GCSE · Chemistry · Edexcel · Spec 1CH0

Electronic configuration and position in the periodic table

A configuration like 2,8,6 works like an address: how many numbers there are tells you one thing, the last number tells you another.

Chemistry · The periodic table

Read the numbers, find the address

Place each atom using the rule for the criterion you are on. Try to say the rule to yourself before you pick.

Which group is this atom in?

Still to sort

Group 0 (0)

A full outer shell.

Where the line is: Group 0 is not 'zero outer electrons'. These atoms do have outer-shell electrons; the outer shell is just full.

Group 2 (0)

Two electrons in the outer shell.

Where the line is: Two outer electrons only means group 2 when that outer shell is not already full.

Group 3 (0)

Three electrons in the outer shell.

Group 6 (0)

Six electrons in the outer shell.

6 of 6 still to sort.

Six atoms, written as electronic configurations. Sort them by group first, then switch the rule and sort the very same six by period.

Watch out: One atom will refuse to follow the outer-electrons rule. Which one, and why?

Group 0

The atom that breaks the rule

You have just learned that the group number is the number of electrons in the outer shell. Then you meet neon (2,8) and helium (2).

Which of these is closest to what you think right now?

Neon has eight outer-shell electrons, so it must be in group 8.Helium has two outer-shell electrons, so it should be in group 2.Group 0 must mean the atoms have no electrons in their outer shell.Helium (2) and neon (2,8) are both in group 0, because each has a full outer shell, even though they have different numbers of outer electrons.

How sure are you?

I'm sureI think soI'm guessing

Neon has eight outer-shell electrons, so it must be in group 8.

This is a common misconception

The counting is right: neon (2,8) does have eight electrons in its outer shell. The trap is the next step. Eight is a full outer shell, and elements with a full outer shell are in group 0. That is why the group is called group 0, not group 8.

Helium has two outer-shell electrons, so it should be in group 2.

This is a common misconception

Helium's only shell holds two electrons, and that shell is full. A full outer shell means group 0, so helium is in group 0, not group 2. Calcium (2,8,8,2) also ends in 2, but its outer shell is not full, so calcium is the one in group 2.

Group 0 must mean the atoms have no electrons in their outer shell.

This is a common misconception

Easy to assume, because 0 looks like 'none'. But helium has two electrons in its outer shell and neon has eight. They are in group 0 because the outer shell is full, not because it is empty.

Helium (2) and neon (2,8) are both in group 0, because each has a full outer shell, even though they have different numbers of outer electrons.

This matches the evidence

Exactly. Two electrons fill helium's shell and eight fill neon's outer shell. The outer-electron count differs, but a full outer shell is what puts both in group 0.

Group = outer-shell electrons, unless the outer shell is full. A full outer shell means group 0.

Worked example: calcium, forwards

Problem

A calcium atom has 20 electrons. Work out its electronic configuration, then its group and period.

Now backwards

From position to configuration

An element is in group 5 and period 3. What is its electronic configuration?

Period 3: the atom spreads its electrons over three shells.
missing step

Which line is step 2?

Group 5: the last of those three shells holds five electrons.Group 5: the atom spreads over five shells, and the last holds the rest.Group 5: the last shell is packed full, so it holds eight.

Check the working

Find the line where it goes wrong

Oxygen has the configuration 2,6. A student works out its group and period. Which line loses the mark?

A student's answer — which line goes wrong?

Oxygen's configuration is 2,6.
There are two numbers, so two shells are occupied.
The last number is 6, so there are six electrons in the outer shell.
Group is the number of occupied shells, so oxygen is in group 2. Period is the number of outer-shell electrons, so oxygen is in period 6.
Answer: oxygen is in group 2 and period 6.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A few numbers are all you need to find an element's address.

What you need to know

A configuration lists the electrons in each shell, starting with the shell nearest the nucleus.
Electrons fill the lowest energy level first. For the first 20 elements, the shells hold 2, then 8, then 8.
Have a goYour friend writes an atom with ten electrons as '10' in a single shell. Rewrite it properly before you look.
2,8
The first shell holds up to 2 electrons, and the second can then take up to 8, so the ten electrons split 2 and 8.
An element's position in the periodic table is linked to its atomic number and its arrangement of electrons.
Group number is the number of electrons in the outer shell. Oxygen (2,6) and sulphur (2,8,6) both have six, so both are group 6.
Have a goAn atom is written 2,5. Which number do you read to find its group, and what is the group?
The last number, 5, so group 5.
The group is the number of electrons in the outer shell, and the outer shell is the last one in the list.
Period number is the number of shells holding electrons. Boron (2,3) and oxygen (2,6) both have two, so both are period 2.
Have a goPriya says: 'Boron is 2,3, so it has two shells and must be in group 2.' Which rule has she borrowed?
She used the period rule for the group. Boron is period 2 and group 3.
Two shells gives the period. The group comes from the outer shell, whose last number is 3.
A full outer shell means group 0. Helium (2) and neon (2,8) are both full, so neon is group 0, not group 8.
Calcium (2,8,8,2) has four occupied shells and two outer electrons, so it is in period 4 and group 2.
Going backwards, the period gives the number of occupied shells and the group gives the electrons in the outer shell.

The big picture

An electronic configuration is an address. Count the numbers to get the period (the occupied shells). Read the last number to get the group (the outer-shell electrons), unless the outer shell is full, which means group 0. You can also work backwards from a position to a configuration.

Key points

1An electronic configuration shows how the electrons are arranged in shells around the nucleus, starting with the lowest energy level, the shell nearest the nucleus.

2Group number = number of electrons in the outer shell (the shell furthest from the nucleus).

3Period number = number of shells occupied by electrons.

4Elements in group 0 have a full outer shell, which is why the group is called group 0, not group 8.

5The configuration gives the group and period, and the position can be used to deduce the configuration.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Sulphur has the electronic configuration 2,8,6. State its period and its group, and say which rule you used for each.

⚠ Watch out

Mixing up which rule gives which. The number of shells gives the period and the outer-shell electrons give the group. The second slip is calling neon group 8 because of its eight outer electrons; a full outer shell means group 0.

🧠

Memory hook

Count the numbers for the period. Read the last number for the group. If the last shell is full, it's group 0.

✓

Check yourself

Without looking back: for neon (2,8) and for boron (2,3), say the group and the period, and name the rule you used each time.

Flashcards

(10)

What does an electronic configuration show?

How the electrons are arranged in shells around the nucleus.

Which shell do electrons fill first?

The lowest energy level, which is the shell nearest the nucleus.

How many electrons can the first, second and third shells hold (first 20 elements)?

Up to 2, then up to 8, then up to 8.

What does an element's group number tell you?

The number of electrons in its outer shell (the shell furthest from the nucleus).

What does an element's period number tell you?

The number of shells occupied by electrons.

What do group 0 elements have in common?

A full outer shell, for example helium (2) and neon (2,8).

Why is the group of neon not called group 8?

Neon's eight outer electrons make a full shell, and full-shell elements are group 0.

Oxygen (2,6) and sulphur (2,8,6): what do they share, and what differs?

Six outer electrons, so both are group 6. Different numbers of shells, so oxygen is period 2 and sulphur period 3.

Boron (2,3) and oxygen (2,6): what do they share?

Two occupied shells, so both are in period 2.

How do you work from a position back to a configuration?

The period gives the number of shells and the group gives the outer-shell electrons; the shells before the outer one are filled first.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Read the numbers, find the address

The atom that breaks the rule

From position to configuration

Find the line where it goes wrong

More Edexcel GCSE Chemistry topics