# Connexions

You are here: Home » Content » Siyavula textbooks: Grade 10 Physical Science [CAPS] » Isotopes

• Units

• #### Transverse pulses

• Transverse waves

• #### Sound

• The particles that substances are made of

• Magnetism

• #### Mechanical energy

• The Hydrosphere
• Safety in the laboratory

### Lenses

What is a lens?

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

#### Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
• FETChem

This module is included inLens: Siyavula: Chemistry (Gr. 10-12)
By: Siyavula

Review Status: In Review

Click the "FETChem" link to see all content affiliated with them.

Click the tag icon to display tags associated with this content.

### Recently Viewed

This feature requires Javascript to be enabled.

### Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.

Inside Collection:

Collection by: Free High School Science Texts Project. E-mail the author

# Isotopes

## Isotopes

### What is an isotope?

The chemical properties of an element depend on the number of protons and electrons inside the atom. So if a neutron or two is added or removed from the nucleus, then the chemical properties will not change. This means that such an atom would remain in the same place in the Periodic Table. For example, no matter how many neutrons we add or subtract from a nucleus with 6 protons, that element will always be called carbon and have the element symbol CC (see the Table of Elements). Atoms which have the same number of protons, but a different number of neutrons, are called isotopes.

Definition 1: Isotope

The isotope of a particular element is made up of atoms which have the same number of protons as the atoms in the original element, but a different number of neutrons.

The different isotopes of an element have the same atomic number ZZ but different mass numbers AA because they have a different number of neutrons NN. The chemical properties of the different isotopes of an element are the same, but they might vary in how stable their nucleus is. Note that we can also write elements as X - AX - A where the X is the element symbol and the A is the atomic mass of that element. For example, C-12C-12 has an atomic mass of 12 and Cl-35Cl-35 has an atomic mass of 35 u, while Cl-37Cl-37 has an atomic mass of 37 u.

#### Note: Interesting Fact :

In Greek, “same place” reads as ισoςισoςτoπoςτoπoς


(isos topos). This is why atoms which have the same number of protons, but different numbers of neutrons, are called isotopes. They are in the same place on the Periodic Table!

It is important to realise that the atomic mass of isotopes of the same element will be different because they have a different number of nucleons. Chlorine, for example, has two common isotopes which are chlorine-35 and chlorine-37. Chlorine-35 has an atomic mass of 35 u, while chlorine-37 has an atomic mass of 37 u. In the world around us, both of these isotopes occur naturally. It doesn't make sense to say that the element chlorine has an atomic mass of 35 u, or that it has an atomic mass of 37 u. Neither of these are absolutely true since the mass varies depending on the form in which the element occurs. We need to look at how much more common one is than the other in order to calculate the relative atomic mass for the element chlorine. This is the number that you find on the Periodic Table.

#### Note: Interesting fact:

The relative atomic mass of some elements depends on where on Earth the element is found. This is because the isotopes can be found in varying ratios depending on certain factors such as geological composition, etc. The International Union of Pure and Applied Chemistry (IUPAC) has decided to give the relative atomic mass of some elements as a range to better represent the varying isotope ratios on the Earth. For the calculations that you will do at high school, it is enough to simply use one number without worrying about these ranges.

#### Exercise 1: The relative atomic mass of an isotopic element

The element chlorine has two isotopes, chlorine-35 and chlorine-37. The abundance of these isotopes when they occur naturally is 75% chlorine-35 and 25% chlorine-37. Calculate the average relative atomic mass for chlorine.

##### Solution
1. Step 1. Calculate the mass contribution of chlorine-35 to the average relative atomic mass :

Contribution of Cl-35=(75100×35)=26,25uCl-35=(75100×35)=26,25u

2. Step 2. Calculate the contribution of chlorine-37 to the average relative atomic mass :

Contribution of Cl-37=(25100×37)=9,25uCl-37=(25100×37)=9,25u

3. Step 3. Add the two values to arrive at the average relative atomic mass of chlorine :

Relative atomic mass of chlorine=26,25u+9,25u=35,5uRelative atomic mass of chlorine=26,25u+9,25u=35,5u

If you look on the periodic table, the average relative atomic mass for chlorine is 35,5u35,5u. You will notice that for many elements, the relative atomic mass that is shown is not a whole number. You should now understand that this number is the average relative atomic mass for those elements that have naturally occurring isotopes.

This simulation allows you to see how isotopes and relative atomic mass are inter related.

run demo

#### Isotopes

1. Complete the table below:
 Isotope Z A Protons Neutrons Electrons Carbon-12 Carbon-14 Chlorine-35 Chlorine-37
2. If a sample contains 90% carbon-12 and 10% carbon-14, calculate the relative atomic mass of an atom in that sample.

3. If a sample contains 22,5% Cl-37 and 77,5% Cl-35, calculate the relative atomic mass of an atom in that sample.


Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows:

#### Note:

A nuclide is a distinct kind of atom or nucleus characterized by the number of protons and neutrons in the atom. To be absolutely correct, when we represent atoms like we do here, then we should call them nuclides.

For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as:

26 56 Fe 26 56 Fe
(1)

where the atomic number is Z=26Z=26


and the mass number A=56A=56. The number of neutrons is simply the difference N=A-ZN=A-Z.

The following worked examples will help you to understand the concept of an isotope better.

#### Exercise 2: Isotopes

For the element 92234U92234U (uranium), use standard notation to describe:

1. the isotope with 2 fewer neutrons
2. the isotope with 4 more neutrons
##### Solution
1. Step 1. Go over the definition of isotope :

We know that isotopes of any element have the same number of protons (same atomic number) in each atom, which means that they have the same chemical symbol. However, they have a different number of neutrons, and therefore a different mass number.

2. Step 2. Rewrite the notation for the isotopes :

Therefore, any isotope of uranium will have the symbol:

U U
(2)

Also, since the number of protons in uranium isotopes is always the same, we can write down the atomic number:

92 U 92 U
(3)

Now, if the isotope we want has 2 fewer neutrons than 92234U92234U, then we take the original mass number and subtract 2, which gives:

92 232 U 92 232 U
(4)

Following the steps above, we can write the isotope with 4 more neutrons as:

92 238 U 92 238 U
(5)

#### Exercise 3: Isotopes

Which of the following are isotopes of 2040 Ca 2040 Ca ?

• 19 40 K 19 40 K
• 20 42 Ca 20 42 Ca
• 18 40 Ar 18 40 Ar
##### Solution
1. Step 1. Go over the definition of isotope: :

We know that isotopes have the same atomic number but different mass numbers.

2. Step 2. Determine which of the elements listed fits the definition of an isotope. :

You need to look for the element that has the same atomic number but a different atomic mass number. The only element is 2042Ca2042Ca. What is different is that there are 2 more neutrons than in the original element.

#### Exercise 4: Isotopes

For the sulphur isotope 1633S1633S, give the number of...

1. protons
2. nucleons
3. electrons
4. neutrons
##### Solution
1. Step 1. Determine the number of protons by looking at the atomic number, Z. :

Z=16Z=16, therefore the number of protons is 16 (answer to (a)).

2. Step 2. Determine the number of nucleons by looking at the atomic mass number, A. :

A=33A=33, therefore the number of nucleons is 33 (answer to (b)).

3. Step 3. Determine the number of electrons :

The atom is neutral, and therefore the number of electrons is the same as the number of protons. The number of electrons is 16 (answer to (c)).

4. Step 4. Calculate the number of neutrons :
N = A - Z = 33 - 16 = 17 N = A - Z = 33 - 16 = 17
(6)

The number of neutrons is 17 (answer to (d)).

#### Isotopes

1. Atom A has 5 protons and 5 neutrons, and atom B has 6 protons and 5 neutrons. These atoms are...
1. allotropes
2. isotopes
3. isomers
4. atoms of different elements
2. For the sulphur isotopes, 1632S1632S and 1634S1634S, give the number of...
1. protons
2. nucleons
3. electrons
4. neutrons
3. Which of the following are isotopes of 1735Cl1735Cl?
1. 3517Cl3517Cl
2. 1735Cl1735Cl
3. 1737Cl1737Cl
4. Which of the following are isotopes of U-235U-235? (X represents an element symbol)
1. 92238X92238X
2. 90238X90238X
3. 92235X92235X

### Relative atomic mass

It is important to realise that the atomic mass of isotopes of the same element will be different because they have a different number of nucleons. Chlorine, for example, has two common isotopes which are chlorine-35 and chlorine-37. Chlorine-35 has an atomic mass of 35 u, while chlorine-37 has an atomic mass of 37 u. In the world around us, both of these isotopes occur naturally. It doesn't make sense to say that the element chlorine has an atomic mass of 35 u, or that it has an atomic mass of 37 u. Neither of these are absolutely true since the mass varies depending on the form in which the element occurs. We need to look at how much more common one is than the other in order to calculate the relative atomic mass for the element chlorine. This is the number that you find on the Periodic Table.

#### Note: Interesting fact:

The relative atomic mass of some elements depends on where on Earth the element is found. This is because the isotopes can be found in varying ratios depending on certain factors such as geological composition, etc. The International Union of Pure and Applied Chemistry (IUPAC) has decided to give the relative atomic mass of some elements as a range to better represent the varying isotope ratios on the Earth. For the calculations that you will do at high school, it is enough to simply use one number without worrying about these ranges.

#### Exercise 5: The relative atomic mass of an isotopic element

The element chlorine has two isotopes, chlorine-35 and chlorine-37. The abundance of these isotopes when they occur naturally is 75% chlorine-35 and 25% chlorine-37. Calculate the average relative atomic mass for chlorine.

##### Solution
1. Step 1. Calculate the mass contribution of chlorine-35 to the average relative atomic mass :

Contribution of Cl-35=(75100×35)=26,25uCl-35=(75100×35)=26,25u

2. Step 2. Calculate the contribution of chlorine-37 to the average relative atomic mass :

Contribution of Cl-37=(25100×37)=9,25uCl-37=(25100×37)=9,25u

3. Step 3. Add the two values to arrive at the average relative atomic mass of chlorine :

Relative atomic mass of chlorine=26,25u+9,25u=35,5uRelative atomic mass of chlorine=26,25u+9,25u=35,5u

If you look on the periodic table, the average relative atomic mass for chlorine is 35,5u35,5u. You will notice that for many elements, the relative atomic mass that is shown is not a whole number. You should now understand that this number is the average relative atomic mass for those elements that have naturally occurring isotopes.

This simulation allows you to see how isotopes and relative atomic mass are inter related.

run demo

#### Isotopes

1. Complete the table below:
 Isotope Z A Protons Neutrons Electrons Carbon-12 Carbon-14 Chlorine-35 Chlorine-37
2. If a sample contains 90% carbon-12 and 10% carbon-14, calculate the relative atomic mass of an atom in that sample.

3. If a sample contains 22,5% Cl-37 and 77,5% Cl-35, calculate the relative atomic mass of an atom in that sample.


## Content actions

EPUB (?)

### What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

PDF | EPUB (?)

### What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

#### Collection to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

#### Module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks