Groups and periods · printable packet

Why the periodic table has the shape it has, and what an element's position tells you before you know anything else about it.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

By the end of this lesson you will be able to place an element in the periodic table from the arrangement of its electrons, and read a position back as structure: the group number is how many electrons are in the outer shell and the period number is how many shells there are. You will be able to name the four families that have names, say what each of them is forced to do by its outer shell, and say where the simple rule stops — at helium, and at the ten columns in the middle.

2. What you already have

You can read a formula as a count of atoms, and you know that an atom has a nucleus of protons and neutrons with electrons around it, arranged in shells. What this lesson adds is the one further step that turns that picture into a prediction: if you know how many electrons are in an atom's outer shell, you know which column of the periodic table it is in — and the column tells you how it will behave.

3. Words for this lesson

4. The table is a consequence, not a filing system

The group number is the number of electrons in the outer shell and the period number is how many shells there are. The table is not an arbitrary filing cabinet that elements were sorted into; it is what you get when you write the elements out in order of atomic number and start a new row every time a shell fills.

Write the elements out in one long line in order of atomic number — hydrogen, helium, lithium, beryllium — and fill each atom's shells as you go. The first shell holds two electrons, so after helium it is full. Start a new line.

The second shell holds eight, so lithium through neon fills it, and then you start a new line again. Sodium through argon fills the third shell to eight, and you start another.

Now look down the lines you have made rather than along them. Lithium and sodium are one under the other, and both have exactly one electron in their outer shell. Fluorine and chlorine are one under the other, and both have seven. Neon and argon are one under the other, and both have a full shell.

Term	What it means
Group	A column of the periodic table.
Period	A row of the periodic table.
Outer shell	The occupied shell furthest from the nucleus; the part that meets another atom.
Alkali metals	Group 1.
Alkaline earth metals	Group 2.
Halogens	Group 7.
Noble gases	Group 8.
Transition metals	The ten short columns in the middle; a block, not a group.

Nothing was sorted. The columns fell out of counting, and the elements in one column have the same outer shell because that is what a column is.

Those two sentences are the whole of this lesson, and they are exact rather than approximate: sulfur has six outer electrons and is in group 6, chlorine has seven and is in group 7. This course numbers the main groups 1 to 8, which is the numbering that makes the rule true as written. A modern table numbers all eighteen columns instead, counting the transition metals in as it goes, so what is group 7 here is column 17 there and group 8 here is column 18. Same columns, two ways of counting them.

Another way: picture

Think of each period as a floor of an apartment building and each group as the apartment in the same position on every floor. Lithium, sodium and potassium are the same apartment on three different floors: same layout, same one window facing out, further and further up. What happens at the front door depends on the apartment, not the floor — which is why a group behaves as one thing and a period does not.

Another way: steps

To place an element from its structure:

Count the electrons in the outer shell. That number is the group number.
Count how many shells hold any electrons at all. That number is the period number.
Metals are on the left and in the middle, non-metals at the top right, metalloids along the border between.
Check the two exceptions: helium is group 8 rather than group 2, and an element in the middle trough takes no group number at all.

5. The four families worth knowing by name

Each of these is a column, and each behaves the way it does for one reason: what its atoms have to do with their outer electrons.

Read the last column and the first column together and the whole of group chemistry is there. An alkali metal and a halogen meet, one electron moves, and you have a salt — which is why sodium chloride exists and why potassium bromide exists and why they look and behave so much alike.

6. The method, step by step, and how to check it

Family	Group	Outer electrons	What that forces
alkali metals	1	1	One electron to give away, and it goes easily — soft metals that react hard with water
alkaline earth metals	2	2	Two to give away, so the ion carries two positive charges
halogens	7	7	One short of full, so they take one — the ion carries one negative charge
noble gases	8	8 (2 for helium)	Nothing to gain and nothing to give, so almost no reactions at all

Write the electron arrangement. From the atomic number, fill 2, then 8, then 8.

Count the outer electrons. The last number of the arrangement. That is the group.

Count the shells in use. How many numbers the arrangement has. That is the period.

Classify it. Left and middle are metals; top right non-metals; the staircase border between them metalloids.

Predict the ion. Groups 1 to 3 lose their outer electrons and form positive ions with that charge. Groups 5 to 7 take electrons to reach eight and form negative ions with a charge of eight less the group. Group 8 forms none.

Check the work. Is the group between 1 and 8, and the period between 1 and 4 for the elements in this course? Does the predicted ion make sense — would chlorine take one electron rather than lose seven? And have you caught the two exceptions: helium, whose full shell has two, and the transition metals, which take no group number?

7. Why each step is allowed

Reading the group from the outer shell is allowed because the table was built so that it would be true. Each row is one shell filling, so an element's place along its row is how far its outer shell has filled.

Reading the period from the number of shells is allowed for the same reason: a new row begins with the first electron in a new shell.

Predicting that a group 1 atom loses one electron, and a group 7 atom takes one, is allowed because an atom with a full outer shell is particularly stable, and each of these reaches a full shell in the fewest moves. Losing seven electrons, or taking seven, would cost far more energy than moving one.

Trusting a column to behave alike is allowed because chemistry happens where atoms meet, and atoms meet at their outer shells. Elements with the same outer shell offer the same thing to a partner, however different their nuclei.

8. Where the simple rule stops

Helium. Two electrons in its outer shell, and it is in group 8 rather than group 2. The reason is that the first shell holds two and not eight, so helium's outer shell is full — which is what group 8 means. The group is defined by the full shell, not by the number eight, and helium is the one element in the first twenty where the outer-shell count gives the wrong column.

The transition metals. The ten short columns in the middle have no group number worth writing down. When you get to the fourth period the outer shell stops filling at two electrons and the shell underneath it starts taking electrons instead. So iron, copper and zinc all have much the same outer shell, and what distinguishes them is a layer further in. That is why they behave so much alike, why several of them form more than one kind of ion, and why so many of their compounds are colored — none of which the group rule predicts, because the group rule is about the outer shell and this is not happening in the outer shell.

A rule with its limits stated is a rule you can use. A rule without them is one you will apply to helium.

9. In the world: why fluoride and chlorine protect American water

Two members of group 7 do much of the work of keeping American water safe, and their position on the table explains what they do. Both have seven outer electrons, one short of a full shell, so both take an electron readily from almost anything they meet.

Chlorine's eagerness is what makes it a disinfectant. Added to the water supply of most American cities at about 1 to 4 milligrams per liter, it pulls electrons from the molecules in a bacterium's cell wall and kills it. A Houston public pool is dosed the same way, and the chlorine smell at a pool is mostly chlorine that has already reacted with something.

Fluoride is the ion fluorine forms once it has taken its electron: nine protons and ten electrons, a full outer shell, and therefore stable rather than reactive. Denver and many other cities add it to tap water at about 0.7 milligrams per liter, where it becomes part of the mineral of tooth enamel and makes it harder for acids to dissolve. The same column, two very different jobs: the atom that takes electrons is a disinfectant, and the ion that has already taken one is a stable part of a tooth.

10. In the world: helium for MRI magnets

Hospitals cool the magnets of MRI scanners with liquid helium. Helium's full outer shell means it reacts with nothing and stays liquid at the lowest temperature of any substance, so it can chill the magnet without ever forming a compound with it.

11. Where this goes wrong

Group and period get swapped. They are counts of different things: the group counts electrons in one shell, the period counts shells. Chlorine is group 7, period 3 — seven outer electrons in the third shell. Getting them the wrong way round predicts nothing correctly.

The group number is read as the atomic number. Potassium is in group 1 and has nineteen protons. The atomic number is what the elements were put in order of; the group is where that order landed them.

A transition metal is given a group number. Counting along the row and calling iron "group 8" predicts that it forms one ion with a charge of eight, which it does not. It forms two ions, with charges of two and three, and the reason is in the shell beneath the outer one rather than in the outer shell the group number is about.

Helium is put in group 2. It has two outer electrons, and its shell is full at two. It is a noble gas and behaves like one.

The table is treated as a list to memorize. It is the opposite: it exists so that you do not have to memorize the behavior of a hundred elements, because the column predicts it.

12. Placing an element from its electrons

13. Why two elements in one group behave alike

14. Placing magnesium and predicting its oxide

15. Your turn: an atom has electrons arranged 2, 8, 2. Where is it, and what will it do?

16. Guided practice

An atom of potassium has an outer-shell electron count of $1$. Mark the part of the table it belongs to. The picture is the outline of the periodic table with the symbols left out. Reading left to right: the tall column at the far left is group 1; the column beside it is group 2; the ten short columns forming the wide trough in the middle are the transition metals, which take no group number; the four columns after the trough are groups 3, 4, 5 and 6; the column second from the right is group 7; and the tall column at the far right is group 8.

17. Guided practice

Complete the worked solution: an element in the third period has atomic number $15$, with two electrons in its first shell and eight in its second. Find its outer-shell electrons, its group, and how many electrons it takes to fill its outer shell.

18. Guided practice

Each of these elements belongs to a named family of the periodic table. Match them up. Two of them are in the same family.

19. Guided practice

Two elements are in the same group of the periodic table but in different periods. Which of these must they have in common?

20. Practice

bromine is in group $7$ and period $4$. How many electrons does one atom of it take to fill its outer shell?

21. Practice

The fluoride added to Denver's tap water is there as the ion. fluorine is in group 7, with atomic number $9$. How many electrons does one of its ions carry?

22. Somewhere new

Somebody needs a metal whose ions carry two positive charges, and whose carbonate is the main substance of limestone. Mark the part of the periodic table they should go looking in. The picture is the outline of the periodic table with the symbols left out. Reading left to right: the tall column at the far left is group 1; the column beside it is group 2; the ten short columns forming the wide trough in the middle are the transition metals, which take no group number; the four columns after the trough are groups 3, 4, 5 and 6; the column second from the right is group 7; and the tall column at the far right is group 8.

23. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

24. Test question

For each of these three elements, give the group number, the period number and the number of electrons in the outer shell, and say whether it is a metal, a non-metal or a metalloid.

25. What you can do now

You can place an element from its electron arrangement, and read its position as a count of outer electrons and a count of shells. Say out loud why fluorine and chlorine react so much alike when almost everything else about their atoms is different. Next: the trends that run across a period and down a group, and the exceptions that show what causes them.

Working for the steps left to you

15. Your turn: an atom has electrons arranged 2, 8, 2. Where is it, and what will it do?, step 3