Electron Configuration

Why we study electrons Knowing the position of electrons helps us with many topics: Why chemical reactions occur Why some atoms are more stable than others Why some elements react with only certain atoms We want to know how many electrons an atom has, and where they’re located.

The Electron Cloud Bohr’s model of the atom only showed us that electrons have different energy levels Our current “quantum mechanical” model of the atom goes even further and talks about the different orbital shapes inside these energy levels. Quantum numbers link The world of quantum mechanics 1 2 3 4 5

4 QUANTUM NUMBERS n l m s N - Principal Energy Level Higher values of n mean more energy for the electron and the corresponding radius of the electron cloud or orbital is further away from the nucleus. Values of n start at 1 and go up by integer amounts. The higher the value of n, the closer the corresponding energy levels are to each other.

QUANTUM NUMBERS n l m s L - Angular Quantum Number In chemistry, there are names for each values of ℓ. The first value, ℓ = 0 called an s orbital. s orbitals are spherical, centered on the nucleus. The second, ℓ = 1 is called a p orbital. p orbitals are usually polar and form a teardrop petal shape with the point towards the nucleus. ℓ = 2 orbital is called a d orbital. These orbitals are similar to the p orbital shape, but with more 'petals' like a clover leaf. They can also have ring shapes around the base of the petals. The next orbital, ℓ=3 is called an f orbital. These orbitals tend to look similar to d orbitals, but with even more 'petals'.

QUANTUM NUMBERS n l m s M - Magnetic Quantum Number The third quantum number is the magnetic quantum number, m. These numbers were first discovered in spectroscopy when the gaseous elements were exposed to a magnetic field. This relationship shows for every value of ℓ, a corresponding set of values of m ranging from -ℓ to ℓ is found. This number determines the orbital's orientation in space. For example, p orbitals correspond to ℓ=1, can have m values of -1,0,1. This would represent three different orientations in space for the twin petals of the p orbital shape. They are usually defined to be px, py, pz to represent the axes they align with.

QUANTUM NUMBERS n l m s S - Spin Quantum Number The fourth quantum number is the spin quantum number, s. There are only two values for s, +½ and -½. These are also referred to as 'spin up' and 'spin down'. This number is used to explain behavior of individual electrons as if they were spinning in a clockwise or counterclockwise. The important part to orbitals is the fact that each value of m has two electrons and needed a way to distinguish them from one another..

QUANTUM NUMBERS

QUANTUM NUMBERS SUMMARY Principal Angular Momentum Magnetic Spin Symbol N l Ml Ms Values N-1 -l to l What it describes How far the electron is f rom nucleus Describes the shape or type of orbital Describes the electron’s position in the orbital Describes the spin N=1, 2, 3, 4, 5, 6, 7 l=0, 1, 2, 3 s, p, d, f Ml= s 1 position 0 p 3 positions -1, 0, 1 D 5 positions -2, -1, 0, 1, 2 F 7 positions -3, -2, -2, 0, 1, 2, 3 +1/2 counter-clockwise -1/2 clockwise Maximum # e- 2N2 2e for each position S=2, p=6, d=10, f=14 Maximum # sub levels Eg. e level 2 l=0 or l=1 s, p Pauli Exclusion Principle: NO 2 ELECTRONS in the same atom have the same 4 quantum numbers

Orbital – region where electrons of a specific energy are likely to be. -up to 2 electrons per orbital

Summary of Principal Energy Levels, Sublevels, Orbitals Principal E Level N Number of Sublevels =N Types of Sublevels Orbitals =N2 # Electrons in Energy Level =2N2 N=1 1 1s (1 orbital) 2 N=2 2s (1 orbital), 2p (3 orbitals) 4 8 N=3 3 3s (1 orbital) 3p (3 orbitals) 3d(5 orbitals) 9 18 N=4 4s (1 orbital) 4p (3 orbitals) 4d(5 orbitals) 4f (7 orbitals) 16 32 N=5 N=6 N=7 5 25 50 6 36 72 7 49 98

Orbital Diagrams The order in which will fill a sublevel all comes down to energy! Electrons fill the lowest energy sublevels first The periodic table helps to show this order!

C. Periodic Patterns s p d (n-1) f (n-2) 1 2 3 4 5 6 7 6 7 © 1998 by Harcourt Brace & Company

Help a brother out…. When working out electron configuration, it’s important to understand orbital diagrams. Here are 3 rules that will help us understand and interpret orbital diagrams.

A. General Rules Electrons fill the lowest energy sublevel available. Aufbau Principle Electrons fill the lowest energy sublevel available. You cannot skip any sublevels

A. General Rules Pauli Exclusion Principle Each orbital can hold only TWO electrons with opposite spins. Represents an electron rotating clockwise Represents an electron rotating counter - clockwise

A. General Rules WRONG RIGHT Hund’s Rule Within a sublevel, place one e- per orbital with the same spin before pairing them. “Empty Seat Rule” WRONG RIGHT

A Few more things…. Each arrow you draw represents an electron. (remember the three rules!) Each orbital holds 2 electrons and has different shapes The S level has only 1 orbital= total of 2 e- The P level has 3 orbitals = total of 6 e- The D level has 5 orbitals = total of 10 e- The F level has 7 orbitals = total of 14 e-

Practice - Fill in the orbital diagram for Magnesium *Use your PT to help you out!

Electron Configuration An atom’s electron configuration… Describes the location of electrons within the atom Identifies the shape of the electron clouds - regions where the electrons are held. Uses numbers and letters to describe electrons’ location and which electron cloud it is part of. Essentially, summarizes the orbital diagram!

1s2 An example The “s” tells you the electron’s cloud shape. In this case it’s a spherically shaped cloud. It is called the s sublevel. Sublevel = Shape 1s2 The “2” simply tells you how many electrons are in this cloud. The “1” tells you how far from the nucleus the electrons can go. In this case 2 electrons are creating the cloud. In this case, its in the 1st energy level, which is the closest level to the nucleus. Note: Every orbital can only hold 2 electrons. They must have opposite spins.

1s2 2s2 2p4 O B. Notation 8e- 1s 2s 2p Orbital Diagram Electron Configuration 1s2 2s2 2p4

C. Periodic Patterns Shorthand Configuration This is a shorter way to do e- configurations Steps… 1) Write the symbol of the element your doing 2) Find the noble gas (group 18) that comes before the element you are doing. 3) To begin, put that noble gas in [brackets] 4) Starting with that noble gas, finish the electron configuration

S 16e- 1s2 2s2 2p6 3s2 3p4 S 16e- [Ne] 3s2 3p4 B. Notation Longhand Configuration S 16e- 1s2 2s2 2p6 3s2 3p4 Core Electrons Valence Electrons Shorthand Configuration S 16e- [Ne] 3s2 3p4

C. Shorthand practice Example - Germanium [Ar] 4s2 3d10 4p2

Practice these in shorthand Li N Mg C

Draw this! This will help It is a way to remember the order in which electrons fill their orbitals. When you reach the far Left side, you reach a “wall” and must go back to The right hand side.