What is the mass of oxygen atom? 

16

The correct answer is option 3. 16.

The mass of an oxygen atom is a fundamental property that represents the total amount of matter contained within the atom. In the context of atomic mass units (amu), which are commonly used to express atomic masses, the mass of an oxygen atom is approximately 16 amu.

This value is determined by summing the masses of the protons, neutrons, and electrons that make up the atom. However, since the mass of an electron is much smaller compared to that of a proton or neutron, it is often neglected when calculating atomic masses.

The atomic mass of an element, such as oxygen, is typically listed on the periodic table. For oxygen, the atomic mass is approximately 15.9994 amu, which is rounded to 16 amu for simplicity.

In summary, the mass of an oxygen atom is approximately 16 atomic mass units (amu), representing the total mass of its constituent protons and neutrons. This value is crucial in various scientific contexts, including chemistry, physics, and materials science.