What is the atomic mass of oxygen (expressed in'u')?

16

The correct answer is option (3) 16.

The atomic mass of an element, expressed in atomic mass units (u), is the average mass of the atoms of that element taking into account the relative abundance of its isotopes. For oxygen, the atomic mass is approximately 16 u.

Here's a breakdown:

1. Isotopes of Oxygen:

Oxygen has several isotopes, but the two most abundant and significant ones are oxygen-16 (\(^{16}O\)) and oxygen-18 (\(^{18}O\)). Oxygen-16 is the most abundant isotope, making up about 99.76% of naturally occurring oxygen. Oxygen-18 is present in trace amounts.

2. Calculation of Atomic Mass:

The atomic mass of oxygen is a weighted average of the masses of its isotopes.

The atomic mass is calculated using the formula:\[ \text{Atomic Mass (u)} = (\text{Fractional Abundance}_{^{16}O} \times \text{Mass}_{^{16}O}) + (\text{Fractional Abundance}_{^{18}O} \times \text{Mass}_{^{18}O}) \]

3. Substitute Values:

For oxygen:
\[ \text{Atomic Mass (u)} = (0.9976 \times 16 \, \text{u}) + (0.0004 \times 18 \, \text{u}) \]
\[ \text{Atomic Mass (u)} \approx 15.999 \, \text{u} \]

4. Rounded Value:

The calculated atomic mass is often rounded to a whole number, and in this case, it is commonly expressed as \(16 \, \text{u}\).

So, the atomic mass of oxygen, expressed in atomic mass units (u), is approximately \(16 \, \text{u}\). This value is a standard reference used in chemistry to represent the mass of one mole of oxygen atoms.