1. Section 2.2 Electrons in Atoms
Objective
Relate the electron to modern atomic theory.
Compare electron energy levels in an atom.
Illustrate valence electrons by Lewis electron dot structures.

2. Electron Motion and Energy
Opposites attract…
So, why are the negative electrons not pulled into the positive nucleus of an atom?

3. Niels Bohr ( )
A Danish scientist who worked the Rutherford solved this very question.
He proposed the the electrons have enough energy to keep them in constant motion around the nucleus.
He compared the motion of the electrons to the motion of the planets around the sun.

4. Electromagnetic Spectrum
Electromagnetic spectrum – the whole range of electromagnetic radiation.
Radio waves
Microwaves
Infrared rays
UV rays
X rays
Gamma rays

5. Electromagnetic Spectrum

6. Electrons and Light
When electrons of an atom are excited they emit light.
Emission spectrum – the spectrum of light released from excited atoms of an element.

7. Emission Spectrum of Hydrogen

8. Evidence for Energy Levels
When electrons absorb energy, the only absorb the amount needed to move to a specific higher energy state.
It is when the electrons fall back to the lower energy state that they emit only certain amounts of energy and therefore only certain colors of light.
Because of this Bohr reasoned that electrons move around the nucleus only at distances that correspond with the those amounts of energy.
These regions of space in which electrons can move around the nucleus of an atom are called energy levels.

9. Energy Levels of Hydrogen

10. The Electron Cloud Model
Because of on going research scientists today realize that energy levels are not neat, planet-like orbits around he nucleus of an atom.
They are spherical regions of space around the nucleus in which electrons are most likely to be found.

11. Electron Cloud
Electron Cloud – The space around the nucleus of an atom where the atom’s electrons are found.

12. Electrons in Energy Levels
The lowest energy level is the smallest and the closest to the nucleus.
The first energy level holds a maximum of two electrons
The second energy level hold a maximum of 8 electrons
The third energy level holds 18 electron
You can use the formula 2n2 to determine how many electrons each level holds

13. Valence Electrons
Valence Electrons – the electrons in the outermost energy level
You can use the periodic table to determine how many valence electrons some elements have

14. Valence Electrons Group 1 – 1 valence electron
Group 2 – 2 valence electrons
Group 13 – 3 valence electrons
Group 14 – 4 valence electrons
Group 15 – 5 valence electrons
Group 16 – 6 valence electrons
Group 17 – 7 valence electrons
Group 18 – 8 valence electrons

15. Valence Electrons
Why is it important to know how many valence electrons an element has?
It is the electrons of an atom that interact.
Therefore, many of the chemical and physical properties of an element are directly related to the number and arrangement of valence electrons.

16. Oxygen

17. Lewis Dot Diagrams
Lewis Dot Diagram – a diagram where dots or other small symbols are placed around the chemical symbol of an element to illustrate the valence electron.

18. Lewis Dot

19. Let’s Try It
Oxygen’s Lewis Dot
Carbon’s Lewis Dot
Sodium’s Lewis Dot