SUBATOMIC PARTICLES
The three main subatomic particles found in atoms are protons, neutrons, and
electrons. These particles are compared by mass, charge, and location below.
Mass
Proton
1 amu
Neutron
1 amu
Electron
1/1837 amu
surrounding the nucleus
Charge
+1
0
-1
Location
In the nucleus
In the nucleus
In the cloud
The actual mass of a proton or a neutron is 1.67 X 10-24 grams. Since this is
such a small number, scientists have come up with a relative scale for
measuring the mass of protons and neutrons. The units of this scale are called
atomic mass units (amu). Originally scientists based this scale on the mass of
a hydrogen atom before finally deciding than one atomic mass unit is equal to
1/12 the mass of a carbon atom with 6 protons and 6 neutrons. Electrons have
almost no mass in comparison to protons and neutrons and their mass is usually
not even considered when calculating the mass of an atom.
Atoms are identified by their atomic number (Z). The atomic number of an
atom is the same as the number of protons found in the atom. The periodic
table is arranged in order of increasing atomic number, beginning with
hydrogen (atomic number 1) and moving down the table to larger atoms such
as uranium (atomic number 92). If an atom is electrically neutral, the number
of protons (+) will equal the number of electrons (-). Therefore, if you know
the atomic number of an atom you also know how many protons and electrons
are in that atom. All atoms of the same element have the same number of
protons since this is what identifies an atom. If you know the atomic number of
an atom you can look on the periodic table and find the symbol of that atom.
The Mass Number (A) of an atom is the total number of protons and neutrons
found in that atom. The mass number of an atom is not found on the periodic
table. By knowing the atomic number and the mass number of an atom you can
calculate the number of neutrons. #Neutrons = Mass Number - Atomic
Number
All atoms of the same element must have the same number of protons, but they
may vary in their number of neutrons. Atoms of the same element with
different numbers of neutrons are called isotopes. All atoms found on the
periodic table have two or more isotopes. For example, hydrogen has 3
naturally occuring isotopes. They all have 1 proton but they vary in their
number of neutrons from 0 to 2. Isotopes can be shown by writing out the
name of the element with the mass number following or by using the symbols
shown below.
1
2
3
hydrogen-1
H
hydrogen-2
H
hydrogen3
H
1
1
1
When using the symbols for isotopes, the atomic number is shown on the
bottom beside the symbol and the mass number is shown on top beside the
symbol. Hydrogen-1 has zero neutrons and hydrogen-2 has one neutron and
hydrogen-3 has two neutrons. They are all different isotopes of hydrogen and
only differ by their number of neutrons.
Periodic tables show the atomic number of atoms and they also show the
atomic mass of atoms. The atomic mass of an atom is the weighted average of
the different isotopes of an element. The atomic mass for hydrogen is 1.007
amu. If you look at the mass of the 3 isotopes of hydrogen shown above you
would expect the atomic mass to be 2 amu (the average of the 3 isotopes). The
reason for the discrepancy is that hydrogen-1 makes up over 99% of all
hydrogen atoms and this skews the average closer to 1 amu than the expected 2
amu. The atomic mass of an element is found by multiplying the mass of each
isotope by its percentage and then adding them up.
Example:
There are two common isotopes found for boron atoms, boron-10 and boron11. Boron-10 comprise 19.78% of all boron atoms and the other 80.22% of
boron atoms are boron-11. Calculate the atomic mass of boron.
boron-10 = 10 amu X 19.78% = 197.8
boron-11 = 11 amu X 80.22% = 882.42
197.2 + 882.42 = 1079.62 /100 = 10.8 amu
When calculating atomic mass you can check your answer by comparing it to
the atomic mass found on the periodic table. The number on the periodic table
should be very close to your answer. Your answer might very slightly because
of rare isotopes that may not be included in the percentages when calculating
atomic mass.