Understanding Average Atomic Mass of Chlorine

The question asks for the average atomic mass of a chlorine atom. The average atomic mass of an element is a weighted average of the atomic masses of its naturally occurring isotopes. It takes into account the mass of each isotope and its relative abundance in nature.

Chlorine Isotopes and Abundance

Chlorine ($\text{Cl}$) exists primarily as two stable isotopes in nature:

• Chlorine-35 ($^{35}\text{Cl}$)
• Chlorine-37 ($^{37}\text{Cl}$)

Each isotope has a different mass number because they have a different number of neutrons. While the number of protons (atomic number, 17 for chlorine) is the same, the number of neutrons differs.

These isotopes are found in different proportions (natural abundance) in any natural sample of chlorine. The approximate natural abundances and isotopic masses are:

Calculating Average Atomic Mass

The average atomic mass is calculated using the following formula:

$\text{Average Atomic Mass} = \Sigma (\text{Isotopic Mass}_i \times \text{Natural Abundance}_i)$

Where $\text{Isotopic Mass}_i$ is the mass of isotope $i$ and $\text{Natural Abundance}_i$ is its fractional abundance (abundance divided by 100).

Let's calculate the average atomic mass for chlorine using the values above:

Average Atomic Mass of $\text{Cl} = (\text{Mass of } ^{35}\text{Cl} \times \text{Abundance of } ^{35}\text{Cl}) + (\text{Mass of } ^{37}\text{Cl} \times \text{Abundance of } ^{37}\text{Cl})$

Average Atomic Mass of $\text{Cl} = (34.9689 \text{ u} \times 0.7577) + (36.9659 \text{ u} \times 0.2423)$

Average Atomic Mass of $\text{Cl} \approx (26.4958 \text{ u}) + (8.9568 \text{ u})$

Average Atomic Mass of $\text{Cl} \approx 35.4526 \text{ u}$

Comparing with Options

The calculated average atomic mass is approximately 35.45 u. When we look at the given options:

1. 35.9 u
2. 35.5 u
3. 35.0 u
4. 37.0 u

The value 35.4526 u is closest to 35.5 u. The average atomic mass of chlorine is widely known and accepted as approximately 35.5 u.

Conclusion on Chlorine's Average Atomic Mass

The average atomic mass of a chlorine atom, taking into account the natural abundance of its isotopes, is approximately 35.5 u.

Revision Table: Key Atomic Mass Concepts

Additional Information on Atomic Mass Units

The unit 'u' used for atomic mass stands for the unified atomic mass unit (sometimes called the Dalton, Da). It is defined as one-twelfth of the mass of an unbound atom of carbon-12 in its nuclear and electronic ground state. This unit is used to express atomic and molecular masses.

Why do we use average atomic mass?

• Most elements exist as a mixture of isotopes.
• In chemical reactions, we deal with a large number of atoms, which is a statistical mixture of these isotopes.
• Using the average atomic mass allows us to work with the element as a whole in stoichiometric calculations, rather than having to consider each isotope separately. This simplifies calculations significantly in chemistry.

The value of the average atomic mass for each element is typically found on the periodic table.