Unit 5.1 Chemical Composition – The Atom
How many gallons of gas can fill a car's tank? The current
system of measurement in the Unites States is a hodge-podge
of different units, many of which are hard to interconvert,
unlike the metric system. We at least have standardized units
these days, unlike centuries past. At one time, measurements
of length often were defined as the distance from the end of
one appendage to another. For example, the yard would be
defined as the distance from the king’s nose to the tip of his
thumb when his arm was stretched out.
Standardized measurements make it possible for people everywhere to get the same amount of
something. Note the red and white labels on the gas pump above. These labels certify that the gas pump
has been checked and is pumping an accurate gallon of gas. Standard measurements in science are very
important so that we can compare experimental data from one lab to another and make sure we all are
talking about the same thing.
When working with a large amount of objects, say a bag of jelly beans, a fairly accurate count of the
number of jelly beans can be obtained using counting by mass. Counting by mass is a process using the
average mass of each object and the total mass of all of the objects. For the jelly beans, a mass would
be measured for a few jelly beans. An average mass is calculated from those beans. Finally, the total
mass of all of the jelly beans is divided by the average mass of a jelly bean, which will give the number of
jelly beans in the bag. A bag of jelly beans has a mass of 150.26 grams. The following is the calculations
to determine the number of jelly beans.
Mass of 10 jelly beans mass individually:
The average mass of jelly beans is calculated:
Jelly bean
Mass of bean
number
(g)
1
2.45
2
2.46
3
2.37
4
2.33
5
2.39
6
2.41
7
2.29
8
2.40
Counting by mass, works for large objects. Working with
9
2.37
something as small as atoms, scientists have had to develop
10
2.38
different techniques to determine the number of atoms in a given
Total mass
23.85
23.85 g
= 2.385 g each
10 beans
To find the number of beans in the bag with a mass of 150.26 g,
divide the mass of the bag by the mass of the average jelly bean.
150.26 g
= 63 jelly beans
2.385 g average
mass of element.
Atomic Mass
Masses of individual atoms are very, very small. Using a modern device called a mass spectrometer, it is
possible to measure such minuscule masses. An atom of oxygen-16, for example, has a mass of
2.66 × 10-23 g. While comparisons of masses measured in grams would have some usefulness, it is far more
practical to have a system that will allow us to more easily compare relative atomic masses. Scientists
decided on using the carbon-12 nuclide as the reference standard by which all other masses would be
compared. By definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units (amu). An
atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12. In grams,
an amu is equal to 1.67 x 10-24 g. The mass of any isotope of any element is expressed in relation to the
carbon-12 standard. For example, one atom of helium-4 has a mass of 4.0026 amu. An atom of sulfur-32
has a mass of 31.972 amu.
The carbon-12 atom has six protons and six neutrons in its
nucleus for a mass number of 12. Since the nucleus accounts
for nearly all of the mass of the atom, a single proton or
single neutron has a mass of approximately 1 amu. However,
as seen by the helium and sulfur examples, the masses of
individual atoms are not whole numbers. This is because an
atom’s mass is affected very slightly by the interactions of
the various particles within the nucleus, and the small mass
of the electron is taken into account.
Subatomic Particles
Below are listed the subatomic particles along with the characteristics of each.
Particle
amu
mass (g)
Atomic charge
Electrical charge (Coulomb)
Proton (p+)
1.0073
1.673 x 10-24
+1
+1.602 X 10-19
Electron (e-)
.0005486
9.109 x 10-28
-1
-1.602 X 10-19
Neutron (n)
1.0087
1.675 x 10-24
0
0
What is unique about each one of us? For the vast majority of people, it is
not their name, because it is quite possible for others in the world to have
the same name (check it out by doing an internet search for your name and
see how many other of “you” there are). It is not your physical
description. Eye-witnesses to crime scenes often pick the wrong person when
trying to identify the criminal.
There may be some unique identifiers for us. If you have a cell phone in your
name, nobody else in the world has that number. Email addresses are
different for each of us, which is a good thing since we can email almost
anywhere in the world. Our DNA is unique, but getting a DNA analysis is
expensive and time-consuming, so we really don’t want to have to explore that.
Organizing the Elements
One of the goals of science is to discover the order in the universe and to organize information that
reflects that order. As information about the different elements was made known, efforts were made
to see if there were patterns in all of the data. An early attempt to organize data was made by
Mendeleev, who developed the first periodic table. His data set was based on atomic weights (the sum
of the p+ and n) and was instrumental in providing clues as to the possible identity of new elements. Once
we learned the details of the atomic nucleus, the table was based on the number of protons in the
nucleus, called the atomic number (the number of p+) of the element. This table was designed by Henry
Moseley.
The atomic number (Z) of an element is the number
of protons in the nucleus of each atom of that
element. This means that the number of protons is
the characteristic which makes each element unique
compared to all other elements. Elements are
different because of their atomic number.
The periodic table displays all of the known elements
and is arranged in order of increasing atomic
number. In this table, an element’s atomic number is
indicated above the elemental symbol. Hydrogen, at
the upper left of the table, has an atomic number of
1. Every hydrogen atom has one proton in its nucleus.
Following on the table is helium, whose atoms have
two protons in the nucleus. Lithium atoms have three
protons, and so forth.
Nuclear notation is a way of showing both
the atomic number (Z) and the atomic
mass (A) for an element.
Since atoms are neutral, the number of
electrons is equal to the number of
protons. Hydrogen atoms all have one
electron occupying the space outside of
the nucleus. Manganese (atomic number
25) would have twenty-five protons and
twenty-five electrons.
The classification of elements by atomic number allows us to understand many properties of the atom
and makes it possible to predict behaviors instead of just having to memorize everything.
Summary


Carbon-12 is the reference for all atomic mass calculations.
An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12.
It is equal to the mass of approximately 1 proton or 1 neutron.


The mass of an atom is the number of protons plus neutrons.
The atomic number (Z) of an element is the number of protons in the nucleus of each atom of
that element

The number of electrons is equal to the number of protons in an element.
Review
1.
What is the reference standard for atomic mass units?
2.
What is the current periodic table based upon?
3.
What does the atomic number represent?
4.
Why are the numbers for atomic mass of individual atoms not whole numbers?
5.
How many protons and electrons are in the following elements:?
6.
7.
a.
Ne
b.
Ca
c.
Pt
Write the symbol for the element with the following atomic number:
a.
18
b.
41
c.
82
Write the nuclear notation for the following elements.
a.
Carbon
b.
Helium
c.
Osmium
d.
Bromium
Answers
1.
1/12th the mass of one carbon atom
2.
Atomic number
3.
4.
The number of p+
Because an atom’s mass is affected very slightly by the interactions of the various particles
within the nucleus, and the small mass of the electron is taken into account.
5.
6.
7.
How many protons and electrons are in the following elements:?
a.
Ne
10 p+
b.
Ca
20 p+
c.
Pt
78 p+
Write the symbol for the element with the following atomic number:
a.
18
Ar
(Argon)
b.
41
Nb
(Niobium)
c.
82
Pb
(Lead)
Write the nuclear notation for the following elements.
a.
Carbon
12
C
b. Helium
6
c. Osmium
190
76
4
He
2
Os
d. Bromium
80
35
Br