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Atomic Structure
Werner Heisenberg
http://www.science-class.net/8th_Notes/8th_Notes_MainPage.htm
1. John Dalton (1766-1844) thought that an atom is a sphere of matter that is the same
throughout.
2. J.J. Thomson (1856-1940) discovered that all atoms contain electrons, which are tiny,
negatively charged particles. Thomson proposed that an atom is a sphere of positive charge. The
electrons are mixed uniformly in the sphere.
3. Rutherford (1871-1937) updated the model of the atom. He hypothesized that almost all the
mass and all the positive charge of an atom is concentrated in an extremely tiny nucleus at the
center of the atom.
4. Bohr (1885-1962) described the atom as a planetary arrangement: electrons orbiting the
nucleus.
http://www.epa.gov/
5. Today's model of the atom has a concentrated nucleus containing the protons and neutrons
surrounded by a cloud representing where the electrons are likely to be found.
a) Protons are in the nucleus (center) of the atom; positive charge
b) Neutrons are in the nucleus (center) of the atom; neutral (no charge)
c) Electrons move in energy levels outside of the nucleus; negative charge
6. The number of protons in the nucleus of an atom is its atomic number
a) The atomic number identifies which element you have. For example, the oxygen atom
has atomic number 8, which means that an oxygen atom has 8 protons.
7. In a stable atom, the number of protons equals the number of electrons.
a) Atoms that have lost or gained electrons are called ions.
b) Atoms that lost electrons form positive ions, e.g. Na+, Ca2+, Al3+
c) Atoms that gained electrons form negative ions, e.g. Cl –, O2–, F –
d) Note: To form ions, atoms lose and gain electrons (not protons).
8. The number of protons plus the number of neutrons is the mass number.
9. The atomic weight or atomic mass is the average of the mass numbers of all of the isotopes of
an element.
10. Isotopes are atoms of the same element (that have the same number of protons) but that have
different numbers of neutrons. The mass of each isotope is equal to the sum of protons and
neutrons.
11. Electrons can spin in different directions; scientists cannot tell exactly where an electron is at a
given moment or where it is going. They can calculate the probability that an electron will be
found in a given space. This is quantum mechanics.
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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12. The electron position as a function of distance from the nucleus is called its energy level, or
orbital. The farther the electron is away from the nucleus the higher its energy.
a) Each energy level can only hold a certain number of electrons
i) The first energy level (1) can hold 2 electrons
ii) The second energy level (2) can hold 8 electrons
iii) The third energy level (3) can hold up to 18 electrons
iv) The fourth energy level (4) can hold up to 32 electrons
v) The fifth energy level (5) can hold up to 50 electrons
vi) The sixth energy level (6) can hold up to 72 electrons
b) Usually, each energy level is completely filled before electrons fill the next level.
c) The electrons in the outermost level are called valence electrons.
3
•
Valence shell
(outermost
energy level)
2
•
•
•
1
•
•
•
•
• e-
nucleus
•
•
•
•
•
This schematic diagram of an
atom indicates that this atom
has 6 valence electrons in its
outermost energy level
(energy level 3).
•
•
13. An atom's nucleus is held together by the strong nuclear force.
If the numbers of neutrons and protons are very different, the nucleus can become unstable
and undergo radioactive decay.
The Periodic Table
The PERIODIC TABLE is an organizational system for elements. Elements are arranged in horizontal
rows going from right to left called Periods and columns going up and down called Families or
Groups.
• Elements in the same period have their valence electrons in the same energy level.
For example, elements in period 3 have valence electrons in energy level 3.
The start of each new period in the periodic table is the start of a new energy level (it is the start
of a new valence shell).
Atoms generally get more massive as you move from left to right across a period
The atomic radii tend to decrease as you move from left to right across a period
Atoms get more massive as you move down a group from top to bottom in the periodic table
• Elements in the same family / group have similar properties because they have a similar electron
arrangement in their outer shell.
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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PERIODIC TABLE
1A
1
H
1.008
1
2
Period
3
4
5
6
7
1A
3
Li
6.941
11
Na
22.99
19
K
39.10
37
Rb
85.47
55
Cs
132.9
87
Fr
(223)
2A
4
Be
9.012
12
Mg
24.30
20
Ca
40.08
38
Sr
87.62
56
Ba
137.3
88
Ra
(226)
3
4
5
21
22
23
Sc
Ti
V
44.96 47.88 50.94
39
40
41
Y
Zr
Nb
88.91 91.22 92.91
72
73
La-Lu Hf
Ta
178.5 180.9
104
105
Ac-Lr Rf
Db
(261) (262)
Lanthanides
Actinides
6
7
24
25
Cr
Mn
52.00 54.94
42
43
Mo
Tc
95.94 (98)
74
75
W
Re
183.8 186.2
106
107
Sg
Bh
(263) (264)
8
26
Fe
55.85
44
Ru
101.1
76
Os
190.2
108
Hs
(265)
9
27
Co
58.93
45
Rh
102.9
77
Ir
192.2
109
Mt
(268)
10
28
Ni
58.69
46
Pd
106.4
78
Pt
195.1
110
Uun
(269)
11
29
Cu
63.55
47
Ag
107.9
79
Au
197.0
111
Uuu
(272)
12
30
Zn
65.39
48
Cd
112.4
80
Hg
200.6
112
Uub
(269)
3A
5
B
10.81
13
Al
26.98
31
Ga
69.72
49
In
114.8
81
Tl
204.4
4A
6
C
12.01
14
Si
28.09
32
Ge
72.61
50
Sn
118.7
82
Pb
207.2
114
Uuq
5A
7
N
14.01
15
P
30.97
33
As
74.92
51
Sb
121.8
83
Bi
209.0
6A
8
O
16.00
16
S
32.07
34
Se
78.96
52
Te
127.6
84
Po
(209)
116
Uuh
7A
9
F
19.00
17
Cl
35.45
35
Br
79.90
53
I
126.9
85
At
(210)
8A
2
He
4.003
10
Ne
20.18
18
Ar
39.95
36
Kr
83.80
54
Xe
131.3
86
Rn
(222)
118
Uuo
57
58
59
60
La
Ce
Pr
Nd
138.9 140.1 140.9 144.2
61
Pm
(145)
62
63
Sm
Eu
150.4 152.0
64
Gd
157.2
65
66
67
Tb
Dy
Ho
158.9 162.5 164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0
89
90
91
92
Ac
Th
Pa
U
(227) 232.0 231.0 238.0
93
Np
(237)
94
Pu
(244)
96
Cm
(247)
97
98
Bk
Cf
(247) (251)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(262)
95
Am
(243)
99
Es
(252)
Metals are on the left hand side of the table (to the left of the metalloids)
Non-metals are on the right-hand side of the table (to the right of the metalloids).
Metalloids (semi-metals) are between the metals and non-metals (shaded).
Group 1A
Alkali metals - Elements whose atoms have 1 valence electron; they are very reactive
Note: Hydrogen although nominally a member of Group 1A, very rarely exhibits
behavior comparable to the alkali metals. Hydrogen is a non-metal.
Group 2A
Alkaline Earth Metals - Elements whose atoms have 2 valence electrons (2 electrons in
the outermost energy level)
Group 3A
Elements whose atoms have 3 valence electrons
Group 4A
Elements whose atoms have 4 valence electrons
Group 5A
Elements whose atoms have 5 valence electrons
Group 6A
Elements whose atoms have 6 valence electrons
Group 7A
Halogens - Elements whose atoms have 7 valence electrons
Group 8A
Noble Gases (also called inert gases) - Elements whose atoms have full outer shells so
they are very unreactive.
Reference: Devlin Gualtieri http://www.science-class.net/8th_Notes/8th_Notes_MainPage.htm
VIDEO: Tom Lehrer’s video on the elements. http://www.privatehand.com/flash/elements.html
The video starts as soon as you double click on the connection speed: dial up or broad band.
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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Questions:
1. Consider the fluorine atom, F, which is atomic number 9 in the periodic table.
•
•
•
•
•
9
F
19.00
9 P+
10 N
• e•
Atomic number = number of protons = 9 protons (P+)
Atomic Symbol
Atomic mass = mass of protons + mass of neutrons
Number of neutrons = Atomic mass – Number of protons
Number of neutrons = 19 – 9 = 10 neutrons (N)
Number of electrons = Number of protons = 9 electrons (e-)
•
•
How many valence electrons does fluorine have?
3
2. Consider the atomic structure of an atom as follows:
2
•
•
•
•
•
•
• e-
nucleus
•
•
1
•
•
•
•
•
•
a) What is the total number of electrons in the atom shown?
b) If this diagram represents a neutral atom (no charge), the number of protons (each proton has a
+1 charge) equals the number of electrons (each electron has a –1 charge). How many protons
does this atom have?
c) The number of protons is equal to the atomic number. Find the atomic number of this atom in
the periodic table. Identify the element.
3. The sodium atom has the symbol Na. Find sodium in the periodic table. The atomic number for
sodium is 11. The atomic number equals the number of protons in the atoms of a given element.
The mass of protons and neutrons are very nearly equal to one another. The mass of an electron is
1846 times less than a proton. In the periodic table, the atomic mass of sodium is given to be 22.99.
The mass of the electrons is negligible in the atomic mass of an atom.
Atomic Mass = Mass of the protons + Mass of the neutrons
a) How many protons does a sodium atom have?
b) Round up the atomic mass given to a whole number. The number of neutrons equals the atomic
mass minus the number of protons. How many neutrons does a sodium atom have?
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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c) On the diagram below, draw the number of electrons (e –), protons (p+) and neutrons (n) for the
sodium atom.
d) How many valence electrons does sodium have?
4. The chlorine atom has the symbol Cl. Find chlorine in the periodic table.
a) How many protons does a chlorine atom have?
b) How many neutrons does a chlorine atom have?
c) On the diagram below, draw the number of electrons (e –), protons (p+) and neutrons (n) for the
chlorine atom.
d) How many valence electrons does the chlorine atom have?
e) How many electrons does chlorine need to complete its valence shell?
5. When sodium and chlorine react to form the common table salt, NaCl, sodium gives up one electron to
form a positive ion with a +1 charge, Na+, and chlorine accepts one electron to form a negative ion with
a –1 charge, Cl–. The ions Na+ and Cl– combine in a 1:1 ratio to form ionic bonds in an ionic solid.
a) How many valence electrons does the ion Cl– have?
b) How many valence electrons does the ion Na+ have?
c) In your diagram of the sodium atom in question 3c, circle the valence electron and draw an
arrow from this electron to the outer shell of the chlorine atom diagram in question 4c. This is
how the electron is transferred to form the ionic bond between Na+ and Cl–. Common table salt,
NaCl, is a stable compound because both Na+ and Cl– have a filled valence shell.
Note: The atomic mass given in the periodic table is the weighted average mass of the naturally
occurring isotopes for a given element. Sodium has only one isotope that occurs in nature.
However, chlorine has two naturally occurring isotopes: one isotope has a mass of 35.0 (75.8%),
the other isotope has a mass of 37.0 (24.2%). The weighted average mass of these two isotopes is
35.45 as indicated in the periodic table. Thus, the method we are using to determine the number of
neutrons by rounding the mass to the nearest whole number is an approximate method.
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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Chemical Bonding
Atoms can react with one another to form new substances called compounds. When two or more atoms
chemically bond together, the resulting compound is unique both chemically and physically from its
parent atoms.
But why do atoms react to form compounds?
Chemical bonds are formed between atoms because electrons from different atoms interact with each
other. G. N. Lewis observed that many elements (in the second and third period of the periodic
table) are most stable when they contain eight electrons in their valence shell. He suggested that
atoms with fewer than eight valence electrons bond together to share electrons and complete their
valence shells.
Remember that noble gases (or inert gases) are generally not reactive and exist as stable monatomic
gases (single atom gases) at room temperature and pressure. They have the number of electrons they
need to be stable as single atoms and therefore rarely form compounds.
The halogens in Group 7A are in the column right next to the noble gases in group 8A and need only
one electron to end up with the same number of electrons as a noble gas element. For example, the
atomic number for chlorine is 17, which means it has 17 protons and 17 electrons. When the chlorine
atom gains one electron to form Cl–, it has 18 electrons, the same number of electrons as the noble gas
element argon, Ar. Note: Cl– has a minus one charge but the one is usually not shown.
The alkali metals in group 1A have only one electron in their valence shell. They lose one electron to
end up with the same number of electrons as a noble gas element. For example, the atomic number for
sodium is 11, which means it has 11 protons and 11 electrons in the neutral atom. When a sodium atom
loses one electron to form Na+ it has 10 electrons which is the same number of electrons as the noble
gas element neon, Ne.
Cl-1
Na
Cl-1
Na
Cl-1
Cl-1
+1
+
+1
Na
sodium metal
Na (s)
chlorine gas
Cl2 (g)
table salt
NaCl (s)
sodium (on the left) loses its one
valence electron to chlorine (on the right),
Sodium Chloride Crystal
Na
Cl-1
Na
Cl-1
Na
Na
Cl-1
Na
Cl-1
Cl-1
Na
Cl-1
Na
+1
+1
+1
+1
+1
+1
+1
+1
NaCl Crystal Schematic
to form a positively charged sodium ion (left)
and a negatively charged chlorine ion (right).
Reference: Anthony Carpi http://www.visionlearning.com/library/module_viewer.php?mid=55
CaMSP Science Matters Summer Institute 2007 Petra van Koppen
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