Modern Atomic Theory

How do electrons create the chemical trends on the

Periodic Table

Wave-Mechanical ModelAssumptions

• We never know exactly how electrons move in an atom

• At any point in time there are areas around the nucleus where the electron is more likely to be (higher electron probability)

• We can make a probability map of where the electron is more likely to be– The more intense the color on the map, the more

likely the electron will be there

Wave-Mechanical ModelAssumptions (cont)

• We call the probability maps orbitals– An orbital is the three dimensional space

around the nucleus where electron probability is high

Wave-Mechanical ModelAssumptions (cont)

• The “edge” of the orbital is fuzzy, like the outer edge of the earth’s atmosphere

• Chemists and physicists arbitrarily define the orbital as containing 90% electron probability

Three Rules for electron orbital filling

• Aufbau Principle• Pauli Exclusion Principle• Hund’s Rule

– When electrons occupy orbitals of equal energy, one electron enters each orbital until all of the orbitals contain one electron with parallel spins

– Then the second electron enters the orbitals with opposite spins

Principal Components of the Wave-Mechanical Model

1. Atoms have a series of energy levels called Principal Energy Levels

A. Designated by whole numbers

B. Symbolized by n

- n=1, n=2, n=3, n=4

Principal Components cont

2. The energy level increases as the value of n increases

3. Each Energy Level contains one or more types of orbitals called sub-levels

4. The number of sub-levels (or types of orbitals) equals n (number of the energy level)

Principal Components cont

5. The number of the energy level, and the letter of the sub-level (orbital shape) are used to designate the energy level, sub-level, and orbital shape (I.e. 1s, 2p)

6. An orbital can be empty or it can contain up to two electrons as long as they have opposite spins

Principal Components cont

7. The shape of the orbital indicates 90% electron probability

-NOT the details of the electron’s movement

Energy Levels

• Electrons fill the orbitals in order of energy, from lowest energy to highest

• The order of energy is:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

What this means is….• 3d has a higher energy than 4s• Even though 3d is closer to the nucleus as far

as the number of the energy level• Electrons will fill 4s before 3d• 4s is still the outer orbital compared to 3d• It is not Mrs. Erfft’s fault• Mrs. Erfft is not confused

This creates a pattern on the Periodic Table

• Explains the trends seen in much of the chemical behavior of the elements

Electron Configuration Patterns

The whole Periodic Table

Short Form Electron Configuration

The electron configuration for Sr is1s22s22p63s23p64s23d104p65s2

The electron configuration for Kr is1s22s22p63s23p64s23d104p6

Note that everything is the same through the fourth energy level (4p6 is complete)

Short Form Cont

We can shorten the electron configuration for Sr by writing the electron configuration for Kr as [Kr]

1s22s22p63s23p64s23d104p6=[Kr]

Then we can write the configuration for Sr as [Kr]5s2

To write the short form:1. Take the electron configuration for the noble

gas on the period above the element you are focusing on

2. Shorten the electron configuration of the noble gas by putting the symbol of the noble gas into brackets

3. Write the rest of the electron configuration to the right of the bracketed noble gas symbol in the correct order

Valence Electrons

Valence Electrons are the electrons in the outermost energy level of an atom

These are the electrons in the s and p orbitals specifically

All of the other electrons are called the core electrons

Valence Electrons• Are the most important electrons to

chemists• Being the outermost electrons, they are

the electrons that govern the chemical behavior of the element/atom

• Valence electrons are the electrons that form bonds and are lost or gained to form ions

Valence Electrons of Groups

• Note that the atoms in the same vertical groups on the periodic table have the same numbers of electrons in the same orbitals

• Just higher energy levels as you go down a column

• Therefore elements in the same groups show the same valence electron configuration

Valence electron configurations

For Chlorine

Electron Configuration:

[Ne]3s23p5

Valence Configuration: 3s23p5