Chapter 5 of 8

All 8 chapters
  1. 1Inside the atom, readunderstood
  2. 2How the model of the atom changed, readunderstood
  3. 3Isotopes, readunderstood
  4. 4Electron shells, readunderstood
  5. 5The periodic table, readunderstood
  6. 6Ionic bonding, readunderstood
  7. 7Covalent bonding, readunderstood
  8. 8Metals and alloys, readunderstood

S4.6.5 Chapter 5

The periodic table

How did one chemist predict elements that nobody had found yet?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how Mendeleev arranged the elements and left gaps
  • Describe trends in groups 1, 7 and 0

By 1869 chemists knew about 60 elements. They had measured how heavy the atoms of each element are and how each element behaves. Many tried to find a pattern.

Dmitri Mendeleev wrote each element on a card. He put the cards in order of atomic mass, then began a new row whenever the properties began to repeat. Elements with similar properties fell in one column. This was his periodic table.

Mendeleev trusted the pattern more than the order of masses. Tellurium has a slightly greater mass than iodine. But iodine behaves like chlorine and bromine, so he put iodine after tellurium so that it sat under them. He swapped the pair.

Some places in his table fitted no known element. Mendeleev did not move the others to close the gap. He left it empty and said an element there had not been found yet. From the elements above and below it, and on each side, he predicted its properties.

The main groups of the periodic table up to krypton, with group numbers 1, 2, 3, 4, 5, 6, 7 and 0 across the top and period numbers 1, 2, 3 and 4 down the side. Two cells in period 4, in group 3 and group 4, show a question mark, ?. They are two of the gaps that Mendeleev left, under aluminium and under silicon. The other cells hold the symbols H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, As, Se, Br and Kr. A label says transition metals go here, for the narrow gap between groups 2 and 3.HHeLiBeBCNOFNeNaMgAlSiPSClArKCa??AsSeBrKr123456701234transition metals go here
The modern shape of the table, with two of the gaps that Mendeleev left marked with a question mark. His own table looked different.

The element under silicon he called eka-silicon. The element under aluminium he called eka-aluminium. Within about twenty years gallium and then germanium were found. They fitted his predictions closely.

A table that compares Mendeleev's prediction for the element under silicon with the real germanium. The first column is headed Value and the other columns are Mass and Density. In the row predicted, the mass is 72 and the density is 5.5 g/cm³. In the row found, the mass is 72.6 and the density is 5.3 g/cm³.ValueMassDensitypredicted725.5 g/cm³found72.65.3 g/cm³
Germanium was found in 1886. The mass has no unit because it is a relative mass.

Worked example

Mendeleev predicted a mass of 72 for the element under silicon. Germanium has a mass of 72.6. By what percentage was his prediction below the real value? Give it to one decimal place.

  1. The difference is 72.6 − 72 = 0.6.
  2. As a fraction of the real value: 0.6 ÷ 72.6 = 0.0083.
  3. As a percentage: 0.0083 × 100 = 0.8% to one decimal place.

Answer: 0.8%

A prediction that comes true is strong evidence. After germanium, most chemists accepted the table. Mendeleev ordered by atomic mass because atoms were not yet known to have protons.

Today the table is ordered by atomic number, which is the number of protons. This removes the need to swap pairs. Tellurium has 52 protons and iodine has 53, so iodine follows tellurium in the order.

The shape of the periodic table in 18 columns and 7 periods, without the two rows usually printed below it. Group 1 is hatched at the left, labelled group 1. Hydrogen, at the top left, is left white. Group 2 is cross hatched, labelled group 2. The block of transition metals in the middle is dotted, labelled transition metals. Group 7 is cross hatched, labelled group 7. Group 0 at the right is black, labelled group 0. A staircase line separates the metals, labelled metals, from the non-metals, labelled non-metals.metalsnon-metalsgroup 1group 0group 2group 7transition metals
The modern table. Most elements are metals. They are on the left of the staircase. Hydrogen, at the top left, is a non-metal. Elements next to the staircase, such as boron, silicon and germanium, are metalloids, with some properties of each. The 28 elements usually printed in two rows below the table, the lanthanides and actinides, are left out.

Elements in a group have the same number of outer electrons. This is why they react in a similar way. Within a group the properties change step by step. A step by step change like this is a trend.

Group 1 is the alkali metals: lithium, sodium and potassium are the first three. They are soft enough to cut with a knife, and they have a low density. They tarnish quickly in air. They react with water to make hydrogen and an alkali. For example: sodium + water → sodium hydroxide + hydrogen.

The trend is that group 1 metals become more reactive going down. Lithium fizzes steadily in water. Sodium melts into a ball and fizzes fast. Potassium reacts so fast that its hydrogen catches fire with a lilac flame. Reactivity is how readily an element takes part in reactions.

Why does reactivity rise? A group 1 atom reacts by losing its one outer electron. Going down, the outer electron is further from the nucleus, and more inner shells lie between them. The pull of the nucleus on it is weaker. So the electron is lost more easily.

Two atoms of group 1 side by side. Lithium, Li, has two shells, and below it the words lithium and held tightly. Potassium, K, has four shells, and below it the words potassium and held loosely. Each has one outer electron.Lilithiumheld tightlyKpotassiumheld loosely
The outer electron of potassium is far from the nucleus, so it is lost more easily than the outer electron of lithium.

Group 7 is the halogens: fluorine, chlorine, bromine and iodine. They are non-metals. Their atoms pair up in molecules of two: F₂, Cl₂, Br₂ and I₂. Their colours get darker going down, and their boiling points rise.

A table of the halogens with the columns Halogen, Colour and State at room temperature. Fluorine is a yellow gas. Chlorine is a green gas. Bromine is a red-brown liquid. Iodine is a dark grey solid.HalogenColourStatefluorineyellowgaschlorinegreengasbrominered-brownliquidiodinedark greysolid
The state is for room temperature. The yellow of fluorine is pale and the green of chlorine is a yellow green. The colours deepen going down, and the state moves from gas to solid.
A table of boiling points of the halogens with the columns Halogen and Boils at in °C. Fluorine boils at −188 °C. Chlorine boils at −34 °C. Bromine boils at 59 °C. Iodine boils at 184 °C.HalogenBoils atfluorine−188 °Cchlorine−34 °Cbromine59 °Ciodine184 °C
Boiling points rise going down group 7.

Going down group 7 the halogens become less reactive. A halogen atom reacts by gaining one electron. Further down, the outer shell is further from the nucleus and more shielded. The nucleus pulls an incoming electron less strongly, so the electron is gained less easily.

Two atoms of group 7 side by side. Fluorine, F, has two shells, and below it the words fluorine and pulls strongly. Chlorine, Cl, has three shells, and below it the words chlorine and pulls less. Each has seven outer electrons.Ffluorinepulls stronglyClchlorinepulls less
A space in the outer shell of fluorine is close to the nucleus. The same space in chlorine is further away.

Group 0 is the noble gases: helium, neon, argon and krypton. Each has a full outer shell. They do not need to gain, lose or share electrons, so they are very unreactive. Only a few compounds of the heavier noble gases, such as krypton, are known. Their atoms stay single, not joined in molecules.

A table of boiling points of the noble gases with the columns Noble gas and Boils at in °C. Helium boils at −269 °C. Neon boils at −246 °C. Argon boils at −186 °C. Krypton boils at −153 °C.Noble gasBoils athelium−269 °Cneon−246 °Cargon−186 °Ckrypton−153 °C
All are gases at room temperature. Boiling points rise going down group 0.

Worked example

Argon boils at −186 °C and krypton at −153 °C. By how many degrees is the boiling point of krypton higher?

  1. Subtract the lower boiling point from the higher one.
  2. −153 − (−186) = −153 + 186.
  3. Krypton is 33 °C higher.

Answer: 33 °C

Try it

Sort the elements like Mendeleev

You need: a sheet of paper, torn into 18 cards, a pencil, the table of electron arrangements in Chapter 4, a flat table or floor.

  1. Write on each card a symbol from Li to Ca: that is elements 3 to 20. Write its outer electrons, from Chapter 4.
  2. Put the cards in order of atomic number, from lithium.
  3. Start a new row after each card with 8 outer electrons: after neon, and after argon.
  4. Slide the rows so that cards with the same number of outer electrons are in one column.
  5. Look down the columns. Which cards are together?

What you will see: Each column holds elements with the same outer electrons. Lithium, sodium and potassium are together in group 1. Fluorine and chlorine are together in group 7. Neon and argon are together in group 0. Their properties are alike too. The order of atomic number makes the pattern appear.

If you cannot try it, this is what happens.
The periodic table up to calcium, with the cells for lithium, Li, sodium, Na, and potassium, K, hatched. All three are in group 1. The other cells hold the symbols H, He, Be, B, C, N, O, F, Ne, Mg, Al, Si, P, S, Cl, Ar and Ca. The group numbers across the top are 1, 2, 3, 4, 5, 6, 7 and 0.HHeLiBeBCNOFNeNaMgAlSiPSClArKCa123456701234

A wrong idea many people have

Every group becomes more reactive going down.

Why it feels right: Group 1 does, and the dramatic reactions of potassium are the ones people remember.

The test that breaks it: Check group 7. Fluorine reacts with almost everything, and iodine reacts slowly. Going down, the halogens become less reactive, not more.

What is true: Metals lose electrons, and that is easier when the outer electron is further from the nucleus. Non-metals gain electrons, and that is harder when the shell is further away. So group 1 rises and group 7 falls.

Key words

periodic table
the elements in order of atomic number, in rows and columns so that similar elements are in one column.
predicted
said before it is found, from a pattern.
trend
a change that goes step by step down a group or along a period.
alkali metals
the metals of group 1.
reactivity
how readily an element takes part in reactions.
halogens
the non-metals of group 7.
noble gases
the unreactive gases of group 0.

Back to your guess

The answer is A: He left gaps in his table and worked out what the missing elements would be like.

Mendeleev left gaps where no known element fitted the pattern. From the elements round each gap he predicted the missing element. Later it was found, and the predictions held.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. How did Mendeleev arrange the elements?

    Tap your answer.

    1. Not this one. That is how the modern table is arranged. Protons were not known in his day. Try another.

    2. Not this one. He did not use the date of discovery. He used atomic mass and behaviour. Try another.

    3. Yes, that is right. He put the elements in order of atomic mass and began a new row so that similar elements lined up. He swapped a pair where the properties demanded it.

    See the answer

    By atomic mass, in columns of similar elements

    He put the elements in order of atomic mass and began a new row so that similar elements lined up. He swapped a pair where the properties demanded it.

    • If you chose A: That is how the modern table is arranged. Protons were not known in his day.
    • If you chose B: He did not use the date of discovery. He used atomic mass and behaviour.

  2. This table has two gaps, marked with a question mark. Mendeleev left a gap under silicon. What did he predict about the missing element?

    The main groups of the periodic table up to krypton, with group numbers 1, 2, 3, 4, 5, 6, 7 and 0 across the top and period numbers 1, 2, 3 and 4 down the side. Two cells in period 4, in group 3 and group 4, show a question mark, ?. The other cells hold the symbols H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, As, Se, Br and Kr. A label says transition metals go here.HHeLiBeBCNOFNeNaMgAlSiPSClArKCa??AsSeBrKr123456701234transition metals go here
    Tap your answer.

    1. Not this one. Sodium is in group 1. The gap is in the same column as silicon, so it should be like silicon. Try another.

    2. Yes, that is right. Elements in a column have similar properties. The gap is in the silicon column, so the missing element should be like silicon.

    3. Not this one. Argon is at the far right, in group 0. The gap is far from it, in the silicon column. Try another.

    See the answer

    It would behave like silicon, the element above it.

    Elements in a column have similar properties. The gap is in the silicon column, so the missing element should be like silicon.

    • If you chose A: Sodium is in group 1. The gap is in the same column as silicon, so it should be like silicon.
    • If you chose C: Argon is at the far right, in group 0. The gap is far from it, in the silicon column.

  3. Going down the alkali metals and going down the halogens, how does reactivity change?

    Tap your answer.

    1. Yes, that is right. A group 1 atom loses an electron, and that is easier further down. A group 7 atom gains an electron, and that is harder further down.

    2. Not this one. That is true of the alkali metals only. The halogens become less reactive going down. Try another.

    3. Not this one. It is the other way round. Potassium is more reactive than lithium, and fluorine is more reactive than iodine. Try another.

    See the answer

    It rises for the alkali metals and falls for the halogens.

    A group 1 atom loses an electron, and that is easier further down. A group 7 atom gains an electron, and that is harder further down.

    • If you chose B: That is true of the alkali metals only. The halogens become less reactive going down.
    • If you chose C: It is the other way round. Potassium is more reactive than lithium, and fluorine is more reactive than iodine.

  4. Why is potassium more reactive than lithium?

    Tap your answer.

    1. Not this one. Both have one outer electron, as both are in group 1. The distance from the nucleus is what changes. Go to S4.6.4. Try another.

    2. Not this one. The pull is weaker in potassium, because the electron is further away and more inner shells shield it. Try another.

    3. Yes, that is right. Potassium has four shells and lithium has two. The outer electron of potassium is held more loosely.

    See the answer

    Its outer electron is further from the nucleus, so it is lost more easily.

    Potassium has four shells and lithium has two. The outer electron of potassium is held more loosely.

    • If you chose A: Both have one outer electron, as both are in group 1. The distance from the nucleus is what changes. Go to S4.6.4.
    • If you chose B: The pull is weaker in potassium, because the electron is further away and more inner shells shield it.

  5. Chlorine is more reactive than bromine. Why?

    Tap your answer.

    1. Not this one. Both are in group 7 and have seven outer electrons. Go to S4.6.4. Try another.

    2. Yes, that is right. A halogen reacts by gaining one electron. The nearer the outer shell is to the nucleus, the more strongly the electron is pulled in.

    3. Not this one. Bromine atoms are heavier. A reaction depends on how strongly the nucleus pulls an extra electron, not on mass. Try another.

    See the answer

    Its outer shell is closer to the nucleus, so it pulls an extra electron in more strongly.

    A halogen reacts by gaining one electron. The nearer the outer shell is to the nucleus, the more strongly the electron is pulled in.

    • If you chose A: Both are in group 7 and have seven outer electrons. Go to S4.6.4.
    • If you chose C: Bromine atoms are heavier. A reaction depends on how strongly the nucleus pulls an extra electron, not on mass.

  6. Why is argon unreactive?

    Tap your answer.

    1. Yes, that is right. A full outer shell is a steady arrangement. Argon has 2, 8, 8, so it keeps its electrons as they are.

    2. Not this one. Argon has a full outer shell of eight electrons. One outer electron is group 1. Go to S4.6.4. Try another.

    3. Not this one. Size is not the reason. Neon is smaller and argon is bigger, yet both are unreactive because their outer shells are full. Try another.

    See the answer

    Its outer shell is full, so it has no need to gain, lose or share electrons.

    A full outer shell is a steady arrangement. Argon has 2, 8, 8, so it keeps its electrons as they are.

    • If you chose B: Argon has a full outer shell of eight electrons. One outer electron is group 1. Go to S4.6.4.
    • If you chose C: Size is not the reason. Neon is smaller and argon is bigger, yet both are unreactive because their outer shells are full.

  7. From an earlier chapter

    An atom has the electron arrangement 2, 8, 7. Which group is it in?

    Tap your answer.

    1. Not this one. Three is the number of shells, which gives the period. The group is the number of outer electrons. Go to S4.6.4. Try another.

    2. Yes, that is right. The outer shell holds 7 electrons, so the atom is in group 7. It is chlorine, a halogen.

    3. Not this one. Eight is the electrons in shell 2. Count only the outer shell. Go to S4.6.4. Try another.

    See the answer

    Group 7

    The outer shell holds 7 electrons, so the atom is in group 7. It is chlorine, a halogen.

    • If you chose A: Three is the number of shells, which gives the period. The group is the number of outer electrons. Go to S4.6.4.
    • If you chose C: Eight is the electrons in shell 2. Count only the outer shell. Go to S4.6.4.

  8. Neon boils at −246 °C and krypton boils at −153 °C. By how many degrees is the boiling point of krypton higher?

    See the answer

    93 °C

    −153 − (−246) = −153 + 246 = 93.

  9. Chlorine boils at −34 °C and iodine boils at 184 °C. By how many degrees is the boiling point of iodine higher?

    See the answer

    218 °C

    184 − (−34) = 184 + 34 = 218.

Keep it

Copy this into your exercise book.

Mendeleev ordered by atomic mass, grouped by properties, swapped pairs and left gaps. Today: atomic number. Group 1 reacts more going down. Group 7 reacts less. Group 0 is unreactive, with a full outer shell.

Draw: The table shape with group 1, group 7 and group 0 marked, and an arrow down each for the trend.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (826 KB). It opens on any phone.