Name ________________________
Honors Chemistry
___/___/___
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.
Subatomic Particle Symbol
Location
Charge
Mass
Mass
+
-27
proton
p
nucleus
+1
1.673 x 10
kg
1.00728 amu
0
-27
neutron
n
nucleus
0
1.675 x 10
kg
1.00866 amu
-
–31
-4
electron
e
outside nucleus
-1
9.11 x 10
kg 5.4880 x 10
amu
*
-24
1 amu = 1.66 x 10
g
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
Isotopes
Isotopes are two or more atoms of the same element with the same number of protons but a
different number of neutrons. The existence of isotopes proves that another part of Dalton's atomic
theory is incorrect. Dalton wrote that atoms of the same element have the same physical and chemical
properties. Although isotopes have the same chemical properties, they do not have the same physical
properties.
Isotopes are written in two different ways. They can be written using
their symbol with the mass number (to the upper
left) and atomic number (to the lower left) or the
isotope name is written with a dash and the mass number.
For example:
Two naturally occurring isotopes of chlorine are chlorine-35 & chlorine-37. Thirty-five
and thirty-seven are the mass numbers for the two isotopes. Both isotopes have the same atomic number,
number of protons and electrons.
isotope name
atomic number
# protons
# neutrons
mass number
# electrons
chlorine-35
17
17
18
35
17
chlorine-37
17
17
20
37
17
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 1/1836th 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).
Summary: Mass Number = # protons + # neutrons