Electron Configuration History of Modern Atomic Theory Energy Levels and Sublevels The Energy Levels www.nkschools.org/1599206375924150/lib/.../25.1-2_Electron_Structure.ppt

History of Modern Atomic Theory Niels Bohr noticed that electrons could only gain certain amounts of energy This implied that there were energy levels in the electron cloud Electrons could jump from one energy level to another, but could not exist between levels. The levels were still pictured as circular orbits. This model of the atom was called the Bohr model and agree with experimental data for hydrogen atoms. It failed to explain other atoms though.

History of Modern Atomic Theory In the modern description of the electrons in atoms, the quantum mechanical model, the electron does not travel in a circular orbit, instead it exists somewhere within a region of space called an orbital We don’t know it exact position only the probability of finding it We should no longer picture the electron as tiny particle whizzing around the nucleus The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

Energy Levels and Sublevels Remember that e- are what hold atoms together to form molecules. Understanding more about how they are arranged in the atom will help us better understand chemical reactions The electron cloud is divided into energy levels Theoretically there are an infinite number of levels, but even the biggest atoms only use seven The energy levels closest to the nucleus are lower in energy Electrons always try to achieve the lowest energy state possible; this is called the Ground State. This tendency is known as the Aufbau Principle (which means to “Build up”) When we start to place electrons into energy levels we begin with the lowest levels and build our way up as needed. Each energy level contains one or more Sublevels The first energy level has only one sublevel (called the s sublevel) The second energy level has two sublevels (called s and p sublevels) The third energy level has three sublevels (called s, p, and d sublevels) The fourth energy level has four sublevels (called s, p, d, and f sublevels)

Energy Levels and Sublevels Each sublevel is divided into Orbitals which are 3 dimensional regions of space where a pair of e- is most likely to reside The s sublevel (the first sublevel in every energy level) has one orbital The p sublevel (the second sublevel in energy levels 2-7) has three orbitals The d sublevel (the third sublevel in energy levels 3-7) has five orbitals The f sublevel (the forth sublevel in energy levels 4-7) has seven orbitals Each orbital can house two electrons The Apartment Building Analogy: You can think of the electron cloud as an apartment building The energy levels are the same as the floors The sublevels are the same as the apartments The orbitals are the same as a bunk bed for two people Each person represents an orbital

4 3 2 1 f s p s p s p s 4 sublevels 1+3+5+7=16 orbitals d 4 s p 4 sublevels 1+3+5+7=16 orbitals 2 + 6 + 10 + 14 = 32e- d 3 3 sublevels 1+3+5=9 orbitals 2 + 6 + 10 = 18e- s p 2 sublevels 1+3=4 orbitals 2 + 6 = 8e- 2 1 sublevel 1 orbital 2e- s 1

Comprehension Check Knowing that each orbital can hold 2 e-, how many electrons can the following sublevels hold? s sublevel 1 orbital x 2 e- = 2 e- p sublevel 3 orbitals x 2 e- = 6 e- d sublevel 5 orbitals x 2 e- = 10 e- f sublevel 7 orbitals x 2 e- = 14 e-

Comprehension Check How many electrons can each energy level hold? 1st Energy Level 2nd Energy Level 3rd Energy Level 4th Energy Level

Answers to Comprehension Check How many electrons can each energy level hold? 1st Energy Level 2 (2) 2nd Energy Level 8 (2+6) 3rd Energy Level 18 (2+6+10) 4th Energy Level 32 (2+6+10+14)

Energy Levels and sublevels Fill out the following table using what you know about electrons and energy levels Energy Level Sublevels Number of Orbitals Total Number of Electrons 1 2 3 4 s 1 2 s p 1 + 3 = 4 2 + 6 = 8 s p d 1 + 3 + 5 = 9 2 + 6 +10 = 18 s p d f 1 + 3 + 5 + 7 = 16 2 + 6 +10 +14 = 32

The First Energy Level The first energy level is the lowest energy level in all atoms This level only has an s sublevel and can only hold two electrons because it has only 1 orbital

The Second Energy Level Second lowest energy level Has as s sublevel and a p sublevel The 2s sublevel has one orbital and can therefore hold 2 electrons The 2p sublevel has 3 orbitals and can therefore hold up to 6 electrons

The Third Energy Level The third energy level has three sublevels s, p, and d The 3s and 3p sublevels have the same number of orbitals as 2s and 2p The 3d orbital has 5 orbitals and Can therefore hold up to 10 electrons

The Fourth & Higher Energy Levels The 4th energy level has 4 sublevels: s, p, d, and f The 4s, 4p, and 4d are all the same shape, except bigger than lower energy levels The 4f sublevel has 5 orbitals

Electron Arrangement in Atoms There are three rules to help us understand where electrons are in the electron cloud Aufbau principle – electrons will occupy the lowest energy level possible Pauli Exclusion Principle – only two electrons may occupy an orbital and they must have opposite spin (clockwise and counterclockwise) Hund’s Rule – when filling a sublevel such as 3p, e- will go to an empty orbital prior to pairing up (spread out before pairing up)

Writing Electron Configurations Electron Configurations tell you the ground state location of each electron in an atom Example 1: What is the Electron Configuration of nitrogen First of all, how many electron does a nitrogen atom have? 7 Example 2: What is the Electron Configuration of silicon First of all, how many electron does a silicon atom have? 14 # of e- Energy Level 1s2 2s2 2p3 Sublevel 1s2 2s2 2p6 3s2 3p2

The Energy of Orbitals Higher sublevels do not necessarily have a higher energy The order from lowest to highest energy is as follows Lowest Energy Highest Energy 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s

A Trick 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f 7s 7p 7d Fill the sublevels in the order that the arrows pass through, beginning with the top arrow. 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f 7s 7p 7d 7f

Comprehension Check Write the electron configurations for the following elements Be, Mg, S, Ar, Sc, Fe, Ge, Kr, Tc

Comprehension Check Write the electron configurations for the following elements Be 1s2 2s2 Mg 1s2 2s2 2p6 3s2 S 1s2 2s2 2p6 3s2 3p4 Ar 1s2 2s2 2p6 3s2 3p6

Comprehension Check Write the electron configuration for the following elements. Sc 1s2 2s2 2p6 3s2 3p6 4s2 3d1 Fe 1s2 2s2 2p6 3s2 3p6 4s2 3d6 Ge 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2 Kr 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 Tc 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 5d10

Congratulations, you can now write long electron configurations!