# Electron Structure

In this worksheet, students will learn how electrons are arranged in shells for different elements.

### QUESTION 1 of 10

Niels Bohr worked out that electrons can only be in certain parts of the atom. In this activity, you will learn how to apply his rules, and start to understand how useful they are.

Bohr worked out that electrons move around in shells, around the nucleus. For example, a potassium atom is drawn like this:

Potassium is atomic number 19 and mass number 39, so there are 19 protons and 20 neutrons in the nucleus. Because there are 19 protons, there must be 19 electrons to make a neutral atom. They are spread across four shells, according to these rules:

The first shell, closest to the nucleus, can only take 2 electrons.

The second shell can only take the next 8 electrons.

The third shell takes the next 8 electrons.

The fourth shell takes the remaining electrons.

These rules start to work differently if there are more than 20 electrons, but you won't be asked about that situation in a GCSE exam.

So, for potassium, we think about the electrons this way;

"Potassium is atomic number 19, so there are 19 electrons to put in place. The first two fill up the first shell, then there are 17 left. The next 8 fill up the second shell, then there are 9 left. The next 8 go in the third shell, then there's one left, and it must go in the fourth shell."

Sometimes people draw the atom out with the nucleus and shells. You will also see the electron structure written as numbers, separated by dots or commas. The numbers start with the first shell, then come in order, so potassium is 2.8.8.1 or 2,8,8,1.

It took a while for scientists to work out these rules, but they became very useful. It turned out that a lot of patterns in how different elements behave and react depend on how many electrons there are in the outermost shell. For example, lithium, sodium and potassium all have 1 electron in their outermost shells. The next few activities will help you to understand this even better.

How many electrons are there, and what element is it?

How could we write the electron structure in numbers? Tick all the right answers.

281

2.8.1

2,8,1

1,2,8

Here is the electron structure for another atom:

This atom has 13 electrons

This atom has 22 protons.

It is an atom of aluminium

The electron arrangement is 2.8.3.

We could put another six electrons in the first shell

Which of these is the right electron structure for aluminium (atomic number 13)?

 A B C D

Picture A

Picture B

Picture C

Picture D

Draw out the electron structure for silicon (atomic number 14), then check it against these possibilities.

Don't peek at the answers until you've tried drawing it for yourself.

 A B C D

A

B

C

D

Which element has electron structure 2.8.2? Select the right atomic number and element symbol.

A

B

C

D

What is the electron structure of chlorine?

What is the electron structure of boron?

How can you work out the electronic structure of an atom? Sort these steps into correct sentences.

## Column B

First, check the atomic number,
in the fourth shell
Then, put the first two electrons in the first she...
and up to 8 in the third
Next, put up to 8 electrons in the second shell,
because that tells you how many electrons there ar...
Put any remaining electrons
because that fills the shell
Put full stops or commas between the numbers,
because you need to keep the numbers separate.

Fill in the gaps in this paragraph.

## Column B

First, check the atomic number,
in the fourth shell
Then, put the first two electrons in the first she...
and up to 8 in the third
Next, put up to 8 electrons in the second shell,
because that tells you how many electrons there ar...
Put any remaining electrons
because that fills the shell
Put full stops or commas between the numbers,
because you need to keep the numbers separate.
• Question 1

How many electrons are there, and what element is it?

EDDIE SAYS
Start by counting the dots in the shells- there are 11 of them. If there are 11 electrons, there must be 11 protons as well, and the element with atomic number 11 is sodium.
• Question 2

How could we write the electron structure in numbers? Tick all the right answers.

2.8.1
2,8,1
EDDIE SAYS
When you use the number notation, start with the first shell (which has 2 electrons in it for sodium) and work out from there. It really doesn't matter whether you separate the numbers with commas or full stops (and you'd probably get marks in exams for other punctuation), but you need some clear separation between the numbers.
• Question 3

Here is the electron structure for another atom:

This atom has 13 electrons
It is an atom of aluminium
The electron arrangement is 2.8.3.
EDDIE SAYS
If it's an atom, the electron and proton numbers must be the same; you will learn about what happens when these numbers are different in another activity. Atomic number 13 is aluminium, and the number of electrons in each shell is right. Remember that the first shell is different to the others; it can only take 2 electrons before it is full up.
• Question 4

Which of these is the right electron structure for aluminium (atomic number 13)?

 A B C D

Picture D
EDDIE SAYS
Aluminium is atomic number 13, so the electrons go: 2 in the first shell (11 left) 8 in the second shell (3 left) 3 in the third shell.
• Question 5

Draw out the electron structure for silicon (atomic number 14), then check it against these possibilities.

Don't peek at the answers until you've tried drawing it for yourself.

 A B C D

B
EDDIE SAYS
Silicon is atomic number 14, so the electrons go: 2 in the first shell (12 left) 8 in the second shell (4 left) 4 in the third shell.
• Question 6

Which element has electron structure 2.8.2? Select the right atomic number and element symbol.

EDDIE SAYS
Start by adding up the numbers to get the total number of electrons; 2+8+2 = 12. Then check on a periodic table for element 12, which is magnesium.
• Question 7

What is the electron structure of chlorine?

2.8.7
2,8,7
EDDIE SAYS
Chlorine is atomic number 17. The first two electrons fill up the first shell (so there are 15 left). The next 8 electrons fill up the second shell (so there are 7 left). The final 7 electrons go into the third shell (because there are 8 spaces).
• Question 8

What is the electron structure of boron?

2.3
2,3
EDDIE SAYS
Boron is atomic number 5, so there are five electrons. The first two go in the first shell, leaving three to go in the second shell.
• Question 9

How can you work out the electronic structure of an atom? Sort these steps into correct sentences.

## Column B

First, check the atomic number,
because that tells you how many e...
Then, put the first two electrons...
because that fills the shell
Next, put up to 8 electrons in th...
and up to 8 in the third
Put any remaining electrons
in the fourth shell
Put full stops or commas between ...
because you need to keep the numb...
EDDIE SAYS
If you're not sure about the steps, check the introduction- especially the example of silicon at the end. The important thing to remember is the maximum numbers of electrons you can put in each shell (2, 8, 8; at least for the elements you have to know).
• Question 10

Fill in the gaps in this paragraph.

EDDIE SAYS
There's another activity to help you learn about lithium, sodium and potassium; they're part of a group called alkali metals. For now, the important thing is that their electron structures have this similarity, which will help explain why they behave the way they do.
---- OR ----

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