Mass Number Chemistry Example 2

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Example 2

medium
Why is the atomic mass of chlorine listed as 35.4535.45 amu on the periodic table, rather than a whole number like 3535 or 3737?

Solution

  1. 1
    Chlorine exists as two stable isotopes: 35Cl{}^{35}\text{Cl} (75.77%) and 37Cl{}^{37}\text{Cl} (24.23%).
  2. 2
    The atomic mass is a weighted average: 35×0.7577+37×0.242335 \times 0.7577 + 37 \times 0.2423.
  3. 3
    Calculate: 26.52+8.97=35.49≈35.4526.52 + 8.97 = 35.49 \approx 35.45 amu.

Answer

Atomic mass=35.45 amu (weighted average of isotopes)\text{Atomic mass} = 35.45\,\text{amu (weighted average of isotopes)}
The periodic table lists the weighted average atomic mass based on the natural abundance of all isotopes, which is why it is rarely a whole number.

About Mass Number

The total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify specific isotopes of an.

Learn more about Mass Number →

More Mass Number Examples

Example 1 easy

An atom has 17 protons and 18 neutrons. What is its mass number and what element is it?

Example 3 medium

An isotope has 26 protons and 30 neutrons. What is its mass number? Identify the element.

Example 4 easy

Find the mass number of an atom with 26 protons and 30 neutrons.

Example 5 easy

An atom has 17 protons and 18 neutrons. What is its mass number?

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