the group 1 elements which are known as alkali metals are a bit different from most other metals they include lithium sodium potassium rubidium cesium and francium instead of being strong having a high density and having a high melting point like you expect of a metal the alkali metals are in fact relatively soft with low densities and low melting points they are also much more reactive than other metals and react vigorously with water oxygen and group 7 elements such as chlorine as we'll see later as you go down the group the elements become even more reactive with cesium being so reactive that they can ignite spontaneously at room temperature another trend as you go down the group is that the melting points and the boiling points decrease now an easy way to remember this trend in reactivity is that common table salt which has sodium in it doesn't just suddenly catch fire as you sprinkle it on your potatoes or cesium probably would now a big part of chemistry is explaining the properties and trends that we see for example how would you explain the fact that alkali metals are so reactive well if you remember from our electron arrangement video which we'll link down below all atoms want to have a completely full outermost shell in order to be stable and because the alkali metals are all in group 1 we know that they all have one electron in the outermost shell so in order to react and be stable all they need to do is lose that one single electron which is pretty easy because there's only one electron to lose and because they lose a one negative charge they'll become a one positive ion what about the trend that we mentioned though that they become more reactive as you go down the group well as you go down the group the atom's radius gets bigger and so the outermost shell gets further and further away from that positive nucleus now as this positive nucleus is the only thing holding the outermost electron in place the increased distance weakens the attractive force between our positive nucleus and that outer negative electron meaning that the electron can be lost more easily increasing the atom's reactivity remember the term reactivity isn't anything special it's just related to how easily atoms can lose or gain electrons and so react with other atoms because alkali metals can lose their outer electrons so easily they almost always form ionic compounds with non-metals we take a closer look at ionic compounds in another video but it's basically where a metal atom like sodium donates an electron to a non-metal like chlorine so that both of them have full outer shells and because these two atoms are now oppositely charged ions they'll be attracted to each other by electrostatic forces which we call an ionic bond and so together they make an ionic compound this particular example of sodium chloride actually happens to be common table salt but regardless of the alkali metal used the ionic compounds are generally white solids and they tend to dissolve in water form colorless solutions so now that we've covered alkali metals in general we need to consider how they react with water chlorine and oxygen when an alkali metal is put in water it tracks vigorously to produce a metal hydroxide and hydrogen gas for example sodium plus water will form sodium hydroxide plus hydrogen as we mentioned earlier the metals become more reactive as we go down the group and so the reactions will release more energy in fact from potassium onwards so much energy is released that it ignites the hydrogen gas that gets produced which we can see in the form of flames groupon metals also react vigorously when heated in chlorine gas and in this case they form white metal chloride salts so sodium plus chlorine will form sodium chloride and again the reactions will become more vigorous as you go down the group when alkali metals react with oxygen they form metal oxides however the type of oxide they form will depend on the particular metal for example lithium plus oxygen forms lithium oxide li2o just like sodium plus oxygen from sodium oxide na2o however sodium and oxygen can also react to form sodium peroxide instead which is na2o2 meanwhile potassium can react with oxygen to form either potassium peroxide k2o2 or potassium superoxide which is ko2 now that's quite a lot to remember and you don't really need to know why these particular compounds form so the best trick is probably to just put them on some flashcards and practice them whenever you need to know them for exams and that's everything you need to know about alkali metals if this helped you at all please give us a like and subscribe and we'll see you next time