Chemistry Worksheet Template
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Chemistry
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Subatomic Particles
In chemistry we study three subatomic particles. These particles are the proton, neutron & electron. The
discovery of these subatomic particles proved part of Dalton’s atomic theory incorrect. Dalton stated that
all matter is made of indivisible atoms. The fact that atoms can be further divided into protons, neutrons
and electrons proves atoms can be broken down into more fundamental particles. The chart below
summarizes their symbol, location, charge, mass & discoverers.
Discovered
Subatomic Particle
Symbol
Location
Charge
Mass
by
Rutherford
+
-27
proton
p
nucleus
+1
1.673 x 10
kg
(1911)
Chadwick
0
-27
neutron
n
nucleus
0
1.675 x 10
kg
(1932)
Thomson
-
–31
electron
e
outside nucleus
-1
9.11 x 10
kg
(1897)
Atomic Number
The number of protons in an atom is called its atomic number. For example, an atom of phosphorus has
15 protons in its nucleus. Phosphorus has an atomic number of 15.
The number of electrons and protons in an atom determine its charge. Since protons have a +1 charge
and electrons have a -1 charge, in a neutral atom the number of protons is equal to the number of
electrons. Thus, in a neutral atom, the atomic number also indicates the number of electrons in an atom.
(For now, assume all atoms are neutral. We will discuss charged atoms later this year.)
Summary: In a neutral atom: Atomic Number = # protons = # electrons
Mass Number
The mass of an atom is almost entirely due to the mass of its neutrons and protons. The mass of an
–31
-27
electron (9.11 x 10
kg) is 1/1800th of the mass of both a proton (1.673 x 10
kg) and a neutron (1.675
-27
x 10
kg). As a result, the mass of the electrons factors little in the total mass of the atom (it would take
over 1800 electrons to equal the mass of 1 neutron).
The mass of the protons + the mass of the neutrons is an element's mass number.
Element
Atomic Number
# protons
# neutrons*
mass number
# electrons
Hydrogen
1
1
0
1
1
Helium
2
2
2
4
2
Lithium
3
3
4
7
3
Beryllium
4
4
5
9
4
Boron
5
5
6
11
5
*Number of isotopes of the most stable isotope. Isotopes are covered below.
Summary: Mass Number = # protons + # neutrons
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