in this video i want to show you how you can identify a substance as an ionic solid a molecular solid or even atomic solids such as a group eight solid or metallic solid or even a network covalent solid so ionic solids composed of ions and a good way to identify it is to see if you have a substance that is composed of a metal and a nominal like sodium chloride sodium is a metal chlorine is a nonmetal some other examples include magnesium oxide and ammonium chloride now ammonium chloride doesn't have any metals in it so this is one of those exceptions however it is composed of ions ammonium is a polyatomic ion and chloride is an ion so that's how you can identify an ionic solid typically it contains a metal and a non-metal but it's composed of ions which are particles with positive and negative charges ionic solids for the most part have relatively high melting points the melting point for sodium chloride is let's see if i put in a different color 646 celsius for magnesium oxide it's a lot higher it's like 2852 so some ionic solids have very high melting points others are relatively average but ionic compounds with very high charges tend to have higher melting points for example magnesium has a positive two charge but sodium has a plus one charge and so due to the high charge on magnesium that's why it has a higher melting point now this is a term that you need to be familiar with when dealing with ionic solids and that is lattice energy magnesium oxide has a much higher lattice energy than sodium chloride and that's one reason why it has such a high melting point lattice energy increases with increase in charge so if you increase the charge on an ionic compound the lattice energy will increase and typically the melting point is higher the second thing is size if you can increase the atomic radius of an atom the lattice energy decreases and as a result the melting point decreases so ionic compounds with very high melting points tend to have small ions with high charges now the next category are molecular solids and for the most part compared to ionic solids and metallic solids molecular solids have very low melting points carbon dioxide for instance has a melting point of negative 56 degrees celsius water melts at zero degrees celsius now a good way to identify a molecular solid is for the most part it's composed of non-metals it's composed of molecules as opposed to ions and these molecules tend to be small some other examples include fluorine chlorine bromine and iodine now when comparing molecular solids in a let's say in the same column or group in the periodic table the melting point will increase as you go down the group iodine for example has a relatively high melting point compared to fluorine because it has a higher molecular weight and it has more electrons so therefore it has more london dispersion forces the melting point of fluorine is negative 219.7 degrees celsius for chlorine it's negative one or one point five bromine is negative 7.2 and iodine is 113.7 so as you can see as you go down a group the melting point will increase due to the fact that iodide has more ldf forces or van der waals forces fluorine is a gas at room temperature chlorine is a gas bromine is a liquid iodine is a solid so if you know the physical states of the compounds or molecules you're dealing with solids usually have higher melting points than liquids or gases so if you know a substance to be a solid and another one is a gas at room temperature the solid is for the most part going to have a higher melting point than the gas now let's move on to atomic solids now there's three different categories that you need to be familiar with for atomic solids so these include group 8 a atomic solids that is the noble gases and then you have the metallic solids and then the network covalent solids so as we said before ionic solids are composed of ions molecular solids are composed of molecules and atomic solids are composed of atoms now sometimes you may have like giant molecules as well but we'll talk about that later for the most part these are made up of atoms so let's talk about the group 8a atomic cells first these include like neon argon krypton and so forth now group 8a atomic solids have very low melting points the melting point for neon is extremely low it's negative 249 degrees celsius even lower than most molecular solids argon is negative 189 krypton is negative 157 and xenon is negative 112. but notice that the trend is the same for the halogens as you go down a group the melting point increases as in the case of molecular solids and the same is true for group 8a solids krypton has a higher melting point than neon so that's another trend you want to keep in mind next we have the metallic solids which the melting points can vary greatly so zinc has a relatively average melting point the melting point of zinc is 420 degrees celsius mercury is a liquid at room temperature it's a liquid metal and so it has a very low melting point of negative 38.8 celsius tungsten has a very high melting point and that's like 3422 so as you can see from metals the melting points can vary now you want to be familiar with the metals that have very low melting points because the rests for the most part usually high gallium is one of those metals that have a low melting point it's about 30 celsius in fact you can hold a solid chunk of gallium in your hand and the heat from your hand is enough to melt it into a liquid cesium is another metal with a low melting point the melting point is 28.5 copper has the melting point of 1085 so the melting points for metals varies a great deal now you need to be familiar with certain properties of metals metals can conduct electricity they can conduct heat very well and they're malleable they could be hammered into thin sheets and are also ductile they can be pulled into wires so make sure you know those properties of metals molecular solids tend to be soft ionic solids tend to be hard but brittle now let's move on to the network covalent solids also known as the network atomic solids the first element we need to consider is carbon now there's two main allotropes of carbon that you need to be familiar with there's others but the main two are diamond and graphite diamond has a nice crystal clear structure whereas graphite is black and slippery diamond is very hard it's one of the hardest naturally current substances it doesn't connect electricity it's an insulator but it does conduct heat very well it's an excellent thermal conductor graphite on the other hand does conduct electricity and the melting points of these two solids they can vary with pressure but it's about above 3000 celsius it could be 3 500 4 000 celsius but it's over 3 000 celsius and to melt these substances the pressure has to be very high above normal atmospheric pressure another example of a network atomic solid is silicon carbide now this is composed of two different atoms silicon and carbon but it is a network covalent solid just like carbon it's a metalloid it doesn't conduct electricity very well and silicon carbide does have some thermal conductivities to it like diamond carbon its melting point is pretty high it's about 2 800 celsius another network solid they need to be familiar with is silicon dioxide also known as quartz and also in the form of silica glass is another form of silicon dioxide and also the sand that you see in the beach the melting point of silicon dioxide for the most part averages around 1700 degrees celsius boron nitride is also another network atomic solid and its melting point is about 3000 celsius so the network atomic solids they're composed of giant molecules they have strong directional covalent bonding whereas metallic solids they have strong non-directional covalent bonding and the electrons in atomic or rather metallic solids the electrons are delocalized and that's why they can conduct electricity very well the electrons in a metal they can be shared among many atoms now in graphite as well the electrons in graphite the pi electrons are also delocalized as well so not all of the electrons but some of them and that's why graphite can conduct electricity whereas in diamond the electrons are localized they're stuck to the atoms that they're attached to and so they can't move from one atom to the next and so diamond doesn't conduct electricity but metals graphite they can conduct electricity because the electrons are free to move from one atom to the next and so it's said that the electrons are delocalized they're not stuck to one atom they're free to move around from thousands of atoms now how can we identify a network covalent solid the best way to identify it is to look for elements that are close to the metalloids if you see elements such as carbon silicon these are metalloids boron nitrogen elements in that region like group 3 group 4 group 5 elements if you see that chances are it might be a network covalent solid so his silicon in group four here's carbon and then below that is germanium and then here you have boron nitrogen aluminum phosphorus and then this is indium and arsenic so if you see carbon by itself it's a network covalent solid or silicon by itself if you see boron paired up with nitrogen that's uh that's going to be a network covalent solid even aluminum paired up with phosphorus that can also form a network covalent solid or even aluminum with nitrogen so if you see elements in this vicinity there's a good chance it can form a network covalent solid now let's work on some multiple choice problems to see if you remember what you've learned now there might be some other questions that we haven't covered in the beginning of this video so hopefully you've studied this section so which of the following is an ionic solid so now recall ionic solids are composed of ions typically made from metals and non-metals a is an ionic solid calcium is a metal chlorine is a non-metal so that solid is composed of the calcium plus two ion and the chloride ion nickel is a metallic solid nickel is a metal c6h12o6 which typically can represent glucose or fructose that is a molecular solid it's composed of molecules helium is a noble gas so that's a group 8 a atomic solid silicon carbide that is a network covalent solid so a is the answer to this problem number two which of the following elements is ductile now feel free to try this problem if you want to you can pause the video and select your answer ductility is a property of metals metals can be pulled into wires sodium fluoride that is an ionic solid sodium is a metal fluorine is a nominal argon is a group 8a atomic solid boron nitride is a network covalent solid copper is a metal so copper can be pulled into wires so copper is ductile it's also malleable it can be hammered into sheets diamond can't be pulled into wires i never seen a wire that's composed of diamond that'll be interesting though but that's just not a property of that substance number three which the following substances can conduct electricity in the molten state is it silicon carbide sodium bromide xenon water or carbon dioxide water is a liquid and water doesn't conduct electricity in the liquid state pure water is not an electrical conductor however if you add salt to water it does conduct electricity because you have free flowing ions for something to conduct electricity you need a mobile charge carrier in the case of metals the electrons conduct electricity because the electrons which have a negative charge are free to move in the case of salt water the ions which carry positive and negative charge are free to move they can freely flow within water and so that's why salt water conducts electricity so you need a mobile charge carrier for electricity to be conducted within the material carbon dioxide is a molecular solid and even in the molten state it doesn't conduct electricity there's no free flowing ions or free flow in charge silicon carbide doesn't connect electricity in the liquid state sodium bromide does conduct electricity in a liquid state now it's important for you to understand that ionic solids or ionic compounds in a solid state do not conduct electricity in the solid state the ions are fixed in place they're not free to move so ionic solids like salt is an insulator however if you can melt it into a liquid or into the molten state the ions are free to move and whenever you have free flow in charge they can conduct electricity and also in the aqueous state when they're dissolved in water they can conduct electricity so just keep that in mind xenon is a noble gas and in a liquid state it doesn't conduct electricity number four which of the following substances have the lowest melting point is it ionic solids molecular solids atomic solids which one is it molecular solids and group 8a solids or noble gases tend to have the lowest melting point zinc at room temperature is a solid mercury is a liquid neon is a gas carbon dioxide is a gas and magnesium oxide is a solid so if you know the physical state of the substance you could eliminate the ones in a solid state solid compounds tend to have higher melting points than liquids and gases so we can eliminate liquid mercury so we're between neon and carbon dioxide noble gases usually have lower melting points than molecular solids the only forces acting on noble gases are london dispersion forces now granted carbon dioxide is nonpolar which does have london dispersion forces but it does contain polar bonds so it turns out that neon is going to have the lowest melting point and i believe we mentioned the values for these substances zinc was about 420 degrees celsius so it's pretty high mercury it's a liquid metal and so it's negative 38.8 celsius so some metals have relatively low melting points neon is negative 249 celsius and carbon dioxide it melts at negative 56 celsius magnesium oxide is very high 2852 celsius so keep this in mind ionic solids and network covalent solids tend to have very high melting points metals the melting points can vary it can be very low or it can be very high like tungsten was over 3000 degrees celsius molecular solids and noble gases those are the ones that have very low melting points but for the most part noble gases have extremely low melting points now what about number five which of the following substances does not conduct electricity is it potassium fluoride in the liquid state graphite aqueous sodium hydroxide silver or diamond potassium fluoride is an ionic substance in the liquid state it's in a molten state and molten ionic compounds do conduct electricity so we can eliminate a graphite is a network covalent solid and it does conduct electricity the pi electrons are free to move sodium hydroxide is ionic and it's an aqueous state which means it's dissolved in water so we have free flow and ions and so therefore that conducts electricity silver is a metal which also conducts electricity and diamond is a network atomic solid like graphite however diamond structure does not allow it to conduct electricity and that's just something you need to know so e is the answer for this problem you