GENERAL KNOWLEDGE

Detailed explanation-1: -Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons, 6 neutrons, and 6 electrons.

Detailed explanation-2: -Reason: Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as standard.

Detailed explanation-3: -Carbon is an element which can react with more elements than oxygen. Carbon can form very large bonds whereas oxygen cannot since oxygen has 2 valence electrons so it can only react with elements which can give 2 electrons to oxygen.

Detailed explanation-4: -Scientists decided on using the carbon-12 nuclide as the reference standard by which all other masses would be compared. By definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units (amu). An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12.

Detailed explanation-5: -In 1961 C-12 isotope was chosen as the standard reference for measuring atomic mass.