Isotopes
Definition of isotopes: Isotopes are different versions of the same type of atom. Even though they all belong to the same element (like all being types of carbon or hydrogen), they don’t have exactly the same parts inside. They always have the same number of protons, which tells us what element they are. But the number of neutrons can be different, and that makes their total mass different.
Proton and nucleon numbers: Every isotope of an element has the same number of protons. This number is called the proton number or atomic number, and we use the symbol Z for it. However, they may have different numbers of neutrons, so their total number of particles in the nucleus (protons + neutrons) is different. This total is called the nucleon number or mass number, and the symbol is A. So, A = Z + number of neutrons.
Alternative definitions: Another way to say it is: isotopes are atoms that
- Have the same number of protons in their nuclei,
- But have different numbers of neutrons,
- So they also have different nucleon or mass numbers.
Hydrogen isotopes: Hydrogen is a very common and simple element that helps us understand isotopes. It has three types:
- Protium (¹₁H): This type has 1 proton and no neutrons.
- Deuterium (²₁H): This one has 1 proton and 1 neutron.
- Tritium (³₁H): This rare type has 1 proton and 2 neutrons.
Hydrogen notation: All hydrogen isotopes have the same atomic number (1), so they are all hydrogen. But their mass numbers are different—1, 2, and 3—because they have different numbers of neutrons. This helps scientists easily tell them apart.
Other isotope examples: Here are more examples of elements that have isotopes:
- Oxygen: ¹⁶O, ¹⁷O, ¹⁸O (three forms of oxygen)
- Carbon: ¹²C, ¹³C, ¹⁴C (used in carbon dating)
- Chlorine: ³⁵Cl, ³⁷Cl
- Bromine: ⁷⁹Br, ⁸¹Br
Chemical properties: All isotopes of the same element react in the same way during chemical reactions. This is because chemical reactions depend on electrons, and all isotopes of an element have the same number of electrons arranged in the same way.
Physical properties: Even though their chemical behavior is the same, isotopes have different physical properties. That’s because they have different masses due to the different number of neutrons. This can change how fast they move (diffusion), their density, and even boiling or melting points.
Relative Atomic Mass (Ar)
Need for Ar: Most elements in nature exist as a mixture of isotopes. So instead of using just one mass number, we use a value called relative atomic mass (Ar) to represent the average mass of the atoms of that element.
Definition of Ar: The relative atomic mass (Ar) of an element is the average mass of one atom of that element. It is compared to a standard, which is one-twelfth (¹/₁₂) the mass of a carbon-12 atom.
Weighted average: Since not all isotopes are equally common, we must consider how much of each one exists in nature. This is called their relative abundance. Ar is the average mass that takes these different percentages into account.
Calculation steps: To calculate the Ar:
- Find all the isotopes that exist for the element.
- Write down how common each one is (as a percentage).
- Multiply the mass number of each isotope by its percentage.
- Add all the answers together.
- Divide the total by 100 to get the average.
Formula: Ar = [(% abundance × mass number of isotope) + (% of other isotope × mass number) + …] ÷ 100
Natural abundance: This term means how often each isotope is found in nature. It’s usually given as a percentage (like 75% or 25%).
Example – Chlorine: Chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). So, Ar = (75×35 + 25×37) ÷ 100 = 35.5
Example – Bromine: Bromine’s two isotopes, ⁷⁹Br and ⁸¹Br, are equally common (50% each). So, Ar = (50×79 + 50×81) ÷ 100 = 80.0
Uses of Isotopes
Wide applications: Isotopes are used in many important areas in real life. We find them in medicine, archaeology, agriculture, industry, and even in nuclear power. Some isotopes are radioactive (they give off energy), and others are stable (they don’t give off radiation).
Radioactive Isotopes (Radioisotopes)
Nature of radioisotopes: These isotopes have unstable atoms. This means their nuclei break down over time, and when they do, they release energy called radiation. Eventually, they turn into stable atoms.
Medicine
Iodine-131: Doctors use iodine-131 to check and treat problems with the thyroid gland, which is found in the neck. It can help show how well the gland is working and can also be used to shrink or destroy unhealthy thyroid tissue.
Cobalt-60: This isotope gives off powerful gamma rays. These rays can kill cancer cells, so cobalt-60 is often used in radiation therapy for cancer. It is also used to sterilize medical equipment and even preserve food by killing germs.
Phosphorus-32: Phosphorus-32 helps in treating some cancers. Scientists also use it to trace how plants take in and use nutrients, helping us understand plant biology better.
Plutonium-238: This isotope produces heat and is used to power small machines like pacemakers for heart patients. It is also used in spacecraft to provide power where solar energy doesn’t work.
Iron-59: Doctors use iron-59 to track how iron moves in the human body. It is helpful in diagnosing conditions like anaemia, where the body doesn’t have enough healthy red blood cells.
Iridium-192: Hospitals use this isotope to treat cancer with radiation. It is especially useful in internal radiation treatments, where a small amount is placed close to the cancer inside the body.
Samarium-153: This isotope is used to reduce pain caused by cancer that has spread to the bones. It helps patients feel more comfortable.
Sodium-24 (medical): Doctors use sodium-24 to find blockages in a person’s blood vessels. It moves with the blood, so it helps doctors see if something is stopping the blood flow.
Archaeology
Carbon-14: Scientists use carbon-14 to find out how old ancient things are, like bones, wooden tools, or cloth. This process is called carbon dating and helps archaeologists study human history.
Agriculture
Carbon-14 in plants: Scientists use carbon-14 to learn how plants make food during photosynthesis and how they build proteins. This helps in improving crop growth.
Phosphorus-32 in plants: This isotope is used to study how plants absorb phosphorus, an important nutrient for healthy plant growth.
Cobalt-60 in crops: Farmers and researchers use cobalt-60 to change plant genes to develop better types of crops. It is also used to kill pests that damage crops.
Industry
Sodium-24 (industrial): Engineers use sodium-24 to find leaks in underground pipes that carry water or oil. The isotope moves with the liquid and shows where the leak is.
Cobalt-60 (industrial): In factories, cobalt-60 helps kill bacteria in food and checks whether materials like metal or plastic sheets have the correct thickness.
Krypton-85: Factories use krypton-85 to help control the thickness of plastic sheets. It ensures that the sheets are not too thick or too thin.
Power Source
Uranium-235: This isotope is the main fuel used in nuclear power stations. When uranium-235 atoms split, they release energy that is turned into electricity.
Plutonium-238 (space): In space missions, plutonium-238 is used to power machines like satellites and space probes. It works well even when sunlight is not available.
Safety Note
Handling caution: Radioactive isotopes release radiation, which can be dangerous to people if not handled properly. However, with the correct safety equipment and careful procedures, they can be used safely in many helpful ways.
Non-Radioactive Isotopes
Stable isotopes: These isotopes do not give off any radiation. Their nuclei are not unstable, so they don’t change over time.
Research use: Scientists use stable isotopes to safely trace where atoms go in chemical reactions or inside the body.
Example tracers:
- Deuterium (²H): Used to track how water moves inside the body.
- Oxygen-18 (¹⁸O): Used in science to study how air and water cycle through the environment and living things.