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Electronsand the Bohr Model
Neils Bohr
• Neils Bohr was convinced that the atom could be pictured as a small positive nucleus with electrons orbiting around it as the planets orbit the sun.
The Bohr Model
• Neils Bohr pictured the electron moving in circular orbits corresponding to the various allowed energy levels.
Bohr proved incorrect
• Although the Bohr model paved the way for later theories, it is important to realize that the current theory of atomic structure is not the same as the Bohr model.
• Electrons do NOT move around the nucleus in circular orbits like planets orbiting the sun.
• Bohr’s model is useful (in the chemistry classroom) for picturing the electrons present in an atom.
The Bohr Model – in use
• The Bohr model does NOT focus on the nucleus, but rather on the electrons.
• The electrons are first placed on the innermost electron shell or energy level.
• Two electrons can fit in this level.
The Bohr Model – in use (cont.)
• The electrons are then placed on the second electron shell or energy level.
• Eight electrons can fit in this level.• The electrons are then placed on the third electron shell or energy level. • 18 electrons can fit in this level.
The Bohr Model – in use (cont.)
• Electrons are represented by dots.• Electrons are usually placed singly at first and
then doubly.• Example: Sodium
Let’s try one…
• Example: Sulfur
Bohr’s model incorrect again!
• We will look at the trend of Ionization Energy to show how the Bohr model is incorrect in this distribution of electrons.
• Placing 2 electrons in the first energy level, 8 in the second energy level, and 18 in the third energy level is not exactly the correct placement.
Electron placement in energy levels
• Electrons should be placed on the energy levels in the order– First – 2 electrons– Second – 8 electrons– Third – 8 electrons– Fourth – 18 electrons– Fifth – 18 electrons– Sixth – 32 electrons
Ionization Energy
• This new pattern can be explained by looking at the periodic trend of ionization energy.
• Ionization energy is the energy needed to remove one or more electrons from a neutral atom to form a positively charged ion.
• Ionization energy depends on – the attraction between the positively charged nucleus
and the negatively charged electron– The repulsion between the negatively charged
electrons in the energy levels.
Ionization Energy Trend
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
This chart shows the trend for elements 1 – 56 on the periodic table.
Ionization energy increases across a period
• As you go across a period, the ionization energy increases as more energy is required to remove an electron.
• Remember as you go across a period, there are more protons.
• And in a neutral atom, proton number is equal to electron number.
• Thus more energy is required to remove an electron from this balance of charge.
But every few elements, I see a slight decrease in the trend?
• There are minor decreases in the energy required as you go across a period.
• We will discuss this in the next unit.
• So how does this trend illustrate the number of electrons in each energy level?
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
2
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8 18
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8 18
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8 18 18
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8 18 18
H
He
Li
Ne
Na
Ar
K
Kr
Rb
Xe
Cs
0
500
1,000
1,500
2,000
2,500
+
82 8 18 18
2
8
8
18
18
32
Stability
• The first period elements become stable with 2 electrons in their outermost energy level.
• The second and third period elements become stable with 8 electrons in their outermost energy levels.
• The fourth and fifth period elements become stable with 18 electrons in their outermost energy levels.
Most stable elements
• The Noble gases are the most stable elements because they have a complete set of electrons in their outermost energy level.
• Examples:– Helium: 2 electrons in the first energy level– Neon: 8 electrons in the second energy level– Argon: 8 electrons in the third energy level– Krypton: 18 electrons in the fourth energy level– Xenon: 18 electrons in the fifth energy level
