Isotopes
Understanding atoms of the same element with different masses
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They explain why atomic masses on the Periodic Table are rarely whole numbers.
1. Definition of Isotopes
Isotopes are atoms of the same element that possess the same number of protons but differ in the number of neutrons.
Same Chemical Identity
Isotopes have identical proton counts and electron counts, so they exhibit the same chemical properties and react identically in chemical reactions.
Different Physical Properties
Different neutron counts lead to different masses and densities among isotopes, affecting physical properties but not chemical behavior.
Example: Hydrogen Isotopes
Protium
1 proton, 0 neutrons
Mass Number: 1
Abundance: ≈99.98%
Deuterium
1 proton, 1 neutron
Mass Number: 2
Abundance: ≈0.015%
Tritium
1 proton, 2 neutrons
Mass Number: 3
Abundance: trace (radioactive)
Isotope Identifier
Which of these are isotopes of the same element?
2. Isotope Notation
Carbon-14
¹⁴₆C
Nuclide Notation: Element symbol with mass number (top) and atomic number (bottom)
Hyphen Notation: Element name followed by mass number
Notation Practice
Write the hyphen notation for ²³⁵₉₂U
3. Relative Atomic Mass (RAM)
Relative Atomic Mass (RAM) is the average mass of an atom of an element, factoring in the different isotopes and their natural abundance.
This is why atomic masses on the Periodic Table are not whole numbers!
Example: Cl = 35.45 amu, not 35 or 37
Formula:
RAM = Σ (Isotope Mass × Abundance)
where abundance is written as a decimal (e.g., 75% = 0.75)
4. Calculating Relative Atomic Mass
Example: Chlorine
Chlorine has two isotopes:
Chlorine RAM Calculator
Relative Atomic Mass =
amu
Step-by-Step Calculation:
RAM = (35 × 0.75) + (37 × 0.25)
= (26.25) + (9.25)
= 35.50 amu
Try It Yourself
Boron has two isotopes: B-10 (20%) and B-11 (80%). Calculate the relative atomic mass.
Isotope RAM Calculator
Create your own element:
Isotopes Summary
| Isotope | Protons | Neutrons | Mass Number | Notation |
|---|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 | ¹²₆C |
| Carbon-13 | 6 | 7 | 13 | ¹³₆C |
| Carbon-14 | 6 | 8 | 14 | ¹⁴₆C |
| Chlorine-35 | 17 | 18 | 35 | ³⁵₁₇Cl |
| Chlorine-37 | 17 | 20 | 37 | ³⁷₁₇Cl |
Test Your Understanding
1. What makes isotopes of an element different?
2. Which notation represents Carbon-14 correctly?
3. Why is the atomic mass of chlorine 35.45 amu on the Periodic Table?
4. Deuterium is an isotope of hydrogen with:
Key Terms
Key Takeaways
- Isotopes: Same protons, different neutrons - same element, different mass
- Chemical properties: Identical (same electron configuration)
- Physical properties: Different (mass, density)
- Notation: Nuclide (¹⁴₆C) or Hyphen (Carbon-14)
- Relative Atomic Mass (RAM): Weighted average of all isotopes
- Formula: RAM = Σ (isotope mass × abundance as decimal)
- Example (Cl): (35 × 0.75) + (37 × 0.25) = 35.50 amu
- This explains non-whole numbers on Periodic Table
