Chapter 7: Completing the Model of the Atom

Section 7.1: Expanding the Theory of the Atom

Electrons Occupy a complex world of energy

levels  electron distribution in energy levels of

an atom account for many of the physical and chemical properties of the element

Energy levelsElectrons with the most energy are

farthest from the nucleus and occupy the outermost level

Review of Electromagnetic Radiation and Energy levels Waves have a range of freq. and

wavelengths Higher freq = Shorter wavelength =

Greater energy Lower freq = Longer wavelength = Lower

energy

Review of Electromagnetic Radiation and Energy levels (cont)• Use to calculate the exact amount of energy released by

electrons in atoms

• By absorbing a specific amount of energy, an e- can jump to a higher energy level

 • When an e- falls back to a lower energy level, it releases the

same amount of energy in the form of radiation (light) with a definite frequency

• The energy (color) of light depends on how far the electron falls

• Greater energy = Color more toward the violet end of spectrum

Heisenberg’s Uncertainty Principle States that it is impossible to measure

exactly both the position and momentum (mass and speed) of an object (electron)

This led to the electron cloud model in atoms

Electron cloud modelBecause we cannot pinpoint exactly

where an electron is on the “surface” of an atom, we refer to its position as an electron cloud.

The chemical behavior and properties of any 2-subtances are determined by the number of these electrons around the nucleus

How do we describe the electron cloud? We use quantum numbers. These

represent the energy states of the electron.

These difference in energy states were “discovered” due to the different spectral lines of an emission spectrum.

Quantum NumbersThere are 4 quantum numbers that

describe the electron distribution of electrons in an atom

They are n, l, m and s

Principal quantum number, n

• Describes the general size of the electron cloud• Numbered levels low to high – 1,2,3,4…(integers)• Electrons may be found in each• The maximum # of electron possible in any one

level is 2n2 • We have been calling these energy levels, 1-7• Each main energy level has sublevels

Angular quantum number, l Describes the shape of the electron

cloud It represents the sublevels within an

energy level The value of l is 0 to (n-1) The number of sublevels is equal to the

value of n The lowest sublevel is s, then p, d and f

Angular quantum number, l (cont) Each sublevel can hold up to a specific

# of electrons: s sublevel can hold 1 pair (2 electrons) p can hold 3 pair (6 electrons) d can hold 5 pair (10 electrons) f can hold 7 pair (14 electrons)

Each pair has a different place in space, this space is called an orbital.

Orbitals

Angular quantum number, l (cont) The sum of all the electron clouds in any

sublevel is a spherical cloud. Electrons are repelled by each other and

attracted to the positive nucleus

Blocks in the Periodic Table

Magnetic Quantum Number, m Describes the orientation in space of a

particular orbital The value is any integer between +l and

-l

Electron Spin Quantum Number, s Describes the spin of the electron within

an orbital The value is +1/2 (clockwise) or -1/2

(counterclockwise) If there are 2 electrons in an orbital,

they must spin in opposite directions.

Hund’s RuleIn a set of orbitals, the electrons will fill the orbitals

in a way that would give the maximum number of parallel spins (maximum number of unpaired electrons).

Analogy: Students could fill each seat of a school bus, one person at a time, before doubling up.

Orbital Diagram for Hydrogen

OrbitalDiagram for Helium

Orbital Diagram for Lithium

Orbital Diagram for Beryllium

OrbitalDiagram for Carbon

Orbital Diagram for Nitrogen

Standard Notation of Fluorine

Main Energy

Level

Numbers

1, 2, 2Sublevels

Number of electrons in the sub level 2,2,5

1s2 2s2 2p5