what is the difference between an atom and a molecule so take a look at these three substances helium hydrogen gas and h2o which of these substances are composed of atoms and which ones are composed of molecules helium is composed of atoms each particle of helium is made up of one atom of helium a particle of hydrogen gas is a molecule one particle contains two atoms a molecule is basically a particle with multiple atoms water is a molecule you have an oxygen atom and two hydrogen atoms so at the center this is the larger oxygen atom and then these are the two hydrogen atoms so a molecule is basically a particle with many atoms it could be made up of the same type of atom or different types of atoms helium is considered to be a pure element it's made up of only one type of atom hydrogen gas is also a pure element water is not a pure element because water has two different types of hydrogen atoms i mean just atoms in general it's composed of hydrogen and oxygen atoms so water is not a pure element but h2 and h2o are considered to be molecules so let's work on some more examples for each of these substances determine if it's composed of atoms or molecules and also determine if it's a a pure element or if it's a compound let's start with o2 is it composed of atoms or molecules one particle of o2 contains two atoms so it's a molecule since it contains multiple atoms now because it only contain one type of atom it's a pure element the only type of atom that we have in o2 are oxygen atoms now what about co2 would you consider a particle co2 as an atom or molecule now co2 contains many atoms so it's considered to be a molecule however it doesn't have one type of atom it's composed of different types of atoms it has carbon atoms and oxygen atoms so therefore it's not a pure element it's a compound a compound is a substance that is composed of many different types of atoms a compound is basically not a pure element neon neon is composed of atoms one particle of neon looks like this it's simply one atom whereas an oxygen atom i mean oxygen molecule is composed of two oxygen atoms so that's why o2 is a molecule and a neon is not and let's say if we wanted to draw co2 there are three atoms we have a carbon atom and two oxygen atoms so this is the carbon and these are the two oxygen atoms neon is also a pure element because we don't have two different elements there's only just one neon now what about fluorine fluorine is very similar to o2 it's composed of molecules because each particle contains multiple atoms and it's a pure element we only have one type of atom so now you know the difference between atoms molecules compounds and pure elements so just to review a particle is considered an atom if there's only just one atom if there's multiple atoms then the particle is considered to be a molecule if there's only one type of atom it's a pure element and if there's different types of atoms you have a compound now what is the difference between an atom and an ion what do you think the difference between these two are atoms are electrically neutral atoms contain equal number of protons and electrons ions on the other hand contain different number of protons and electrons so a good example is the aluminum atom and the aluminum ion which has a three plus charge aluminum has an atomic number 13 and a mass number of 27. on a periodic table the smaller number of the two numbers that you'll see is the atomic number the atomic number is always equal to the number of protons so aluminum has 13 protons the aluminum ion also has 13 protons the difference between a mass number and the atomic number is the number of neutrons so aluminum and aluminum ion contain 14 neutrons now the electrons are different in an atom which is electrically neutral the number of electrons is going to be the same as the number of protons to calculate the number of electrons you can use this formula it's equal to the atomic number minus the charge so in the case of the aluminum atom it's 13 minus a charge of zero so that's where we got 13. now in the case of the aluminum ion it's going to be the atomic number 13 minus the positive 3 charge so there's 10 electrons and it makes sense each proton has a charge of 1 so there's 13 protons that's the net charge of plus 13 in the nucleus each electron has a charge of minus one there's ten of them if we add these numbers this will give us a net charge of positive three ions that have positive charges are known as cations ions with negative charges are anions so let's say if we have phosphorus 31 and the phosphorus 31 ion which has a minus three charge so this is called an anion since it has a negative charge now both of these particles have 15 protons 31 minus 15 is 16 so they both contain 16 neutrons but atoms and ions they differ in the number of electrons that they have on the left because we have an atom it's electrically neutral so the number of protons and electrons are the same in the case of the anion it's going to be the atomic number minus the charge so the atomic number is 15 minus the charge of negative three this will give us 18 electrons 15 minus negative three is the same as 15 plus three so whenever you have a negatively charged ion it means that there's more electrons than protons and when you have a positively charged ion there are more protons than electrons now let's focus on compounds because there's two types of compounds that you need to be familiar with ionic compounds and covalent compounds so let's look at sodium chloride and h2o sodium chloride is basically table salt h2o as you know is water both of these are compounds because they're both made up of different types of atoms sodium is one type of atom cl chlorine that's another type of atom now which compound is ionic and which one is covalent or molecular covalent compounds are known as molecular compounds a quick way to tell is ionic compounds are composed of metals and nonmetals 99 of the time covalent compounds are usually composed of nonmetals sodium is a metal on a periodic table sodium is all the way on the left side the left side contains the metals the upper right corner contains the amounts chlorine is a nonmetal it doesn't conduct electricity hydrogen and water are both nonmetals even though hydrogen is in the upper left corner it's not considered to be a metal like sodium or lithium so water is considered to be a molecular compound or a covalent compound because it's composed of nonmetals in a molecular compound the electrons are shared between the atoms so hydrogen and oxygen they share electrons an ionic compound like nacl the electrons are not shared there's a transfer of electrons sodium gives one of its electrons to chlorine and thus it turns into a sodium cation and the chlorine becomes an anion so ionic compounds are composed of positive and negative charges that's another way to tell and molecular compounds are not composed of full charges as you see here so that's a quick and simple way to determine which one is ionic and which one is covalent so let me give you some more examples carbon monoxide magnesium chloride sulfur hexafluoride and let's use calcium oxide so carbon monoxide is it a molecular compound or is it ionic this compound is molecular or covalent because carbon and oxygen are both nonmetals magnesium chloride is ionic because magnesium is a metal chlorine is a nonmetal here we have two nonmetals i'm going to use n for nonmetals and m for a metal sulfur hexafluoride and that's composed of two nonmetals sulfur and fluorine are both nominal so this is going to be a molecular or a covalent compound calcium oxide is ionic calcium like magnesium is an alkali earth metal it's in the second group of the periodic table that is the second column it's composed of a metal and a nominal making it ionic now there is an exception ammonium chloride nitrogen is a non-metal hydrogen is also a non-metal and the same is true for chlorine so there are no metals in this compound so you might think that this is a molecular compound but you need to be careful because it's composed of ions ammonium contains a net positive charge and the chloride ion contains a negative charge so because this is composed of ions it's considered to be ionic so watch out for exceptions so anytime you see nh4 just know that it has a positive charge so chances are it's an ionic compound and not a molecular compound but for the most part if you see a compound composed of a metal and a non-metal it's typically ionic and two nonmetals are usually molecular compounds or covalent compounds that's just a general rule but as you can see there are some exceptions you