Unformatted text preview: 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors Chemistry for Non-Majors
Atomic Structure Mass Number 1/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors LEARNING OBJECTIVES Define mass number.
Calculate the mass number when given number of protons and neutrons.
Calculate number of neutrons when given atomic
number. HOW CAN YOU DETERMINE THE MASS OF A
CHEMICAL? Often a student will need to weigh out a chemical for an experiment. If he or she uses a watch glass (a small, round piece
that will hold the solid chemical), the weight of the watch glass
must be determined first. Then the solid is added to the glass
and the weight of the glass plus the solid is measured. The
balance reading will be the total of the glass plus the chemical. History of Atomic Weight Determinations 2/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors As a part of his research on
atoms, John Dalton determined a number of atomic
weights of elements in the
early 1800s. Atomic
weights were the basis for
the periodic table that
Mendeleev developed. Originally all atomic
weights were based on a
comparison to hydrogen,
which has an atomic
weight of one. After the
discovery of the proton,
scientists assumed that the
weight of an atom was essentially that of the protons
—electrons were known to contribute almost nothing to the atomic
weight of the element.
This approach worked until we learned how to determine the number of
protons in an element. We then saw that the atomic weight for an element was often twice the number of protons (or more). The discovery of
the neutron provided the missing part of the picture. The atomic mass
now known to be the sum of the protons and neutrons in the nucleus. Mass Number
Rutherford showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons.
The mass number is defined as the total number of protons and neutrons in an atom. Consider Table below which shows data from the first
six elements of the periodic table. 3/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors Atoms of the First Six Elements
Name Symbol Atomic
Number Protons Neutrons Electrons Mass
Num Hydrogen H 1 1 0 1 1 Helium He 2 2 2 2 4 Lithium Li 3 3 4 3 7 Beryllium Be 4 4 5 4 9 Boron B 5 5 6 Carbon C 6 6 6 5 6 11
12 Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since 2 + 2 =
4, we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons since the number of electrons
must equal the number of protons. This example may lead you to believe
that atoms have the same number of protons and neutrons, but further
examination of the Table above will show that this is not the case.
Lithium, for example has three protons and four neutrons, leaving it with
a mass number of 7.
Knowing the mass number and the atomic number of an atom allows you
to determine the number of neutrons present in that atom by
Number of neutrons = mass number − atomic number
Atoms of the element chromium (Cr) have an atomic number of 24 and a
mass number of 52. How many neutrons are in the nucleus of a
chromium atom? To determine this, you would subtract as shown:
52 − 24 = 28 neutrons in a chromium atom 4/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors The composition of any atom can be illustrated with a shorthand notation using the atomic number and the mass number. Both are written before the chemical symbol, with the mass number written as a superscript
and the atomic number written as a subscript. The chromium atom discussed above would be written as 52
24 Cr . Another way to refer to a specific atom is to write the mass number of
the atom after the name, separated by a hyphen. The above atom would
be written as chromium-52. SUMMARY The mass number is defined as the total number of protons and neutrons in an atom.
The number of neutrons = mass number − atomic
number. PRACTICE Use the link below to answer the following questions:
1. What data in the periodic table tells you the number of
protons in an atom?
2. How do you determine the number of neutrons in an
3. What is the mass number for an atom? REVIEW 5/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors 1. Who first determined atomic weights for elements?
2. What were the original atomic weights based on?
3. Why were calculations based on numbers of protons
not valid for determining atomic weights?
4. A tin atom has an atomic number of 50 and a mass
number of 118. How many neutrons are present in this
5. What is the mass number of a cobalt atom that has 27
protons and 30 neutrons? Glossary
mass number: The total number of protons and neutrons in an atom. Show References 6/7 12/8/21, 7:07 PM Mass Number | Chemistry for Non-Majors Previous Next 7/7 ...
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