Question

A)Why is carbon 14 radioactive but carbon 12 is not radioactive

B)Why is the the word average used in defining atomic mass unit​

Answer 1

A) Carbon 12 is stable whereas carbon 14 is unstable.

B) The word average is used for defining atomic mass units due to the presence of Isotopes.

A)

• Carbon 12 is a stable element and therefore does not emit any radiation.
• Carbon 14 is an unstable isotope of Carbon 12 and shows radioactive decay to attain a stable state.
• carbon 14 has a half-life of 5730 years.
• carbon 12 and carbon 14 differ in the number of neutrons and therefore have different atomic masses.  
• Elements with the same atomic number and different atomic mass are called isotopes.
• carbon 14 has 8 neutrons whereas carbon 12 has 6 neutrons, Hence radioactive decay takes place to remove 2 neutrons and reach the stable state.

B)

• Average atomic mass is used due to the existence of nearly stable isotopes of a single element.
• Average is taken for the atomic mass of all existing isotopes of an element.

Answer 2

C14 is not stable, so it is considered radioactive.

The atomic mass of an atom is the average of the atomic masses of its isotope

• Carbon 14 has 6 protons and 8 neutrons.
• But C12 has 6 protons and 6 neutrons.
• This imbalance in the number of neutrons and protons makes C14 unstable and releases beta particles.
• So C 14 is radioactive with a half-life of 5700 years.
• The atomic mass of an atom is the average mass of the isotopes of the atom.
• For example, Chlorine has two isotopes Cl35 and Cl 37 in a ratio of 3:1
• So average atomic mass = ((3*35)+(1*37))/4
•                                            = 35.5 amu

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