Chapter 5

Electrons in Atoms

5.1 Light and Quantized Energy

• Rutherford’s atomic model – nucleus surrounded by fast-moving electrons- not complete model

• Rutherford’s model did not explain why certain elements behave the way they did

• Certain elements emitted light when heated

• Light can behave as a wave and a particle!

• Electromagnetic Radiation- form of energy that exhibits wavelike behavior

• Ex: visible light, microwaves, x-rays

Wave Nature of Light

• Electromagnetic Spectrum- encompasses all forms of EM radiation

Electromagnetic Spectrum

The WAVE!

• Wavelength (λ )- shortest distance between equivalent points on a continuous wave• Measured crest to crest or trough

to trough

• Frequency (f )- number of waves that pass a given point per second

• 1 Hertz (Hz) = 1 wave per second– SI unit of frequency

• Amplitude- wave’s height from the origin to a crest or from origin to trough

• Speed of light = frequency x wavelength C = fλ– C = 3.0 x 108 m/s

• as wavelength increases, frequency decreases & vice versa

(inversely related)

• Sunlight (white light) contains continuous range of wavelengths and frequencies of visible light

• Atomic Emission Spectrum- set of frequencies of the EM waves emitted by atoms of the element• Unique to each element, used for

identification

Particle Nature of Light

• Quantum- minimum amount of energy that can be gained or lost by an atom

• Quantum = hf• E = energy• h = Planck’s constant; 6.626 x 10-34 J•s• f = frequency• J is joule; SI unit for energy

• Photon- particle of EM radiation with no mass that carries a quantum of energy• Ephoton = hf

• WHEN LIGHT BEHAVES LIKE A PARTICLE!

5.2 Quantum Theory and the Atom

• Bohr Model of the Atom- proposed that hydrogen atom has only certain allowable energy states

• Ground State- lowest allowable energy state of an atom• Excited State- when an atom gains

energy

• The lower the energy level, the smaller the orbit• The higher the energy level, the

larger the orbit

• Orbital - A three-dimensional region around the nucleus of an atom that describes an electron’s probable location

S-Orbital

• Spherical in shape

P-Orbital

• Dumbbell shaped

5.3 Electron Configurations

• Electron Configuration- the arrangement of electrons in an atom–Electrons tend to assume

arrangement to have lowest possible energy

• Remember!!! Number of electrons is equal to number or protons, which is equal to the atomic number!!!• Ex: Carbon is atomic number 6, so

it has 6 total electrons!

• Electron configuration notation - describes the principal energy level and energy sublevel associated with each of the atom’s orbitals and includes a superscript representing the number of electrons in the orbital

• Electron Configuration Examples:O 1s2 2s2 2p4

Cl 1s2 2s2 2p6 3s2 3p5

Al 1s2 2s2 2p6 3s2 3p1

• Electron Notation using Noble Gas Notation:O [He] 2s2 2p4

Cl [Ne] 3s2 3p5

Al [Ne] 3s2 3p1

• Valence Electrons- electrons in the atom’s outermost orbitals- generally those associated with the atom’s highest principal energy level

• Electron-Dot Structure- consists of the element’s symbol (represents nucleus & inner electrons) surrounded by dots representing the atom’s valence electrons• Also known as Lewis Structure

# of Valence

Electrons

1 2 3 4

Example Group 1

Group 2

Group 3

Group 4

Lewis Structure

# of Valence

Electrons

5 6 7 8

Example Group 5

Group 6

Group 7

Group 8

Lewis Structure