Atomic structure + periodic table

- Radius of atoms = 0.1 nanometers ( 1 x 10 to neg 10) 

Nucleus:

- Middle of atom 

- Contains proteins and neutrons

- Positive charge bc protons 

- Almost whole mass of atoms is concentrated in nucleus 

Electrons:

- Move around the nucleus in electon shells

- Neg charge and tiny, but cover lots of space

- Vol of their obits determines the size of atom

- Electrons have virtually no mass 

- Atoms are neutral - no overall charge

- Bc they have same number of protons as electrons 

- Charge on the electrons is the same as charge on protons, but opposite so charge doesn't cancel out

- In an ion, the number of protons doesn't equal the number of electrons, means it has an overall charge. E.g. ion with 2- charge has two more electrons than protons 

Atomic number and mass number 

- Nucleur symbol of an atom tells you its atomic (proton no) and mass number 

- Atomic number tells you how many protons there are

- Mass no tells you the total no of protons and neutrons in the atom

- To get the no of neutrons, just subtract the atomic number from the mass number 

- Atoms can have dif numbers of protons, neutrons and electrons. It's the number of protons in nucleus decides what type of atom it is

- For example, an atom with one proton in its nucleus is hydrogen and an atom with 2 protons is helium 

- If a substance only contains atoms with the same number of protons its called an element, there about 100 of these

- So all the atoms of a particular element have the same no of protons and dif elements have atoms with dif no of protons 

- Atoms of each element can be represented by one or two letter symbols - it's a type of shorthand

- Dif forms of the same element which have the same number of protons but a dif no of electrons 

- Isotopes have the same atomic number but dif mass number

- Very popular example of a pair of isotopes are carbon 12 (6 p, n and e) and carbon 13 (6 p and e, 7 n)

- Bc many elements can exist as a no of dif isotopes, relative atomic mass is used instead of mass no when referring to the element as a whole. This is an average mass taking into account the dif masses and amounts of all the isotopes that make up the element

- Use this formula to find relative atomic mass = (sum of isotopes x isotope mass no) / sum of abundances of all the isotopes 

- When elements react, atoms combine with other atoms to form compounds 

- Compounds are substances formed from 2 or more elements, the atoms of each are in fixed proportions throughout the compound and they're held together by chemical bonds

- Making bonds involves atoms giving away, taking or sharing electrons. Only the electrons are involved - the nuclei of the atoms aren't affected at all when a bond is made

- It's usually dif to seperate the original elements of a compound out again - a chem reaction is needed to do this

- A compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to form pos ions, and the non-metal atoms gain electrons to form neg ions. The opposite charges of ions mean that they're strongly attracted to each other. This is called ionic bonding. Examples of compounds which are ionically bonded are sodium chloride, magnesium oxide and calcium oxide

- A compound formed from non-metals consists of molecules. Each atom shares an electron with another atom - this is called covalent bonding. Examples of compounds that are bonded covalently include hyrogen chloride gas, carbon monoxide and water

- The properties of a compound are usually totally dif from the properties of original elements e.g. if irom (lustrous magnetic metal) and sulfur (yellow powder) react, the compound formed (iron sulfate) is a dull grey solid lump amd doesn't behave like it's parents

- Fomulas made up of elemental symbols in the same proportions that the elements can be found in the compound

- E.g. CO2 is a compound formed from a chem reaction between carbon and oxygen. It contains one carbon atom and two oxygen atoms 

E.g. the formula of sulfuric acid H2SO4 so each molecule contains 2 hydrogen atoms, one sulfur and 4 oxygen atoms 

- There might be brackets in a formula e.g. calcium hydroxide is Ca(OH)2. The little no outside the bracket applies to everything inside braket, so there's one calcium atom, 2 oxygen atom and 2 hydrogen atoms in it 

- No chem bonds between dif parts of mixture

- The parts of a mixture can be either elements or compounds, and they can be seperated out by physical methods e.g. filtration, crystallisation, simple distillation, fractional distillation and chromatography

- Air is a mixture of gases, mainly nitrogen, oxygen, carbon dioxide and argon. These gases can be easily seperated

- Crude oil is a mixture of different length hydrocarbon molecules 

- The properties of a mixture are just a mixture of the properties of the seperate parts - the chem properties of a substance aren't affected by it being part of a mixture 

- E.g. a mix of iron powder and sulfur powder will show properties of both iron and sulfur. It will contain grey magnetic bits and bright yellow bits of sulfur 

- Method of seperation in a mix is through chromatography - used to seperate ink 

- Draw a line near the bottom of the filter paper (pencil is insoluble and won't dissolve in solvent

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

- The solvent used depends on what's being tested - some compounds dissolve well in water, but sometimes other solvents, like ethanol, are needed

- Ensure ink isn't touching solvent - don't want it to dissolve into it 

- Place a lid on top of the container to stop the solvent evaporating 

- The solvent seeps up the paper, carrying ink with it 

- Each dif dye in the ink will move up the paper at a different rate so dyes will seperate out. Each dye will form a spot in a dif place - 1 spot per dye in the ink

- If any of the dyes in the ink are insoluble in the solvent, they'll stay on the baseline 

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

- The end result is a pattern of spots called chromatography 

Filtration:

- Can be used if your product is an insoluble solid that needs to be seperated from a liquid reaction mixture

- It can be used in purification as well, for example, solid impurifies in the reaction mixture can be seperated out using filtration

If a solid can be dissolved it is soluble - 2 methods to seperate soluble salt from solution: 

Evaporation:

- Pour solution into evaporating dish

- Slowly heat solution, solvent will evaporate and the solution will get more concentrated, eventually crystals will start to form

- Keep heating evap dish until all that's left are dry crystals 

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

- Once some of the solvent has evaporated, or when you see crystals start to form (the point of crystallisation), remove the dish from the heat and leave the solution 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. Drying oven or desiccator

- Rock salt is a mixture of salt and sand 

- Salt and sand are both compounds - but salt dissolves in water and sand doesn't - this vital difference in their physical properties gives a good way to seperate them

- Grind the mixture to make sure the salt crystals are small, so will dissolve easily

- Put the mixture in water and stir, the salt will dissolve, but the sand won't 

- Filter the mixture, the grains of salt won't fit through the tiny holes in the filter paper, so they collect on the paper instead. The salt passes through the filter paper as it's part of the solution

- Evaporate the water from the salt so that it forms dry crystals 

- Simple distillation is used for seperating out a liquid from a solution

- The solution is heated, the part of the solution that has the lowest boiling point evaps first

- The vapour is then cooled, condenses and is collected

- The rest of solution is left behind in the flask 

- You can use simple distillation to get pure water from seawater. The water evaps and is condensed and collected - eventually you'll end up with just salt in the flask

- The problem with this method is you can only use it to seperate things with very dif boiling points - if the temp goes higher than the boiling point of the substance with the higher boiling point, they will mix again

- If you have a mixture of liquids with similar boiling points you need fractional distillation

- If you've got a mixture of liquids you can seperate it using fractional distillation. e.g. fractional distillation of crude oil at a refinery

- Put mixture in a flask and stick a fractioning column at the top, then heat it

- The dif liquids will all have dif boiling points - so they will evap at dif temps

- The liquid with the lowest boiling point evaps first, when the temp on the thermometer matches the boiling point of this liquid, it will reach the top of the column

- Liquids with higher boiling points might also start to evap. But the column is cooler towards the top. So they will only get part of the way up before condensing and running back down towards the flask

- When the first liquid has been collected, you raise the temp until the next reaches the top 

- At the start of the 19th century, John Dalton described atoms as solid spheres, and said that different spheres made up the dif elements

- In 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 'solid sphere' idea of atomic structure had to be changed. The new theory was known as the 'plum pudding model' 

- The plum pudding model showed the atom as a ball of pos charge with electrons stuck in it 

- In 1909 Ernest Rutherford and his student Ernest Marsden conducted the famous alpha particle scattering experiments, they fired pos charged alpha particles at an extremley thin sheet of gold

- From the plum pudding model, the were expecting the particles to pass straight though the sheet or be slightly deflected at most. This was because the pos charge of each atom was thought to be very spread out though the 'pudding' of the atom. But, whilst most of the particles did go straight though the gold sheet, some were deflected more than expected, and a small number were deflected backwards - so plum pudding couldn't be right

- Rutherford came up with the an idea to explain this new evidence - the nucleur model of the atom. In this, there's a tiny, pos chargded nucleus in the centre, where most of the mass is concentrated. A 'cloud' of neg electrons surrounds this nucleus - so most of the atom is empty space, when alpha particles came near the concentrated, pos charge of the nucleus, they were deflected. If they were fired directly at the nucleus, they were deflected backwards - otherwise they passed through the empty space 

Bohr:

- Scientists realised that electrons in a cloud around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse. Bohr's nucleur model of the atom suggested that all the electrons were contained in shells

- Bohr proposed the electrons orbit the nucleus in fixed shells and aren't anywhere in between. Each shell is a fixed distance from the nucleus 

- Bohr's theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists observations at the time

- Further experiments by Rutherford and others showed that the nucleus can be divided into smaller particles, which each have the same charge as hydrogen nucleus. These particles were named protons 

- About 20 years after scientists had accepted the atoms have nuclei, James Chadwick carried out an experiment which provided evidence for nucleus which are now called neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model 

Electron shell rules:

- Electrons always occupy shells (energy levels)

- The lowest energy levels are always filled first - these are the closted to the nucleus 

- Only a certain number of electrons are allowed in each shell. 1st shell - 2, 2nd shell 8 and 3rd shell, 8 

- Atoms are happier when they have a full electron shell - like the noble gases in group 0 

- In most atoms, the outer shell is not full and this makes the atom want to react to fill it

- Until quite recently, there were 2 obvious ways to categorise elements. Their physcial and chem properties and relative atomic mass

- The scientist has no idea of atomic structures or of protons, neutrons or electrons, so there were no such thing as atomic number to them. 

- Back then, the only thing they could measure was relative atomic mass, and so the known elements were arranged in order of atomic mass. When this was done a periodic pattern was noticed in the properties of the elements. This is were the name periodic table came from        

- Early periodic table was not complete and some elements were placed in the wrong group. This is bc elements were placed in the order of relative atomic mass and did not take into account their properties 

- In 1869, DM overcame some of the problems of early periodic tables by taking 50 known  elements, and arranging them into his Table of Elements - with various gaps

- Mendeleev put the elements mainly in order of atomic mass but did switch that order if the properties meant it should be changed. An example of this can be seen with Te and I - iodine actually has smaller relative atomic mass but its placed after tellurium as it has similar properties to the elements in that group

- 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 existance of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and they fitted the pattern it helped confirm Mandeleev's ideas. For example, Mendeleev made really good predictions about the chemical and physical properties of an element he called ekasilicon, which today we know as germanium 

- The discovery of isotopes in the 20th century confirmed that Mendeleev was correct to not place elements in a strict order of atomic mass but also take account of their properties. Isotopes of the same element have dif atomic masses but have the same chem properties so occupy the same position on the periodic table

- There are around 100 elements, which all materials are made of 

- In the periodic table the elements are laid out in order of increasing atomic number. Arranging the elements like this means there are repeating patterns in the properties of the elements (arranged periodically, hence periodic table)

- If it wasn't for the periodic table organising everything, it would be hard remebering all these properties

- It's a handy tool for working out which elements are metals and which are non-metals. Metals are found to the left and non-metals to the right 

- Elements with similar properties form columns 

- These vertical columns are called groups 

- The group number tells you how many electrons there are in the outer shell. e.g. group 1 elements all have one electron in their outer shell and group 7 all have seven electrons in its outer shell. It's useful as the way atoms react depend upon the number of electrons in their outer shell, so all elements in the same group are likely to react in similar way.

- If you know how the properties of one element, you can predict properties of the other elements in that group - and in the exam, you might be asked to do this. e.g. the group one element are Li, Na, K, Rb, Cs and Fr. They're all metals and they react in a similar way 

- You can also make predictions about trends in reactivity e.g. in group one, the elements react more vigourosly as you go down the group. And in group 7, reactivity decreases as you go down the group

- The rows are called periods. Each new period represents another full shell of electrons

- Metals are elements which can form pos ions when they react

- They're toward the bottom and to the left of the periodic table

 - Most elements in the periodic table are metals 

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

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

Electronic structure of atoms affects reaction:

- Atoms generally react to form the full outer shell - they do this by losing, gaining or sharing electrons

- Metals to the left of the periodic table don't have many electrons to remove and metals towards the bottom of the periodic table have outer electrons which are a long way from the nucleus so feel a weaker attraction. Both these effects means that not much energy is needed to remove the electrons so it's feasible for the elements to react to form pos ions with full outer shell

- For non-metals, forming pos ions is much more difficult. This is as they are either to the right of the periodic table - where they have lots of electrons to remove to get a full outer shell, or towards the top - where the outer electrons are close to the nucleus so feel a strong attraction. It's far more feasible for them to either share or gain electrons to get full outer shell

Metal and non-metal - dif physcial properties 

- All metals have metallic bonding which causes them to have similar basic physical properties

- They strong but can be bent or hammered into dif shapes (malleable)

- They're great at conducting heat and electricity 

- They have high boiling and melting points 

As non-metals don't have metallic bonding, they don't tend to exhibit the same properties as metals. They tend to be dull looking, more brittle, aren't always solids at room temp, don't generally conduct electricity and often have a lower density

Group 1 elements are reactive, soft metals:

- The alkali metals are lithium, potassium, rubidium, caesium and francium.

- They all have one electron in their outer shell which makes them very reactive and gives them similar properties

- The alkali metals are all soft and have low density 

- The trends for the alkali metals as you go down Group 1 include:

- Increasing reactivity - the outer electron is more easily lost as the attracton between nucleus and electron decreases, bc the electron is further away from the nucleus the further down you go

- Lower melting point 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

- It's so easy for them to lose their outer electron that they only ever react to form ionic compounds. These compounds are generally white solids that dissolves in water to form colourless solutions

Reaction with water produces hydrogen gas:

- When group 1 metals are put in water, they react very vigorously 

- The more reactive an alkali metal is, the more violent the reaction

- Lithium, sodium and potassium float and move around the surface, fizzing furiously 

- They produce hydrogen. The amount of energy given out when they react increases down the group. For potassium and below in the group, there's enough energy to ignite hydrogen

- They also form hydroxides that dissolve in water to give alkaline solutions

- Group one metals react vigorously when heated in chlorine gas to form white salts called metal chlorides 

- As you go down the group, reactivity increases so the reaction with chlorine gets more vigorous 

- The group 1 can react with oxygen to form a metal oxide. Dif types of oxide will form depending on the Group 1 metal

- Lithium reacts to form lithium oxide

- Sodium reacts to form a mixture of sodium oxide and sodium peroxide 

- Potassium reacts to form a mixture of potassium peroxide and potassium superoxide

- Fluorine is a very reactive, poisonous yellow gas 

- Chlorine is a fairly reactive. poisonous dense green gas 

- Bromine is a dense, poisonous, red-brown volatile liquid

- Iodine is a dark grey crystalline solid or a purple vapour

They all exist as molecules which are pairs of atoms 

Trends:

- As you go down group 7, the halogens:

- Become less reactive - it's harder to gain an extra electron, bc the outer shell's further from the nucleus 

- Have higher melting and boiling point 

- Have higher relative atomic masses 

Halogens can form molecular compounds:

- Halogen atoms can share electrons via covalent bonding with other non-metals so as to achieve a full outer shell. E.g. HCI, PLC5, HF and CCL4 contain covalent bonds. The compounds that form when halogens react with non-metals all have simple molecular structures

Halogens form ionic bonds with metals:

- The halogens form 1- ions called halides when they bond with metals, for example Na+Cl- or Fe3+Br-3

- The compounds that form have ionic structures 

More reactive halogens will displace less reactive ones:

- A displacement reaction can occur between a more reactive halogen and the salt of a less reactive one e.g. chlorine can displace bromine and iodine from an aqueous solution of its salt (a bromine or iodine) Bromine will also displace iodine bc of the trend in reactivity 

- Group 0 elements are called the noble gases and include the elements helium, neon and argon

- They all have eight electrons in their outer energy level, apart from helium which has two, giving them a full outer shell. As their outer shell is energetically stable they don't need to give up or gain electrons to become stable. This means they're more or less inert - they don't react with much at all

- They exist as monatomic gases - single atoms not bonded to each other

- All elements in group 0 are colourless gases at room temp

- As the noble gases are inert they're not flammable - won't set on fire

- The boiling points of the noble gases increase as you move down the group along with increasing relative atomic gases

- The increase on boiling point is due to an increase in the number of electrons in each atom leading to greater intermolecular forces between them which need to be overcome.

- In the exam you may be given the boiling point of one noble gas and asked to estimate the value of another one. So make sure you know pattern: (increasing bp as bottom)

- Helium

- Neon

- Argon

- Krypton

- Xenon

- Radon