• The number of protons determines the identity of the atom.
• The number and arrangement of electrons determines an atom’s physical and chemical properties.

All electrons in an atom possess energy,
but not all electrons have the same level of energy.

Energy Levels
The ascending levels of energy around a nucleus in which electrons occur

The number representing the energy level of the electron is called the Principle Quantum Number (n).
 

• Of the elements known today, there are seven energy levels (n = 1 to n = 7), with the seventh being the highest.

Each principal energy level can contain up to a maximum 2n² electrons.

Thus, the first level can contain up to 2 electrons, 2(1²) = 2;
the second up to 8 electrons, 2(2²) = 8;
the third up to 18, 2(3²) = 18;
and so on.

Electron Level Arrangements
The electron level arrangement gives the number of electrons in each energy level.

The arrangement of the electrons in an atom can be determined from its position on the periodic table.

Electrons and their arrangements around the nucleus determines the chemical properties of an element.

Example:
Sulfur, S, is in Period 3, which corresponds to Energy Level 3.
.
Therefore, it has 

• two electrons in the first row,
• eight electrons in the second
• six electrons in the third.

The electron level arrangement for sulfur would be written as 2, 8, 6.

What are the energy levels of an atom?
 

Practice Problems Answer the following questions:

1. What determines an atom's physical and chemical properties?

2. What are the energy levels of an atom?

3. What does the Principle Quantum Number, n, of an electron represent? . What would the formula 2n2 tell you about n? . What would be the maximum number of electrons in the third energy level of an atom?

4. What do electron level arrangements tell you?

5. Write the electron level arrangement for each of the following: . a) oxygen b) chlorine c) neon d) argon .

6. What element has the electron level arrangement of 2, 8, 2?

Energy Level Changes Photon a discreet packet of energy without mass that travels at the speed of light, with the properties of both a particle and a wave 1. When a photon strikes an electron, the energy level of the electron is raised in correspondence to the energy of the photon. The electron retains this extra energy only temporarily. Eventually, the electron loses this extra energy. 2. When the electron goes back to its normal energy level, it emits a photon equal to the amount of energy it loses. When the energy of this photon is within the visible range, we see it as a color. This is how things have the colors we see.