. i. Helium atom has an atomic mass of and two protons in to nucleus. How many neutrons does it have? ii. Write down the electronic configuration of oxygen. Material downloaded from myCBSEguide.com. 3 / 3 iii. The atomic mass of an element X is 16.24. What are the percentages of isotopes of 'X' having atomic number 8 and mass numbers 16 and 18.
Answers
i) The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus. Since helium atom has two protons, mass contributed by the two protons is (2 X 1) u = 2 u. Then, the remaining mass (4 — 2) u = 2 u is contributed by 2 neutrons.. Thus helium atom has two neutrons.
ii)[He] 2s2 2p4
iii) Let the % of isotope ₈¹⁶X = x
Let the % of isotope ₈¹⁸X = 100 - x
Average atomic mass of an element X = 16.24
Now,
Average atomic mass=[(Mass of ₈¹⁶X )(% of ₈¹⁶X)+(Mass of ₈¹⁸X)(% of ₈¹⁸X)]/100
16.24 = (x) (16)+(100-x) (18)] / 100
16.24 = (16x + 1800 - 18x) / 100
1624 = 1800 - 2x
2x = 1800 - 1624
2x = 176
x = 176/2
x = 88
Hence, % of isotope ₈¹⁶X = 88%
and % of isotope ₈¹⁸X = 100 - 88 = 12%
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