Question:

A sample of rock from moon contains equal number of atoms of uranium and lead. $t_{1 / 2}$ for $U =4.5 \times 10^{9}$ years. The age of the rock would be :

Updated On: Jul 12, 2022
  • $ 2.25\times 10^{9}$ years
  • $ 13.5\times 10^{9}$ years
  • $ 9.0\times 10^{9}$ years
  • $ 4.5\times 10^{9}$ years
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The Correct Option is D

Solution and Explanation

One atom of ${ }_{92} U ^{238}$ produces one atom of ${ }_{82} Pb ^{206}$ on decay. Therefore, the number of atom of $U ^{238}$ and $Pb ^{206}$ in a rock sample is equal only when half of the number of $U ^{238}$ originally present decay to give same number of $Pb ^{206}$. Thus the time elapsed will be equal to the half-life period of $U ^{238}$
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Concepts Used:

Structure of Atom

Atomic Structure:

The atomic structure of an element refers to the constitution of its nucleus and the arrangement of the electrons around it. Primarily, the atomic structure of matter is made up of protons, electrons and neutrons.

Dalton’s Atomic Theory

Dalton proposed that every matter is composed of atoms that are indivisible and indestructible. 

The following are the postulates of his theory:

  • Every matter is made up of atoms.
  • Atoms are indivisible.
  • Specific elements have only one type of atoms in them.
  • Each atom has its own constant mass that varies from element to element.
  • Atoms undergo rearrangement during a chemical reaction.
  • Atoms can neither be created nor be destroyed but can be transformed from one form to another.

Cons of Dalton’s Atomic Theory

  • The theory was unable to explain the existence of isotopes.
  • Nothing about the structure of atom was appropriately explained.
  • Later, the scientists discovered particles inside the atom that proved, the atoms are divisible.

​​Subatomic Particles

  1. Protons - are positively charged subatomic particles.
  2. Electron - are negatively  charged subatomic particles.
  3. Neutrons - are electrically neutral particles and carry no charge

Atomic Structure of Isotopes

Several atomic structures of an element can exist, which differ in the total number of nucleons.These variants of elements having a different nucleon number (also known as the mass number) are called isotopes of the element. Therefore, the isotopes of an element have the same number of protons but differ in the number of neutrons.  For example, there exist three known naturally occurring isotopes of hydrogen, namely, protium, deuterium, and tritium.