Question

The weight of an atom of Uranium is $ 238 $ amu. Its actual weight is ______ g.
(A) $ 1.43\times {{10}^{26}}. $
(B) $ 3.95 \times {{10}^{-22}}. $
(C) $ 6.99\times {{10}^{-23}}. $
(D) $ 1.53\times {{10}^{-22}}. $

Answer 1

Hint: We know that Atomic number is the number of protons in the nucleus of an atom. Identity of an element is defined by the number of protons it has. Atomic weight is the total weight of an atom that is the total number of neutrons and protons. We can calculate the number of protons, neutrons and electrons if we know atomic number and atomic weight.

Complete answer:
Proton is a particle with a positive charge. It is found in an atom’s nucleus. The total number of protons in an atom is indicative of its atomic number. The property of atomic numbers is that it uniquely identifies chemical elements. In an uncharged atom, the number of electrons is also equal to atomic number. The atomic number of uranium is $ 92 $ .
Hence the number of protons in uranium is $ 92 $ . In an uncharged uranium atom, the number of protons and number of electrons are the same. Hence the total number of electrons in the uranium atom is $ 92 $ . Atomic mass is the total number of protons and neutrons in an atom. Here while calculating atomic mass; electrons are not counted as their weight is negligible.
weight of one atom of uranium $ =~238amu $ and weight of one mole of uranium $ =~238g $
Number of atoms in one mole of uranium $ =~6.023\times {{10}^{23}} $ , thus weight of one atom uranium in $ g=\left( \dfrac{238}{6.023\times {{10}^{23}}} \right)=39.51\times {{10}^{-23}} $
Therefore, the weight of an atom of Uranium is $ 238 $ amu. Its actual weight is $ 39.51\times {{10}^{-23}}g $.

This value can be written as $3.95 \times 10^{-22}$
Thus the correct answer is option B.

Note:
Remember that Protons as positively charged particles were discovered by Goldstein, neutrons as neutral particles were discovered by James Chadwick and electrons were discovered by Thomson. In an uncharged atom, the number of electrons is also equal to atomic number. The atomic number of uranium is $ 92 $.