Isotopes Chemistry Tutorial

Key Concepts

• Isotopes are atoms of the same element that have a different number of neutrons in their nuclei.
• Isotopes of an element will have the same Atomic Number (Z) but a different Mass Number (A).
• A particular isotope of an element can be referred to in a number of ways:

  Symbol	Examples
  name-A	carbon-12
  ASymbol	12C
  A Symbol Z	12 C 6

• Isotopes can be classified as either:

  ⚛ natural (found in nature)

  ⚛ man made (artificial or synthetic)

• Isotopes can also be classified as stable or unstable:

  ⚛ A stable isotope does NOT undergo radioactive (or nuclear) decay.

  ⚛ An unstable isotope undergoes radioactive (or nuclear) decay.
            Unstable isotopes are also known as radioisotopes or radionucleides

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Isotopes of Elements

The number of protons in the nucleus of an atom, its atomic number (Z), tells us which element it is.

If we know the atomic number (Z) of an element, or, if we know how many protons are present in the nucleus of the atom, then we can look it up in the Periodic Table of the Elements to determine which element it is.

For example, hydrogen has an atomic number of 1 (Z = 1).
Every atom of hydrogen has 1 proton in its nucleus.

However, the number of neutrons in the nucleus of an atom of hydrogen can be different.

An atom of deuterium has 1 proton in its nucleus (Z = 1), and 1 neutron in its nucleus.
Although it is given a different name, it is still an atom of hydrogen because it has 1 proton in the nucleus (Z = 1)

Similarly, an atom of tritium has 1 proton in its nucleus (Z = 1), and 2 neutrons in its nucleus.
Although it is given a different name, it is still an atom of hydrogen because it has 1 proton in the nucleus (Z = 1).

Mass number (A) is defined as the number of protons + number of neutrons in the nucleus of an atom:

We say that there are 3 isotopes of hydrogen; "common" hydrogen, deuterium and tritium.
These are isotopes of the same element, hydrogen, because they all have the same number of protons in the nucleus (same atomic number, Z), BUT, they have different numbers of neutrons in the nucleus and hence different mass numbers (A).

Isotopes of an element have:

• same number of protons in the nucleus (same atomic number, Z)
• different number of neutrons in the nucleus (different mass number, A)

Representing Isotopes of the Same Element

The isotopes of hydrogen are quite special because each isotope has been given a different name.
Generally speaking, the different isotopes of each element have not been given a completely different name.
But we do need a way to refer to one particular isotope of an element, and we can do this in a number of different ways.

Remembering that the atomic number (Z) is the same for all atoms of an element regardless of the number of neutrons in the nucleus of an atom, then each isotope of hydrogen can be distinguished from the others using just the name of the element, hydrogen, AND its mass number (A).
We do this by writing the name of the element, followed by a hyphen, following by the mass number (A):

name-A

Some examples for the isotopes of hydrogen are shown below:

Ofcourse we don't really need to write out the whole name for the element, afterall, each element has been given its own chemical symbol.
The chemical symbol for hydrogen is H (which you can find in the Periodic Table of the Elements).

We can represent an atom of any element with chemical symbol E using its atomic number (Z) and mass number (A) as:

So the different isotopes of hydrogen can be represented as shown in the table below:

Since every atom of hydrogen will have the same atomic number (Z = 1), we need not necessarily include it in the symbol for each isotope.
This "short-hand" form for the symbol of each isotope of hydrogen is shown in the table below:

So, an isotope of an element, E, with mass number A can be represented as:

^AE

In general, there are 3 ways to represent the isotope of an element, E, with atomic number Z and mass number A:

The table below gives the naturally occurring isotopes of some elements:

The isotopes of some elements are man-made. These are also referred to as artificial isotopes, or as synthetic isotopes.
These man-made isotopes can be made in a nuclear reactor, or, in a cyclotron.
We can represent these isotopes using the same 3 methods we used for the naturally occuring isotopes in the table above.
Some examples of isotopes that occur naturally on Earth and man-made isotopes are given in the table below:

Isotopes and Chemical Reactions

In a chemical reaction, the number of electrons around the nucleus of an atom may change, but the atomic number (Z) of each atom does NOT change.

For example, a neutral atom of hydrogen (Z = 1), has 1 proton in its nucleus and 1 electron "orbiting" the nucleus.

An atom of hydrogen, H, (Z = 1) can lose an electron, e^-, in a chemical reaction to produce a positively charged ion of hydrogen:

H → e^- + H⁺

this changes the charge on the hydrogen "atom", but it does NOT change the number of protons in the nucleus of the atom.
Both H and H⁺ have the same atomic number 1 (Z = 1)

Similarly, an atom of hydrogen, H, (Z = 1) can gain an electron, e^-, in a chemical reaction to produce a negatively charged ion of hydrogen:

H + e^- → H^-

this changes the charge on the hydrogen "atom", but it does NOT change the number of protons in the nucleus of the atom.
Both H and H^- have the same atomic number 1 (Z = 1)

When we use the chemical symbol H for hydrogen, this refers to the naturally occurring mix of hydrogen isotopes, that is, most of the hydrogen atoms in the reaction will be the hydrogen-1 isotope because this the most common isotope found in nature, but a small number of the hydrogen atoms will be the hydrogen-2 isotope.

If the only isotope present in the chemical reaction is ¹H, then we could re-write the chemical reactions above as:

¹H → ¹H⁺ + e^-

¹H + e^- → ¹H^-

Similarly, if the only isotope present in the chemical reaction is deuterium, ²H, then we could re-write the chemical reactions above as:

²H → ²H⁺ + e^-

²H + e^- → ²H^-

The chemistry of ¹H and ²H will be the same because they are both hydrogen atoms and therefore have the same arrangement of electrons.

Because isotopes of an element undergo the same chemical reactions, this can be used to "tag" atoms in a chemical reaction in order to follow the progress of a reaction.

For example, ¹⁴C undergoes spontaneous radioactive decay by emitting a beta particle which can be detected using a device like a Geiger counter, but ¹²C is quite stable (does not emit radiation).
To follow the progress of a chemical reaction involving carbon atoms, we could replace some, or all of, the carbon atoms in the reactant with ¹⁴C, and then monitor where those unstable isotopes go.

Worked Example of an Isotope Problem

Question : Which of the following species, Q, R or V, is an isotope of oxygen?

Species	Number of neutrons	Mass Number
Q	8	32
R	16	32
V	8	16

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

   Find the isotope(s) of oxygen.

2. What data (information) have you been given in the question?

   Extract the data from the question:

   Name of element: Oxygen
   From the periodic table: Z = atomic number = number of protons = 8

   Species Q: A = mass number = 32, number of neutrons = 8

   Species R: A = mass number = 32, number of neutrons = 16

   Species V: A = mass number = 16, number of neutrons = 8

3. What is the relationship between what you know and what you need to find out?

   (a) Isotopes of oxygen will have the same atomic number (Z = 8), that is same number of protons.

   (b) Isotopes of oxygen will have a different mass number (A), that is different numbers of neutrons.

4. Calculate the number of protons in the nucleus of each atom and determine which is an isotope of oxygen:

   A = number of protons + number of neutrons
   number of protons = A - number of neutrons

   Species Q: number of protons = 32 - 8 = 24

   Species R: number of protons = 32 - 16 = 16

   Species V: number of protons = 16 - 8 = 8

   Oxygen, Z = 8, has 8 protons in its nucleus.
   Species V has 8 protons, therefore it is an isotope of oxygen.

5. Is your answer plausible?

   Work backwards: in order for Q, R and V to be isotopes of oxygen they must each have 8 protons in the nucleus.
   Since mass number (A) = number of protons + number of neutrons
   If any of the species is an isotope of oxygen then
   mass number (A) given in the question = 8 + number of neutrons given in the question.
   Species Q: 8 + 8 ≠ 32 so it is NOT an isotope of oxygen
   Species R: 8 + 16 ≠ 32 so it is NOT an isotope of oxygen
   Species V: 8 + 8 = 16 so it IS an isotope of oxygen

6. State your solution to the problem "which species is an isotope of oxygen":

   Species V is an isotope of oxygen.