# ATOMIC STRUCTURE

## 3.1 - Arrangement of Electrons

we know that the electrons are located outside the nucleus of the atom in mostly empty space. In this section we will learn that associated with this empty space are many different energy levels. Electrons are organized in such a way such that repulsions between them are minimized. Therefore, they are inter dispersed within this space with distinct distances up to 100,000 times away from the nucleus of the atom. Associated with each distinct distance from the nucleus is an energy level.

We will be looking at three models to understand how electrons are arranged inside an atom. Each model will help us gain a better understanding on this complex arrangement. These models are:

1. The Bohr Model of the Atom
2. The Lewis Electron-Dot Formulas of Elements
3. The Quantum Mechanical Model of the Atom

### 3A - 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.)

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 accommodate 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.

### 3B - Bohr Diagram

Let's take a look at how to draw Bohr diagrams:

1. For a hydrogen atom, H, the one electron goes into the first energy level.
 Draw a circle and label it with the symbol of the nucleus, H. Write the number of protons for the nucleus, 1p+. Draw an arc to represent the first energy level. Label the arc 1e- to represent that there is one electron in this energy level. 2. For a helium atom, He, the two electrons go into the first energy level.
 Draw a circle and label it with the symbol of the nucleus, He. Write the number of protons for the nucleus, 2p+. Draw an arc to represent the first energy level. Label the arc 2e- to represent that both electrons reside in this energy level. 3. For a lithium atom, Li, two of the three electrons go into the first energy level. The third electron goes into the second energy level. This electron in the outer energy level is called the valence electrons. The two electrons in the first energy level are called the core electrons.
 Draw a circle and label it with the symbol of the nucleus, Li. Write the number of protons for the nucleus, 3p+. Draw an arc to represent the first energy level. Label the arc 2e- to represent that 2 of the 3 electrons reside in this energy level. Draw a second arc to represent the second energy level. Label the arc 1e- to represent that the 3rd electron resides in this outer energy level. Let's see if you've grasp the idea. You should be able to draw Bohr diagrams for the first 18 elements of the periodic table.

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