Chapter 4 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

S4.6.4 Chapter 4

Electron shells

Sodium has 11 electrons. How are they arranged round the nucleus?

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

  • Write electron arrangements for the first 20 elements
  • Link an element's group and period to its electrons

In the model of Bohr, the electrons of an atom move on shells. Each shell is at its own distance from the nucleus. The shell nearest the nucleus is shell 1.

A shell can hold only so many electrons. Electrons go into the shell nearest the nucleus first. When it is full, the next one starts. The outermost shell that has electrons is the outer shell.

For the first 20 elements the shells fill to 2, 8 and 8. Then shell 4 starts: potassium puts 1 electron on it and calcium puts 2. This rule works well for these elements. In bigger atoms the third shell can hold more than 8. Shell 4 can hold more too, so the pattern is more complex.

Draw it first

Filling the shells

  1. Four atoms, one at a time. First helium, He, with 2 electrons on one shell, which is full. Then lithium, Li, with 2 electrons and 1 on the second shell. Then neon, Ne, with 2 and 8, so the second shell is full. Then sodium, Na, with 2, 8 and 1 on a third shell.He

    1 Helium has 2 electrons. They fill shell 1.

  2. Four atoms, one at a time. First helium, He, with 2 electrons on one shell, which is full. Then lithium, Li, with 2 electrons and 1 on the second shell. Then neon, Ne, with 2 and 8, so the second shell is full. Then sodium, Na, with 2, 8 and 1 on a third shell.Li

    2 Lithium has 3 electrons. Two fill shell 1 and the third starts shell 2.

  3. Four atoms, one at a time. First helium, He, with 2 electrons on one shell, which is full. Then lithium, Li, with 2 electrons and 1 on the second shell. Then neon, Ne, with 2 and 8, so the second shell is full. Then sodium, Na, with 2, 8 and 1 on a third shell.Ne

    3 Neon has 10 electrons. Two fill shell 1 and eight fill shell 2.

  4. Four atoms, one at a time. First helium, He, with 2 electrons on one shell, which is full. Then lithium, Li, with 2 electrons and 1 on the second shell. Then neon, Ne, with 2 and 8, so the second shell is full. Then sodium, Na, with 2, 8 and 1 on a third shell.Na

    4 Sodium has 11 electrons. Shells 1 and 2 are full, so the last electron starts shell 3.

We write an electron arrangement as the numbers on each shell, from the inside, with commas. Sodium is 2, 8, 1. The numbers always add up to the number of electrons, which is the atomic number.

A table of elements 1 to 10 with the columns Atom, Electrons and Per shell, which lists the electrons on each shell. H has 1 electrons, on shells as follows: 1. He has 2 electrons, on shells as follows: 2. Li has 3 electrons, on shells as follows: 2, 1. Be has 4 electrons, on shells as follows: 2, 2. B has 5 electrons, on shells as follows: 2, 3. C has 6 electrons, on shells as follows: 2, 4. N has 7 electrons, on shells as follows: 2, 5. O has 8 electrons, on shells as follows: 2, 6. F has 9 electrons, on shells as follows: 2, 7. Ne has 10 electrons, on shells as follows: 2, 8.AtomElectronsPer shellH11He22Li32, 1Be42, 2B52, 3C62, 4N72, 5O82, 6F92, 7Ne102, 8
The electron arrangements of the first ten elements. Hydrogen has 1 electron, so its arrangement is 1.
A table of elements 11 to 20 with the columns Atom, Electrons and Per shell, which lists the electrons on each shell. Na has 11 electrons, on shells as follows: 2, 8, 1. Mg has 12 electrons, on shells as follows: 2, 8, 2. Al has 13 electrons, on shells as follows: 2, 8, 3. Si has 14 electrons, on shells as follows: 2, 8, 4. P has 15 electrons, on shells as follows: 2, 8, 5. S has 16 electrons, on shells as follows: 2, 8, 6. Cl has 17 electrons, on shells as follows: 2, 8, 7. Ar has 18 electrons, on shells as follows: 2, 8, 8. K has 19 electrons, on shells as follows: 2, 8, 8, 1. Ca has 20 electrons, on shells as follows: 2, 8, 8, 2.AtomElectronsPer shellNa112, 8, 1Mg122, 8, 2Al132, 8, 3Si142, 8, 4P152, 8, 5S162, 8, 6Cl172, 8, 7Ar182, 8, 8K192, 8, 8, 1Ca202, 8, 8, 2
Elements 11 to 20. Argon finishes shell 3 with 8. Potassium starts shell 4, and calcium adds a second electron to it.

Look at the periodic table with these arrangements in mind. A column is a group. A row is a period. The table is laid out so that elements in one group have the same number of outer electrons.

The periodic table up to calcium, with a word group above the columns. The group numbers across the top are 1, 2, 3, 4, 5, 6, 7 and 0. The period numbers down the side are 1, 2, 3 and 4. The cells hold the symbols H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K and Ca. The cell for sodium, Na, is hatched. It is in group 1 and period 3. Hydrogen, H, is above group 1 and helium, He, above group 0.groupHHeLiBeBCNOFNeNaMgAlSiPSClArKCa123456701234
The numbers down the left side are the periods. Sodium is hatched.

The group number is the number of electrons in the outer shell. Sodium has 1 outer electron, so it is in group 1. Sulfur is 2, 8, 6, so it is in group 6. Group 0 is different: its elements have a full outer shell, which is 2 for helium and 8 for the others.

Many tables number the columns 1 to 18 instead. Then group 6 is group 16, group 7 is group 17 and group 0 is group 18. This book uses 1 to 7 and 0.

The period number is the number of shells. Sodium has 3 shells, so it is in period 3. Lithium has 2 shells, so it is in period 2.

Three atoms of group 1 with all their shells, each with one outer electron. Lithium, Li, has 2 shells, with 2 electrons and then 1. Sodium, Na, has 3 shells, with 2, 8 and then 1. Potassium, K, has 4 shells, with 2, 8, 8 and then 1. Lithium and sodium are side by side, with potassium below them. Under them are the words one outer electron each.LiNaKone outer electron each
Group 1: one outer electron each, on a different shell. Lithium is in period 2, sodium in period 3 and potassium in period 4.

Worked example

Phosphorus has atomic number 15. Write its electron arrangement.

  1. A neutral atom has 15 electrons.
  2. Shell 1 takes 2, which leaves 13. Shell 2 takes 8, which leaves 5.
  3. The last 5 go in shell 3. The arrangement is 2, 8, 5.

Answer: 2, 8, 5

Worked example

Calcium has atomic number 20. Write its electron arrangement, then give its group and its period.

  1. A neutral atom has 20 electrons. Fill the shells in turn: 2, 8, 8. That uses 18.
  2. Shell 4 takes the last 2. The arrangement is 2, 8, 8, 2.
  3. The group is the outer electrons: 2. The period is the number of shells: 4.

Answer: 2, 8, 8, 2, group 2, period 4

Helium, neon and argon are in group 0. Their outer shells are full. This is why they hardly ever react. A full outer shell is a very steady arrangement.

See it three ways

As a picture

A sulfur atom in Bohr shells. The nucleus is labelled S. Two electrons are on the first shell, 8 on the second and 6 on the third, which is the outer shell.S

In words

Sulfur has 16 electrons. They fill shell 1, then shell 2, and the last 6 go in shell 3. Six outer electrons put sulfur in group 6. Three shells put it in period 3.

In symbols

Arrangement 2, 8, 6: group 6, period 3

Try it

Fill the shells with seeds

You need: a sheet of paper and a pencil, a plate, a bowl, a cup and a small lid, to draw round, about 20 small seeds or beans.

Be safe: Do not eat the seeds or beans you use for the model. Keep small seeds away from young children.

  1. Draw round the plate, the bowl, the cup and the lid, one inside another, to make four rings. The lid is the middle.
  2. Choose an element, for example magnesium, which has 12 electrons.
  3. Put the seeds on the rings one at a time, starting at the inner ring. Put 2 on ring 1 and 8 on ring 2.
  4. Put what is left on ring 3. Count the seeds on each ring.
  5. Try chlorine with 17 electrons, then calcium with 20 electrons.

What you will see: Magnesium comes out as 2, 8, 2. The outer ring has 2 seeds, so magnesium is in group 2. It uses 3 rings, so it is in period 3. When a ring is full you must start the next one.

If you cannot try it, this is what happens.
A magnesium atom in Bohr shells. The nucleus is labelled Mg. Two electrons are on the first shell, 8 on the second and 2 on the third, which is the outer shell.Mg

A wrong idea many people have

The group number tells you how many shells an atom has.

Why it feels right: Both groups and periods are small whole numbers, so they are often mixed up.

The test that breaks it: Take sodium. It is in group 1, but its arrangement is 2, 8, 1, which has three numbers. Count the shells and you get 3.

What is true: The group number is the number of outer electrons. The period number is the number of shells. Sodium is group 1 and period 3.

Key words

shell
a place at a set distance from the nucleus where electrons move.
outer shell
the outermost shell that holds electrons.
electron arrangement
the number of electrons on each shell, from the inside, such as 2, 8, 1.
group
a column of the periodic table. The number is the outer electrons.
period
a row of the periodic table. The number is the number of shells.
group number
the number of electrons in the outer shell, with group 0 for a full outer shell.
period number
the number of shells in the atom.

Back to your guess

The answer is B: 2 on the first shell, 8 on the second and 1 on the third.

Electrons fill the shells from the inside. The first shell takes 2 and the second takes 8. The last electron goes on a third shell.

Check yourself

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

  1. This is an atom of sulfur. Which group of the periodic table is it in?

    A sulfur atom in Bohr shells with the nucleus labelled S. There are three shells. The outer shell has several electrons.S
    Tap your answer.

    1. Not this one. Three is the number of shells, so it gives the period. The group is the number of outer electrons. Try another.

    2. Yes, that is right. Count the electrons on the outer shell. There are 6, so sulfur is in group 6.

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

    See the answer

    Group 6

    Count the electrons on the outer shell. There are 6, so sulfur is in group 6.

    • If you chose A: Three is the number of shells, so it gives the period. The group is the number of outer electrons.
    • If you chose C: Eight is the electrons in shell 2. Count only the outer shell.

  2. An atom has the electron arrangement 2, 8, 3. In which period is it?

    Tap your answer.

    1. Not this one. You may have added up the electrons. The period is the number of shells. Try another.

    2. Not this one. The first number is the electrons in shell 1, not the number of shells. Count the numbers in the arrangement. Try another.

    3. Yes, that is right. There are three numbers, so the atom has three shells. It is in period 3.

    See the answer

    Period 3

    There are three numbers, so the atom has three shells. It is in period 3.

    • If you chose A: You may have added up the electrons. The period is the number of shells.
    • If you chose B: The first number is the electrons in shell 1, not the number of shells. Count the numbers in the arrangement.

  3. Fluorine has atomic number 9. What is its electron arrangement?

    Tap your answer.

    1. Not this one. Shell 2 holds up to 8, but fluorine has only 9 electrons in all. 2 + 8 would be 10. Try another.

    2. Not this one. The numbers go from the inside out. Shell 1 is filled first, so it comes first. Try another.

    3. Not this one. All 9 cannot go on one shell. Shell 1 holds only 2, so the rest go on shell 2. Try another.

    4. Yes, that is right. Shell 1 takes 2. The other 7 go in shell 2. So the arrangement is 2, 7.

    See the answer

    2, 7

    Shell 1 takes 2. The other 7 go in shell 2. So the arrangement is 2, 7.

    • If you chose A: Shell 2 holds up to 8, but fluorine has only 9 electrons in all. 2 + 8 would be 10.
    • If you chose B: The numbers go from the inside out. Shell 1 is filled first, so it comes first.
    • If you chose C: All 9 cannot go on one shell. Shell 1 holds only 2, so the rest go on shell 2.

  4. Helium has only 2 electrons, yet it is very unreactive. Why?

    Tap your answer.

    1. Yes, that is right. Shell 1 holds at most 2 electrons. Helium has 2, so its outer shell is full. A full outer shell is a very steady arrangement.

    2. Not this one. Helium has only 2 electrons in all. Its outer shell is the first, which holds 2 when it is full. Try another.

    3. Not this one. It does. Its two electrons are on shell 1, which is its outer shell. Try another.

    See the answer

    Its outer shell is full.

    Shell 1 holds at most 2 electrons. Helium has 2, so its outer shell is full. A full outer shell is a very steady arrangement.

    • If you chose B: Helium has only 2 electrons in all. Its outer shell is the first, which holds 2 when it is full.
    • If you chose C: It does. Its two electrons are on shell 1, which is its outer shell.

  5. Which element is in group 7 and period 3?

    The periodic table up to calcium, with a word group above the columns. The group numbers across the top are 1, 2, 3, 4, 5, 6, 7 and 0. The period numbers down the side are 1, 2, 3 and 4. The cells hold the symbols H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K and Ca.groupHHeLiBeBCNOFNeNaMgAlSiPSClArKCa123456701234
    Tap your answer.

    1. Not this one. Sulfur is in period 3, but it is in group 6. It has 6 outer electrons. Try another.

    2. Yes, that is right. Chlorine has the arrangement 2, 8, 7. It has 7 outer electrons and 3 shells, so it is in group 7 and period 3.

    3. Not this one. Fluorine is in group 7, but it is in period 2. It has only 2 shells. Try another.

    See the answer

    Chlorine

    Chlorine has the arrangement 2, 8, 7. It has 7 outer electrons and 3 shells, so it is in group 7 and period 3.

    • If you chose A: Sulfur is in period 3, but it is in group 6. It has 6 outer electrons.
    • If you chose C: Fluorine is in group 7, but it is in period 2. It has only 2 shells.

  6. What is the most electrons the third shell holds for the first 20 elements?

    Tap your answer.

    1. Not this one. In bigger atoms the third shell can hold up to 18. For the first 20 elements it fills to 8, and then shell 4 starts. Try another.

    2. Not this one. Two is what shell 1 holds. Shells 2 and 3 hold 8 each for these elements. Try another.

    3. Yes, that is right. For the first 20 elements the shells fill to 2, 8 and 8, then shell 4 starts. Argon has the arrangement 2, 8, 8.

    See the answer

    8 electrons

    For the first 20 elements the shells fill to 2, 8 and 8, then shell 4 starts. Argon has the arrangement 2, 8, 8.

    • If you chose A: In bigger atoms the third shell can hold up to 18. For the first 20 elements it fills to 8, and then shell 4 starts.
    • If you chose B: Two is what shell 1 holds. Shells 2 and 3 hold 8 each for these elements.

  7. From an earlier chapter

    Chlorine 35 and chlorine 37 are isotopes of chlorine, which has atomic number 17. What is the electron arrangement of both?

    Tap your answer.

    1. Yes, that is right. Both isotopes have 17 protons, so 17 electrons. They fill 2, then 8, then 7 on the third shell.

    2. Not this one. That is argon, with 18 electrons. Chlorine has 17, so the last shell holds 7. Try another.

    3. Not this one. Chlorine 37 has more neutrons, not more electrons. Both isotopes have 17 electrons. Go to S4.6.3. Try another.

    See the answer

    2, 8, 7

    Both isotopes have 17 protons, so 17 electrons. They fill 2, then 8, then 7 on the third shell.

    • If you chose B: That is argon, with 18 electrons. Chlorine has 17, so the last shell holds 7.
    • If you chose C: Chlorine 37 has more neutrons, not more electrons. Both isotopes have 17 electrons. Go to S4.6.3.

  8. Write the electron arrangement of the atom with atomic number 13. Write it like 2, 8, 1.

    See the answer

    2, 8, 3

    Shell 1 takes 2 and shell 2 takes 8. The last 3 go in shell 3.

  9. An atom has 7 electrons. Write its group number and then its period number.

    See the answer

    Group 5, period 2

    The arrangement is 2, 5. There are 5 outer electrons, so it is in group 5, and 2 shells, so period 2.

Keep it

Copy this into your exercise book.

Shells fill from the inside: 2, 8, 8, then shell 4, for the first 20 elements. Group number = outer electrons (group 0 has a full outer shell). Period number = number of shells. Sodium: 2, 8, 1.

Draw: A sodium atom with its three shells and the numbers 2, 8 and 1, and the words group 1 and period 3.

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

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