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How to Find Atomic Mass in the Periodic Table?

Published in Chemistry Basics 2 mins read

The atomic mass in the periodic table is the weighted average of the masses of all the naturally occurring isotopes of an element. It is usually found below the element's symbol.

Here's a breakdown of what that number represents and how it's derived:

Understanding Atomic Mass

  • What it is: Atomic mass, also known as atomic weight, is the average mass of atoms of an element, taking into account the abundance of each isotope.

  • Where to find it: Look for the number typically displayed below the element's symbol on the periodic table. This is usually a decimal number.

  • Units: Atomic mass is expressed in atomic mass units (amu) or Daltons (Da).

How Atomic Mass is Calculated (and why it's not just protons + neutrons)

Atomic mass is not simply the sum of protons and neutrons. This is because:

  1. Isotopes Exist: Most elements exist as a mixture of isotopes. Isotopes are atoms of the same element with different numbers of neutrons. For example, carbon has isotopes carbon-12 (6 neutrons), carbon-13 (7 neutrons), and carbon-14 (8 neutrons).

  2. Weighted Average: The atomic mass reported on the periodic table is a weighted average of the masses of all the isotopes of that element, based on their natural abundance. This means that isotopes that are more abundant contribute more to the average atomic mass.

Example: Chlorine (Cl)

Chlorine has two major isotopes:

  • Chlorine-35 (35Cl), which has a mass of approximately 35 amu and a natural abundance of about 75.77%.
  • Chlorine-37 (37Cl), which has a mass of approximately 37 amu and a natural abundance of about 24.23%.

To calculate the atomic mass of chlorine, you would use the following formula:

Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + ...

Atomic Mass of Cl = (35 amu × 0.7577) + (37 amu × 0.2423) ≈ 35.45 amu

The atomic mass of chlorine listed on the periodic table is approximately 35.45 amu.

Key Takeaways

  • The atomic mass on the periodic table is a weighted average reflecting the natural abundance of an element's isotopes.
  • It is typically found below the element's symbol.
  • It is expressed in atomic mass units (amu).

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