# Explain Electron Structure

In this worksheet, students will learn how electrons are arranged in shells for different elements and the way that ionisation changes the electron structure.

### 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 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. As there are 19 protons, there must also 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 only work exactly for the first twenty elements. For elements heavier than calcium, the rules get more complicated. You won't be asked to work out the electron structure for (say) iron (atomic number 26), but later on, you will find that iron has some odd properties because of the extra rules.

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.

In harder questions, you might be asked about the electron structure for ions. Ions are atoms which have gained or lost electrons. Because electrons are negatively charged, a positive ion has lost electrons, and a negative ion has gained them. For example, a calcium atom has 20 electrons, but a calcium 2+ ion has 18. Once you know how many electrons the ion has, you fill up shells in the same way as for atoms- so Ca2+ is 2.8.8.

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, and they all have similar properties. The next few activities will help you to understand this idea even better.

Use the information in the picture to fill in the gaps in this paragraph.

Here is the electron structure for another atom:

This atom has 13 electrons.

This atom also has 14 protons.

It is an atom of aluminium.

The electron arrangement is 2.8.3.

We could put another six electrons in the first shell.

Draw out the electron structure for silicon, 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

Match these electron structures with their element names.

## Column B

2.3
Argon
2.7
Fluorine
2.8.4
Boron
2.8.8
Silicon

What is the electron structure of chlorine? (You can use full stops or commas to separate the numbers)

What are the electron structures of nitrogen and boron?

Match these different ways of representing atoms into pairs representing the same element.

## Column B

2.6
2.8
atomic number 10
Oxygen
atomic number 17
Argon
2.2
Beryllium

How are you feeling about electron structure?  Have a go at the paragraph below.

## Column B

2.6
2.8
atomic number 10
Oxygen
atomic number 17
Argon
2.2
Beryllium

Some elements form ions; this happens when they gain or lose electrons. A positive ion has lost one or more electrons, and a negative ion has gained one or more electrons.

Which of these statements is true about a Mg2+ ion? Tick all the statements which are true.

Mg2+ has 12 protons

Mg2+ has 10 electrons

Mg2+ has 12 electrons

Mg2+ has 14 electrons

The electron configuration of Mg2+ is 2.8

The electron configuration of Mg2+ is 2.8.4

The electron configuration of Mg2+ is the same as a Ne atom

Some elements form ions; this happens when they gain or lose electrons. A positive ion has lost one or more electrons, and a negative ion has gained one or more electrons.

Which of these statements is true about a S2- ion? Tick all the statements which are true.

S2- has 16 protons

S2- has 14 electrons

S2- has 16 electrons

S2- has 18 electrons

The electron configuration of S2- is 2.8.4

The electron configuration of S2- is 2.8.8

The electron configuration of S2- is the same as an Ar atom

• Question 1

Use the information in the picture to fill in the gaps in this paragraph.

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. To write the electron structure, count the number of electrons in each shell, starting with the innermost one. Use full stops or commas to separate the numbers.
• Question 2

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 3

Draw out the electron structure for silicon, 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 4

Match these electron structures with their element names.

## Column B

2.3
Boron
2.7
Fluorine
2.8.4
Silicon
2.8.8
Argon
EDDIE SAYS
Start by adding up the numbers to get the total number of electrons; for the first one, 2.3 gives 2+3 = 5. Then check for the element with atomic number 5, which is boron. The same idea works for the others.
• Question 5

What is the electron structure of chlorine? (You can use full stops or commas to separate the numbers)

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 6

What are the electron structures of nitrogen and boron?

EDDIE SAYS
Nitrogen is atomic number 7, so there are seven electrons. The first two go in the first shell, leaving five to go in the second shell. The same idea works for boron.
• Question 7

Match these different ways of representing atoms into pairs representing the same element.

## Column B

2.6
Oxygen
atomic number 10
2.8
atomic number 17
Argon
2.2
Beryllium
EDDIE SAYS
You can turn pictures into configurations by counting the electrons in each shell. To turn these into elements, work out the total number of electrons, and look up what that means as an atomic number.
• Question 8

How are you feeling about electron structure?  Have a go at the paragraph below.

EDDIE SAYS
This is an example of something you can learn more about in the next few activities. Elements with similar outermost shells tend to behave and react in similar ways. It's strong evidence for the rules you have learned to apply; the rules explain lots of different effects.
• Question 9

Some elements form ions; this happens when they gain or lose electrons. A positive ion has lost one or more electrons, and a negative ion has gained one or more electrons.

Which of these statements is true about a Mg2+ ion? Tick all the statements which are true.

Mg2+ has 12 protons
Mg2+ has 10 electrons
The electron configuration of Mg2+ is 2.8
The electron configuration of Mg2+ is the same as a Ne atom
EDDIE SAYS
This question is a lot harder than the earlier ones- well done if you got it right, but don't panic if you found it hard the first time. Start by thinking about a magnesium atom- that has 12 protons, so 12 electrons. A 2+ ion means that you lose 2 electrons, so that leaves 10 electrons. Then put those 10 electrons in shells, like before. Notice that 10 electrons is the same as a neon atom; that idea will come up again later on.
• Question 10

Some elements form ions; this happens when they gain or lose electrons. A positive ion has lost one or more electrons, and a negative ion has gained one or more electrons.

Which of these statements is true about a S2- ion? Tick all the statements which are true.

S2- has 16 protons
S2- has 18 electrons
The electron configuration of S2- is 2.8.8
The electron configuration of S2- is the same as an Ar atom
EDDIE SAYS
This is the same idea as the last question, so hopefully you found it a bit easier. Start by thinking about a sulfur atom- that has 16 protons, so 16 electrons. A 2- ion means that you gain 2 electrons, so that makes 18 electrons. Then put those 18 electrons in shells, like before. Notice that 18 electrons is the same as an argon atom; again, that idea will come up again later on.
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