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Question

Which of the following is necessary to calculate the atomic mass of an element?
a.

the atomic mass of carbon-12
b.

the atomic number of the element
c.

the relative masses of the element’s protons and neutrons
d.

the masses of each isotope of the element

Answer 1

b

Answer 2

The relative atomic mass of an element is the weighted average of the masses of the isotopes relative to 1/12 of the mass of a carbon-12 atom


The relative atomic mass (RAM) of an element is given the symbol Ar and is defined as:

The relative atomic mass of an element is the weighted average of the masses of the isotopes relative to 1/12 of the mass of a carbon-12 atom.

A "weighted average" allows for the fact that there won't be equal amounts of the various isotopes. The example coming up should make that clear.

Suppose you had 123 typical atoms of boron. 23 of these would be 10B and 100 would be 11B.

The total mass of these would be (23 x 10) + (100 x 11) = 1330

The average mass of these 123 atoms would be 1330 / 123 = 10.8 (to 3 significant figures).

10.8 is the relative atomic mass of boron.

Notice the effect of the "weighted" average. A simple average of 10 and 11 is, of course, 10.5. Our answer of 10.8 allows for the fact that there are a lot more of the heavier isotope of boron - and so the "weighted" average ought to be closer to that.


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Answer 3

a. # of neutrons and protons
b. 14 amu
c. c
d. p+= 6 amu n= 12 amu
e. the isotope numbers + atomic number

Answer 4

c