'alpha particle scattering experiments'. They fired positively charged alpha particles at an thin sheet of gold.

From the 'Plum Pudding Model' they were expecting the particles to pass through the sheet or be slightly deflected. This was because the positive charge of each atom was thought to be very spread out through the 'pudding' of the atom. Whilst most of the particles did go straight through the gold sheet, some were deflected more than expected and a small number were deflected backwards. 'Plum Pudding Model' = wrong.

Rutherford came up with the 'nuclear model' of an atom to explain this new evidence. Nuclear model - a tiny, positively charged nucleus at the centre (most of the mass is concentrated). A 'cloud' of negative electrons surround nucleus (most of the atom is empty space). When alpha particles came near the concentrated, positive charge of the nucleus, they were deflected.

Bohr's Nuclear Model.

Suggested that all the electrons were contained in shells.

He proposed that electrons orbit the nucleus in fixed shells, each shell is a fixed distance from the nucleus.

Further experiments showed the existence of protons.

Rutherford and others showed that the nucleus can be divided into smaller particles, which have the same charge as the hydrogen nucleus. These are called protons.

About 20 years after scientists had accepted that atoms had nuclei, James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus - neutrons.

Electronic Structure

Electron Shell rules:

Electrons always occupy shells (sometimes called energy levels).

The lowest energy levels are always filled first - closest to nucleus.

Only a certain number of electrons in each shell: 1st shell - 2, 2nd shell - 8, 3rd shell - 8, 4th shell - 8.

Atoms are better when they have full outer shells. For example: noble gases in Group 0.

In most atoms, the outer shell is not full, makes the atom react to fill it.

Development of the Periodic Table

In the early 1800s elements were arranged by atomic mass. Until quite recently there were two ways to categorise elements: their physical and chemical properties and their relative atomic mass.

Scientists had no idea of atomic structure or of protons, neutrons or electrons so there was no atomic number.

Back then, only thing that could be measured was relative atomic mass and so the known elements were arranged in order of atomic mass. A periodic pattern was noticed in the properties of the elements.

Early periodic tables were not complete and some elements were placed in the wrong group - elements were placed in the order of relative atomic mass and did not look at properties.

Dmitri Mendeleev left gaps and predicted new elements.

1869 - Dmitri Mendeleev took 50 known elements and arranged them into his 'Table of Elements' - with various gaps.

He put the elements mainly in order of atomic mass but switched the order if the properties meant it should be changed.

Gaps were left in the table to make sure that elements with similar properties stayed in the same groups. Some of these gaps indicated the existence of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and fitted the pattern it helped confirm Mendeleev's ideas.

The Modern Periodic Table

The periodic table helps to see patterns in properties.

There are about 100 elements which all materials are made from.

Elements are laid out in the order of increasing atomic (proton) number - there are repeating patterns in the properties of the elements.

Metals are found in the left and non-metals are found in the right.

Elements with similar properties - columns. These vertical columns are called groups.

The group number - how many electrons there are in the outer shell - the way atoms react depends upon the number of electrons in the outer shell. Elements in the same group react the same way usually.

You can predict properties and reactivity from one element.

Rows are called periods. Each new period represents another full shell of electrons.

Metals and Non-Metals

Most elements are metals.

Metals are elements - form positive ions when they react.

Towards the bottom and left of the periodic table.

Non-metals are at the far right and at the top of the periodic table.

Non-metals don't generally form positive ions when they react.

The electronic structure of atoms affects how they will react.

Atoms generally react to form a full outer shell - losing, gaining or sharing electrons.

Metals to the left of the periodic table don't have many electrons to remove, metals towards the bottom have outer electrons which are a long way from the nucleus so feel a weaker attraction - not much energy is needed to remove the electrons, feasible for the elements to react to form positive ions.

Non-metals have a lot of electrons to remove to get a full outer shell or there is a strong attraction between the nucleus and the outer most electron. It is easier to gain or share an electron.

All metals have metallic bonding which causes them to have similar properties.

Strong (hard to break), but can be bent or hammered into different shapes (malleable).

Great conductors of heat and electricity.

High boiling and melting points.

Non-metals are the opposite - dull, aren't always solids, don't conduct electricity and have lower density.

Transition metals can be found between Group 2 and Group 3.

Centre of the periodic table.

Typical metals, good conductors of heat and electricity, dense, strong and shiny.

Can have more than one ion.

Often coloured, and compounds that contain them are colourful.

Make good catalysts.

Group 1 Elements

Group 1 elements are known as the alkali metals. They are reactive.

Lithium, sodium, potassium, rubidium, caesium and francium.

They all have one electron in their outer shell - very reactive and similar properties.

As you go down the Group 1 elements...

Reactivity increases - outer electron is more easily lost as the attraction between the nucleus and electron decreases because the electron is further away from the nucleus the further down you go in the group.

Lower melting and boiling points.

Higher relative atomic mass.

The Group 1 elements don't need much energy to lose their one outer electron to form a full outer shell, so they readily form 1+ ions. Easy for them to lose their outer electron so they only ever react to form ionic compounds - generally white solids that dissolve in water to form colourless solutions.

Reaction with water:

React vigorously - produce hydrogen gas and metal hydroxide.

The more reactive an alkali, the more violent the reaction.

Amount of energy given out by the reaction increases down the group.

Reaction with chlorine:

Form metal chloride salts.

Reactivity increases down the group, reaction is more vigorous.

Reaction with oxygen:

Form metal oxide.

Group 1 metals are more reactive than transition metals. They are less dense, strong, hard and have much lower melting points than transition metals.

Group 7 Elements

Group 7 elements are known as halogens. The halogens are all non-metals with coloured vapours.

Fluorine - very reactive, poisonous yellow gas.

Chlorine - fairly reactive, poisonous dense green gas.

Bromine - dense, poisonous,red-brown volatile liquid.

Iodine - dark grey crystalline solid or a purple vapour.

They all exist as molecules which are pairs of atoms.

As you go down the Group 7 elements...

Less reactive - harder to gain an extra electron because the outer shell's further from the nucleus.

Higher melting and boiling points.

Higher relative atomic mass.

Halogen atoms can share electrons - covalent bonding with other non-metals to achieve full outer shell.

The halogens form 1- ions called halides when they bond with metals. The compounds that form have ionic structures.

A displacement reaction can occur between a more reactive halogen and the salt of a less reactive one.

Group 0 Elements

Group 0 Elements are known as noble gases. They don't react very much.

Helium, Neon, Argon, Krypton, Xenon and Radon.

Eight electrons in the outer shell, apart from Helium which has two, giving them a full outer shell - they don't need to give up or gain electrons, don't react that much.

Monatomic gases - single atoms not bonded to each other.

Non-flammable.

As you go down the Group 0 elements...

Boiling points increase - increase in the number of electrons leading to greater intermolecular forces between them.

Increasing relative atomic mass.