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