in today's video we're going to take a look at both the group seven elements known as halogens and group zero elements known as the noble gases and we'll run through the properties and the trends of each of them let's start with the halogens which are all in some way pretty dangerous fluorine is a poisonous yellow gas and is very reactive chlorine is less reactive and this time a poisonous green gas bromine is a ready brown volatile liquid and poisonous again and iodine is a dark gray solid that can form poisonous purple vapors although iodine is also an antiseptic which has saved millions of lives and for those last two elements astatine and tenocine you can basically ignore them because they don't really come up in the course all of the halogens can exist as pairs of atoms that we call diatomic molecules which just means two atom molecules they do this by sharing electrons and forming a covalent bond as we've shown here for chlorine although this outer shell arrangement would be the same for any of the halogens halogens can also form covalent bonds with other non-metals such as hydrogen or carbon to form compounds like hydrogen fluoride or carbon tetrachloride these compounds are known as simple molecular structures and we take a closer look at them and the covenant bonds they use in another video just like the group 1 elements group 7 also has trends as you go down the group but they're the opposite way around so for halogens the melting points and boiling points increase as you go down the group and the reactivity decreases this decrease in reactivity is because as you go down the group the outermost shell gets further and further away from the positive nucleus so the attractive force that's needed to pull in an extra electron from another atom gets weaker and if the halogen can't attract an electron to complete its outer shell then it can't react so the larger atoms at the bottom of the table like iodine are the least reactive the next thing we need to look at is how halogens form ionic bonds with metals when a halogen gains an electron to form a one minus ion we call it a halide and we have to change the name slightly changing the ends to these so we get fluoride chloride bromide and iodide all of which would be ions you'll probably most often see halogens forming ionic compounds with the alkali metals in group 1 like sodium chloride as we have here but they can form ionic bonds with metals of other groups too lastly we need to take a look at displacement reactions these involve a more reactive halogen displacing a less reactive one for example let's imagine that we pumped some chlorine gas into a solution of potassium bromide as the chlorine is more reactive than the bromine because it's higher up in the group it displaces the bromide to bond to the potassium so effectively the chlorine has taken the bromine's place to form potassium chloride the main thing to remember here is just that more reactive halogens will always displace less reactive ones for example fluorine chlorine and bromine could all displace iodine as iodine is the least reactive now we were going to do an entire video on the group zero elements which are also known as a noble gases but there really isn't that much that you need to know they all exist as colorless gases and because they have full outer shells they're inert which just means that they don't really react with anything and because they don't react with other atoms they exist as single atoms and they're also non-flammable which means they can't be set on fire the only other thing that you really need to know is that their boiling point increases as you go down the group just like it does for the halogens and that's all if you enjoyed it then please do share with your friends and we'll see you next time