The periodic table: groups, periods and position

Mercury is a metal you could pour out of a jug. Bromine is a liquid too, and a splash of it would burn a hole in your skin. Two liquids that look alike on a bench, yet they sit far apart on the periodic table and behave nothing alike. Now picture lithium, sodium and potassium, three grey metals you would walk straight past. Drop any one of them into water and it skates across the surface, fizzing. Those three are stacked in a single column of the table. Looking at an element tells you very little about what it will do. What does a column know that your eyes do not?

In the exam you are given a periodic table, printed in the paper itself, so nobody expects you to learn it by heart. Everything in this video is for both tiers, Foundation and Higher. Combined science and separate chemistry students need all of it as well. The table you are given is plain. It has no colours and no shading, and no line marks off the metals, so practise reading it bare. The elements are placed in order of atomic number, which is the number of protons in each atom. You read it like a page of a book: left to right along a row, then down to the start of the next row. It is not in order of mass, and it is not alphabetical. A column is called a group. The groups are numbered one to seven across the top, and the last column on the right is Group zero. A row is called a period. The periods are numbered one to seven down the left-hand side. Period one is short. It holds two elements and nothing else, hydrogen and helium, and every count of periods starts from that top row. Now, which of these three is in Period two: helium, lithium or sodium? Lithium. Helium sits at the far right, but it is up in the top row beside hydrogen. Lithium opens the second row, and sodium opens the third. Each element's box carries two numbers. The smaller one is the atomic number. The bigger one is the relative atomic mass, which you read off the table and never memorise. Neither of those two numbers is the group or the period. Those come from where the box sits, not from anything printed inside it. Finding an element on the table is something most students already do well. One examiner's report on a Foundation paper says "the majority of candidates were able to find the element and give the symbol correctly." Finding the box is the half you probably have. The rest of this video is the other half: what the box's position tells you about the atoms inside it.

Here is what a position tells you. For Groups one to seven, the group number is the number of electrons in the outer shell. The period number is the number of shells that have electrons in them. Period three means three shells. Take sodium, two, eight, one. It has three shells, so it sits in Period three. It has one electron in the outer shell, so it sits in Group one. The reason is the order. Each step to the right adds one proton and one more electron, and that electron goes into the outer shell. When a shell can take no more, the next element has to start a new shell, and the table starts a new row to match. That is why periods and shells line up. Now, lithium is two, one. Why does its outer shell put it in sodium's column? Lithium has one electron in its outer shell, just as sodium does. The group counts those electrons, so both go in Group one, two rows apart. Listen to the word outer. An atom can have several shells, but it has exactly one outer shell, the one furthest from the nucleus. Potassium has four shells and sodium has three, yet each has one outer shell. More shells never means more outer shells. One examiner's report on a Higher paper puts it plainly: "Students should note that there is only one outer shell or level." The way to stay safe is to say the outer shell, singular, every time. Group zero is the one exception to the rule. Its atoms already have a full outer shell, and the video on Group zero, the noble gases, is where that gets explained. Here is a full one to try, using only the table you are given. Group six, Period three: what is it, how many outer electrons and shells, metal or not? Group six first: six electrons in the outer shell. Period three next: three shells. Those two lines come before anything else. Count the rows on the left edge, one, two, three, then go across to Group six, and you land on sulfur. Sulfur sits on the right-hand side, so it is a non-metal. Exactly where the metals stop and the non-metals start is the job of the video on metals and non-metals. One examiner's report on a Foundation paper describes students who used the table, "but most of these students referred to periods or rows and groups or columns and so failed to gain any marks." Those answers said where the element was. They never said what that position meant for its electrons. What earns it instead is a habit. Every time you read a position, turn it straight into electrons: group number, outer electrons; period number, shells.

Now turn it round. An electronic structure lists the electrons in each shell, and from one you can work back to a place on the table. Two, eight, eight, one: which group, which period, and which element? Group one, because the last number is the outer shell, and it holds one electron. Period four, because there are four numbers, which means four shells. Count four rows down, hydrogen's row first, go to Group one, and you reach potassium. The period is how many numbers there are, not the value of the last one. Four numbers in the structure, four shells, Period four. One examiner's report on a Foundation paper records two slips with magnesium, two, eight, two. Some "had ignored hydrogen and helium as forming the 1st period of the periodic table and said that magnesium was in the 2nd period as shown by the '2' in the electronic configuration." Others "had mentioned 3 shells, as taken from the electronic configuration, but had not linked that to the period number." One habit fixes both slips: count the numbers to get the shells, then say the period out loud. Magnesium has three shells, so it sits in Period three. One more, using only the table you are given. This time the question wants a name, written in full. Pause and find the Group two metal in Period four, and write its name. Calcium. Count the rows on the left edge with hydrogen and helium's row as Period one, stop at four, then step across to Group two. Write calcium, the word, rather than C a. When a question asks for the name of an element, the symbol on its own is not the name.

Four quick ones to finish, and every one of them uses the table you are given. Phosphorus is two, eight, five. Which group, and which period? Group five, from the five electrons in the outer shell. Period three, from its three shells. Now go the other way: Group three, Period two. How many shells, and how many outer electrons? Two shells, with three electrons in the outer one. That element is boron. Next, what does the bigger number printed in an element's box tell you? The relative atomic mass. Neither number in the box is the group or the period; those come from where it sits. Try argon, two, eight, eight. Which group is it in, and why is that one different? Group zero. Its outer shell is full, so it is the one column where the group number does not count outer electrons. And that is what a column knows that your eyes do not: how many electrons sit in the outer shell. Mercury and bromine only look alike; their outer shells are nothing alike.

This is video one of nine in The periodic table, and the first of them, so nothing in the chapter comes before it. If you are comfortable with it, a thumbs up marks it as one to skip when you revise. Leave it without one and it stays on your list, saved to a playlist if you like; three questions in, it starts to feel normal.

Next in the chapter: How the periodic table was developed.

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