Atomic Weight NEET Level Challenge

Question 1

Chlorine has isotopes of mass numbers 35 and 37 in an approximate ratio of 3:1. What average atomic mass does this best explain?

Accepted Answer

35.5 u

Answer

36.0 u

Answer

35.0 u

Answer

36.5 u

Question 2

Which statement best explains why the atomic weight of copper is not a whole number?

Accepted Answer

Copper contains isotopes with different natural abundances

Answer

Copper atoms lose fractional electrons in nature

Answer

The nucleus of copper changes mass during bonding

Answer

Copper always exists as a mixture of ions and atoms

Question 3

If an element exists as 20% isotope X-10 and 80% isotope X-11, what is its average atomic mass?

Accepted Answer

10.8 u

Answer

10.2 u

Answer

10.5 u

Answer

11.2 u

Question 4

The standard atomic weight scale assigns exactly 12 u to which species?

Accepted Answer

One atom of carbon-12

Answer

One carbon atom in graphite

Answer

The average carbon atom in nature

Answer

One mole of carbon-12

Question 5

An element has two isotopes of masses 24 u and 26 u present in equal abundance. What is its average atomic mass?

Accepted Answer

25 u

Answer

24 u

Answer

25.5 u

Answer

26 u

Question 6

Which pair is most likely to show the greatest difference between mass number and atomic weight of the element?

Accepted Answer

Elements with two isotopes in nearly equal abundance

Answer

Monoisotopic elements

Answer

Noble gases only

Answer

Elements with atomic number below 3

Question 7

Natural boron contains about 20% B-10 and 80% B-11. Its atomic weight is therefore closest to:

Accepted Answer

10.8 u

Answer

10.2 u

Answer

11.0 u

Answer

11.8 u

Question 8

What happens to the average atomic mass of an element if the abundance of its heavier isotope increases?

Accepted Answer

It increases

Answer

It decreases

Answer

It becomes equal to the lighter isotope

Answer

It remains unchanged

Question 9

Which quantity is numerically equal to the molar mass of an element in g/mol?

Accepted Answer

Atomic weight in u

Answer

Atomic number

Answer

Valency

Answer

Mass number of the most abundant isotope

Question 10

Why is the atomic weight of most elements listed on the periodic table as a decimal number?

Accepted Answer

It is an average over naturally occurring isotopes

Answer

It includes electron mass and bond energy corrections only

Answer

It is rounded from the atomic number

Answer

It refers to the mass of ions in compounds

Question 11

An element has isotopes of masses 63 u and 65 u, and its average atomic mass is 63.5 u. The lighter isotope is approximately what percent abundant?

Accepted Answer

50%

Answer

25%

Answer

65%

Answer

75%

Question 12

Which of the following best distinguishes atomic weight from mass number?

Accepted Answer

Atomic weight is an average value, mass number refers to one isotope

Answer

Atomic weight is always an integer, mass number may be fractional

Answer

Atomic weight counts neutrons only, mass number counts protons only

Answer

Atomic weight is for molecules, mass number is for atoms

Question 13

If the isotopic composition of a naturally occurring sample changes, which property of the element can change measurably?

Accepted Answer

Average atomic mass

Answer

Atomic number

Answer

Number of protons in each atom

Answer

Position of electrons in the K shell

Question 14

Which element is classically cited in NEET-level chemistry for having an atomic weight very close to 35.5 due to isotopic abundance?

Accepted Answer

Chlorine

Answer

Sulfur

Answer

Argon

Answer

Phosphorus

Question 15

An artificial sample contains 30% isotope A-28 and 70% isotope A-30. What is the average atomic mass of the sample?

Accepted Answer

29.4 u

Answer

28.6 u

Answer

29.0 u

Answer

29.8 u

Question 16

Which term refers specifically to the weighted mean of the masses of naturally occurring isotopes of an element?

Accepted Answer

Atomic weight

Answer

Atomic number

Answer

Mass defect

Answer

Equivalent mass

Question 17

If an element had only one naturally occurring isotope, its atomic weight would be expected to be:

Accepted Answer

Exactly equal to the mass number of that isotope in simple school-level treatment

Answer

Always less than its atomic number

Answer

Half of its molar mass

Answer

Independent of isotopic mass

Question 18

A sample of neon contains a greater fraction of Ne-22 than usual. Compared with standard neon, the sample’s average atomic mass will be:

Accepted Answer

Higher

Answer

Lower

Answer

Exactly 20 u

Answer

Unchanged because neon is inert

Question 19

Which calculation is required to determine atomic weight from isotopic data?

Accepted Answer

Multiply each isotopic mass by its fractional abundance and sum

Answer

Add all mass numbers and divide by atomic number

Answer

Subtract electron mass from the heaviest isotope

Answer

Average only the two most abundant isotopes equally

Question 20

Why does the periodic table usually list chlorine as 35.5 and not 35 or 37?

Accepted Answer

35.5 is the weighted average of isotopic masses in natural chlorine

Answer

35.5 is the atomic number adjusted for bonding

Answer

35.5 is the mass number of the most stable chlorine ion

Answer

35.5 is the average number of neutrons in all halogens

Atomic Weight NEET Level Challenge

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