Download neutrons

BHS Chemistry
Fall 2013
• Except for the nucleus of H simplest type of hydrogen
atom, all atomic nuclei are made of two kinds of particles,
protons and neutrons.
• Proton has a positive charge equal in magnitude to the
negative charge of an electron. Atoms are electrically
neutral because they contain equal numbers of protons and
electrons.
• A neutron is neutral.
Particle
Electron
Charge Mass #
-1
0
Proton
+1
1
Location
Electron
cloud
Nucleus
Neutron
0
1
Nucleus



Protons and neutrons are densely packed into the
nucleus.
Electrons orbit around the nucleus in a loose electron
cloud. (click)
Note the positive charge of the nucleus and the
negative charge of the electron cloud. Also, note that
overall, the charges cancel!

The periodic table contains
information about each element.
Atomic Number
The number of protons
1
H
Hydrogen
1.00797
Symbol
Name
Atomic Mass
The number of protons + neutrons
Atomic number (Z) of an element is the number
of protons in the nucleus of each atom of that
element.
Element
Carbon
Phosphorus
Gold
# of
protons
6
15
Atomic # (Z)
79
79
6
15
•Elements are placed in order of increasing atomic
number
•Atomic number identifies an element
•Ex: all silver atoms contain 47 protons
Mass number is the number of protons and
neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Symbol
p+
n0
e-
Mass #
Oxygen
O
8
8
8
16
Sulfur
S
16
16
16
32
Tin
Sn
50
69
50
119
Element
*So how can you calculate the number of neutrons in an element?
Simple! Take the mass number and subtract the atomic number!
Tin: 119-50 = 69
*If there is no charge indicated, the number of electrons is the same
as the number of protons!

mass # = protons + neutrons
• always a whole number
• NOT on the
Periodic Table!
© Addison-Wesley Publishing Company, Inc.
Isotopes are atoms of the same element having different
masses due to varying numbers of neutrons.
(all have same number of protons and electrons but different number of
neutrons)
Look at Fig
3-9 in book
(p 76)
Isotope
Protons
Electrons Neutrons
Hydrogen–1
(protium)
99.985%
1
1
0
Hydrogen-2
(deuterium)
0.015%
1
1
1
Hydrogen-3
(tritium)
radioactive
1
1
2
Nucleus
© Addison-Wesley Publishing Company, Inc.
235
92
U



Isotopes are atoms of the same element having different mass
number due to varying numbers of neutrons
Isotopes are usually identified by specifying their mass number.
Two methods for specifying isotopes:
 The mass number is written with a hyphen after the name of
the element
ex: hydrogen-3 is tritium
 Show the composition of a nucleus as the isotopes nuclear
symbol
ex: uranium-235 is written
(Superscript is mass number,
subscripts is atomic number)
235
92
U
Mass number – atomic number = number of neutrons
Mass number
(p+ + no)
235
92
U
Atomic number
(number of p+)
Element symbol

Chlorine-37

atomic #:

mass #:

# of protons:

# of electrons:

# of neutrons:
17
37
17
17
20
37
17
Cl
Average Atomic mass is the average of all the
naturally isotopes of that element.
Isotope
Carbon-12
Carbon = 12.011
Symbol Composition of % in nature
the nucleus
12C
6 protons
98.89%
6 neutrons
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%




For most chemical calculations it is more convenient
to use relative atomic masses
Standard used by scientists to govern units of atomic
mass is the carbon-12 nuclide.
Assigned a mass of 12 atomic mass units (12 amu)
One atomic mass unit, or 1 amu, is exactly 1/12 the
mass of a carbon-12 atom.
 12C
atom = 1.992 × 10-23 g
• atomic mass unit (amu)
• 1 amu = 1/12 the mass of a 12C atom
• 1 p = 1.007276 amu
1 n = 1.008665 amu
1 e- = 0.0005486 amu
© Addison-Wesley Publishing Company, Inc.