Section 2.2 Electrons in Atoms

ObjectiveRelate the electron to modern atomic theory.Compare electron energy levels in an atom.Illustrate valence electrons by Lewis electron dot structures.

Electron Motion and Energy Opposites attract…

So, why are the negative electrons not pulled into the positive nucleus of an atom?

Niels Bohr (1885-1962) 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.

Electromagnetic Spectrum Electromagnetic spectrum – the

whole range of electromagnetic radiation. Radio waves Microwaves Infrared rays UV rays X rays Gamma rays

Electromagnetic Spectrum

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.

Emission Spectrum of Hydrogen

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.

Energy Levels of Hydrogen

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.

Electron Cloud Electron Cloud – The space around

the nucleus of an atom where the atom’s electrons are found.

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

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

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

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.

Oxygen

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.

Lewis Dot

Let’s Try It Oxygen’s Lewis Dot

Carbon’s Lewis Dot

Sodium’s Lewis Dot