Isotopes

What Are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Let's break this down:

• All atoms of the same element must have the same number of protons in their nucleus. This is what makes an element what it is. For example, all carbon atoms have 6 protons. If an atom has 6 protons, it must be carbon.

• However, atoms of the same element can have different numbers of neutrons. These different versions of the same element are called isotopes.

• Because isotopes have different numbers of neutrons, they have different mass numbers (also called nucleon numbers).

• But because they have the same number of protons, they have the same atomic number (also called proton number).

Example: Hydrogen Isotopes

Hydrogen has three naturally occurring isotopes:

1. Protium (Hydrogen-1): 1 proton, 0 neutrons, 1 electron
2. Deuterium (Hydrogen-2): 1 proton, 1 neutron, 1 electron
3. Tritium (Hydrogen-3): 1 proton, 2 neutrons, 1 electron

All three are hydrogen because they all have 1 proton. But they have different numbers of neutrons (0, 1, and 2), so they are isotopes of each other.

Example: Carbon Isotopes

Carbon has several isotopes, including:

• Carbon-12: 6 protons, 6 neutrons
• Carbon-13: 6 protons, 7 neutrons
• Carbon-14: 6 protons, 8 neutrons (this one is radioactive)

All have 6 protons (so they're all carbon), but different numbers of neutrons.

Isotope Notation

We write isotopes using a special notation that shows both the mass number and the atomic number.

The Standard Notation:

^A_ZX

Where:

• X = the chemical symbol of the element (e.g., H for hydrogen, C for carbon)
• A = the mass number (or nucleon number) = total number of protons + neutrons
• Z = the atomic number (or proton number) = number of protons

How to Read This:

• The top number (A) tells you the total number of particles in the nucleus (protons + neutrons)
• The bottom number (Z) tells you how many protons are in the nucleus
• To find the number of neutrons: neutrons = A - Z (mass number minus atomic number)

Examples:

1. Boron-11: Written as ¹¹₅B

   • Mass number = 11 (total protons + neutrons)
   • Atomic number = 5 (number of protons)
   • Number of neutrons = 11 - 5 = 6 neutrons

2. Hydrogen-2 (Deuterium): Written as ²₁H

   • Mass number = 2
   • Atomic number = 1
   • Number of neutrons = 2 - 1 = 1 neutron

3. Carbon-14: Written as ¹⁴₆C

   • Mass number = 14
   • Atomic number = 6
   • Number of neutrons = 14 - 6 = 8 neutrons

Alternative Naming:

Isotopes can also be named by writing the element name followed by a dash and the mass number. For example:

• Carbon-12 (instead of ¹²₆C)
• Chlorine-35 (instead of ³⁵₁₇Cl)
• Uranium-238 (instead of ²³⁸₉₂U)