dmitri mendeleev was a russian chemist and inventor and back in the mid nineteenth century he devised the periodic table which organized all of the known elements in fact when he made it it even predicted new elements that we hadn't discovered yet as we saw in our video on elements everything in the universe is created from different combinations of the elements in this table just like a book is read from left to right and from top to bottom with the elements arranged in order of increasing atomic number which represents the number of protons in the atoms of that element within each of these boxes is the nucleus symbol for that element which has three components on the right we have the elemental symbol which is the one or two-letter symbol for that element in this case na which stands for sodium in the bottom left we have the atomic number which tells us how many protons the atoms of that element have and then top left we have the mass number which tells us the total number of protons and neutrons in that atom while he was organizing the table Mendeleev found that the elements displayed a pattern of chemical properties that repeated at regular intervals so instead of arranging the elements and worn along a line he formed a new row every time the pattern repeated which we now call periods because the patterns repeat away each new period the elements in each vertical column have similar chemical properties and we call each of these columns a group starting with one on the far left and going to seven so here we're looking at Group two there are a couple of exceptions to this though one is that the far-right group is called group 0 rather than group 8 and we'll take a closer look at why this is in a minute the other is that the block of met doors between group two and three don't have group numbers don't worry about this for now though just remember to ignore them when counting your groups the reason that elements in each group have similar chemical properties is that they have the same number of electrons in their outermost shell and this is largely what determines how an element reacts in fact the group number actually tells you exactly how many electrons they have in outer shell so as we're looking at group two we know that every element in this group has two electrons in its outermost shell just make sure you don't get confused here all the elements in a given group have different total numbers of electrons they just have the same number of electrons in their outermost shell so if we move over to the group one elements which we call it alkali metals we can tell that they all have one electron in the outer shell whether it's lithium which has three electrons overall or sodium which has 11 electrons overall and if this feature that gives all group one elements similar properties for example they all react violently with water there were also trends within groups for example the Group one elements become more reactive as you're down the group has also you late in a video specifically on group war and elements group seven elements meanwhile which we call halogens all have the seven electrons in the outer shell although these elements become less reactive as you go down the group as you said before group zero is a bit different these elements are called the noble gases and they all have full outer shells so argon which has three shells has eight out of eight in its outer shell while helium which only has one shell has two out of two because if you remember the first shell I can only hold up to two electrons because they don't need to gain or lose any electrons the noble gases are all very unreactive sometimes you might see a zigzag line running here from aluminium to polonium this line separates the metals which include everything to the left in orange from the nonmetals on the right still in blue so as you can see there are lots more metals than there are nonmetals and in the middle of the metals there's a special block of elements which are known as transition metals we'll take a closer look at what design in another video just like we'll take a closer look I have the group 1 group 7a group is zero elements and the difference is between metals and nonmetals before we finish we just wanted to point out that if you google periodic table you'll probably see loads of different types for example you might find that the position of the mass and atomic numbers has been reversed like the example of iron here don't worry about any of this though the tables all through the same information just presented in a slightly different way and you can always tell which is the atomic number because it will be the smaller of the two and in your exam the periodic table you'll get given will be similar to the one we've been using throughout the video another if you enjoyed it then please give us a like and subscribe and we'll see you next time