UNIT # 3 Introduction Objectives 3.1 Structure of the Atom 3.2 3.3 Arrangement of Electrons 3.4 3.5 Atomic Mass Problems 1 | 2 | 3

ATOMIC STRUCTURE

3.3 - Arrangement of Electrons

3.3.1 - The Bohr Model of the Atom

Electrons are always moving around the nucleus and so possess potential and kinetic energy. But they can only possess certain values of energy, or specific energy levels. (Credit should be given to Niels Bohr for proposing this theory.) Click here if you need an illustration of this concept.

According to Bohr's model of the atom, electrons orbit about the nucleus much like the way planets orbit the sun. Different energy levels are associated with the different orbits. The diagram below shows the Bohr model for fluorine. The nucleus of fluorine has 9 protons. Surrounding the nucleus of fluorine is 9 electrons. The electrons arrange themselves in 3 orbits:

• In the first orbit, there are 2 electrons.
• In the second orbit, there are 7 electrons.
• In the third orbit, there are no electron.

Bohr deduced that:

1. electrons inside an atom possess different energies
• electrons in the first orbit belong to the first energy level
• electrons in the second orbit belong to the second energy level
• electrons in the third orbit belong to the third energy level...... etc ......
2. each energy level of an atom could only accomodate a certain number of electrons. The maximum number of electrons that can populate a certain energy level is given by the following formula.

where n = the specific energy level

For example:
• The maximum number of electron in the first energy level (n = 1) is 2 (1)2 = 2 electrons
• The maximum number of electron in the second energy level (n = 2) is 2 (2)2 = 8 electrons
• The maximum number of electron in the third energy level (n = 3) is 2 (3)2 = 18 electrons .... etc ...

See if you can figure out the maximum numbers of electrons in the 4th, 5th, 6th and 7th energy level.

3. Electrons fill the principal energy levels starting from n = 1 to n = 7.

Section 4.6
Arrangement of Electrons in Principal Energy Levels ..p87

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