substances stuff are made of atoms the different types or elements of atoms there are are represented in the periodic table by a symbol a compound is a substance that contains two or more different types of atoms chemically bonded together for example the chemical formula for water is H2O is made up of hydrogen and oxygen atoms for every one oxygen atom there are two hydrogen atoms if there's no number after a symbol there's an invisible one there these atoms change what they're bonded to and how they're bonded through chemical reactions we can represent a reaction with a word equation and a chemical equation using symbols as atoms are not created or destroyed in any chemical reaction there must be the same number of each type of atom on both sides so sometimes we must balance equations Pro tip start balancing atoms that are only in compounds so with this one let's go with the carbons first there's one on the left one on the right so that's all good hydrogens there are four on the left only two on the right now we can't change the small numbers because that would change what the compound is so what we can do is put numbers in front of elements or compounds to multiply them up stick at two in front of the H2O we now have 2 * 2 hydrogens so that's four that's also double de oxygen in it however so now we have four oxygens on the right still only two on the left so doubling this O2 on the left takes care of that if there's an element in a reaction Like Oxygen here we always finish balancing that as there's no KnockOn effect a mixture is any combination of any different types of elements and compounds that aren't chemically bonded together for example air is a mixture of oxygen nitrogen and more solutions are mixtures too like salt water a mixture of water and sodium chloride you can separate large insoluble particles from a liquid using filtration like sand from water as sand can't dissolve crystallization can leave a solute that's the solid dissolved in a liquid behind after you evaporate the solvent from a solution like salt from water similarly distillation involves heating the solution as well but this time the gas is cooled so it condenses back into a liquid you can can also do this at different temperatures to separate the different liquids of a mixture as they will have different boiling points this is called fractional distillation these are all physical processes though and not chemical reactions because no new substances are being made solid liquid and gas are the three main states of matter for example water can be ice a solid where the particles or molecules in this case vibrate around fixed positions it can also be liquid water where the molecules are still touching but are free to move past each other and it can also be a gas water vapor call it when it's water where the particles are far apart and move randomly and they also have the most energy and so move quickly as molecules in a gas are far apart gases can be compressed while solids and liquids cannot to melt or evaporate a substance you must supply energy usually in the form of heat to overcome the electrostatic forces of attraction between the particles we don't say we're breaking Bonds in this case note that none of these make a new substance so these have to be physical changes again not chemical reactions we're not breaking any chemical bonds in chemical re reaction equations we indicate what state of substance is in with state symbols brackets s for solid L for liquid G for gas and also AQ for aquous that means dissolved or in solution again like salt in water the idea of what atoms are like came about gradually JJ Thompson discovered that atoms are made up of positive and negative charges he came up with the plum pudding model of the atom a positive charge with lots of little electrons dotted around it it was Ernest Rutherford who found that the positive charge must actually be incredibly more we now call this the nucleus and the electrons must orbit relatively far away from it he discovered this by finding that most alpha particles fired at a thin Leaf of gold atoms went straight through proving that atoms must be mostly empty space Neils bore later discovered that electrons exist in shells or orbitals then James Chadwick discovered that the nucleus must also contain some neutral charges he called them neutrons while the positive charges are called protons protons and electrons have equal and opposite charges so we just say they're Plus one and minus one relatively speaking neutrons have a charge of zero protons and neutrons have essentially the same mass so we say they have a relative mass of one electrons are very light in comparison so we say they have a mass of zero or just very small depending on the situation the periodic table tells us everything we need to know about an atom the bottom number is the atomic number that's the number of protons in the nucleus this is what determines what elements you have every atom has an overall neutral charge so that means they must have the same number of electrons as protons if an atom gains or loses electrons it's now called an ion not an atom the top number is the mass number or relative atomic mass or RAM for short it tells you how many protons and neutrons are in the nucleus so that must mean that this carbon atom carbon 12 has six neutrons on top of its six protons to make that 12 however you can get a carbon atom with seven neutrons instead so its relative mass is 13 these are what we call Isotopes atoms of the same element but different numbers of neutrons you might see number that isn't a whole number for the mass this is because periodic tables sometimes show the average mass for all of the Isotopes of that element found in the world for example if you have some chlorine gas it turns out that 75% of the atoms will have a mass of 35 while 25% of the atoms will be 37 these are what we call their relative abundance to find the average we just pretend that we have 100 atoms we add up the total masses of all the Isotopes then just divide by 100 that's why chlorine average relative atomic mass is 35.5 the periodic table is incredibly useful but how was it made before it scientists just put elements in order of their atomic weights some were then grouped together if they were seen to have similar properties but still using the atomic weight order Dimitri Mev then came along and grouped elements together based on their properties even if the order didn't follow atomic weight using this method he found there were gaps in his table he asserted that these elements were yet to be discovered in time he was proven correct showing that his table was indeed correct like we said electrons exist in shells around the nuclear ucus the shells fill up from the inside with a maximum of two on the first shell eight on the second and third shells then we only go to two on the fourth shell that's 20 electrons Al together which brings us to a calcium atom after this we get into the transition metals where things get a little bit crazy so we leave that until a level chemistry so we only care about the electron configuration going up to 2882 magnesium has 12 electrons so its electron configuration for example would be 2 8 2 the modern periodic table can be split up into different sections for example everything to the left of this staircase is called a metal metal atoms always donate electrons to gain an empty out of shell of electrons again slightly weird with transition metals but we don't think about their shells to the right of the staircase non-metals they always accept electrons to gain a full outer shell the column an atom is in is called the group it tells you how many electrons an atom has in its outer shell again the transition metals work in a really weird way so they don't get their own group in fact it turns out this is because they can donate a different number of El Rons when they bond to different things the atoms in group one are called the alkal metals they all have one electron in their outer shell which they give away donate when they bond to something so they have similar properties like when they react with water the further down the group you go though the further that outer electron is from the nucleus so the electrostatic attraction is weaker between the negative electron and the positive nucleus this means that the electron is more readily donated this means the metals get more reactive as you go down the group group seven or what we call the halogens they're essentially the opposite they have seven electrons in their outer shell so they need one more to gain a full outer shell the further down the group you go the less readily an electron is accepted onto that shell that's further away from the nucleus so they get less reactive down the group The Boiling Points also increase down the group two group zero sometimes referred to as group eight are called the noble gases they already have an empty or full outer shell just depends on your perspective so they don't react in reality they can react under special I conditions so we just say they're very unreactive we don't really say group eight anymore though because some people thought that helium might feel a little left out as it only has two electrons in its outer shell as electrons are negative themselves Metals become positively charged when they lose them they always form positive ions all of group one lose one electron when they turn into an ion so all of their ions are one plus but again we don't write the one we just put plus group two lose two electrons to get an empty out of shell so their ions are all two plus group seven gain one electron each so all their ions are minus group six's ions are all two minus the atoms in group three four and five don't really form ions except for aluminium which is 3+ like we said transition metals can donate different numbers of electrons for example an ion ion can be fe2+ or fe3+ it can donate two or three electrons so we give them the names Ion 2 and ion 3 to distinguish between them transition metals are generally harder and less reactive than the alkaline metals they also form colored compounds so I hope you found that helpful leave a like if you did and pop any questions or comment below I'll see you in the next video