Electron Configurations

Objectives List the atomic orbitals of an atom. List the total number of electrons needed to fully occupy each main energy level. State the Aufbau principle, the Pauli Exclusion principle and Hund’s rule. Write the electron configuration for any element.

Atomic Orbitals Quantum Mechanical Model A more complex, highly mathematical model was developed to explain observations of atoms containing more than one electron. This model works for all the elements and not just for hydrogen as in the Bohr model.

Electronic Configuration – describes the arrangement of electrons in an atom. Because atoms of different elements have different number of electrons, a distinct electron configuration exists for each element.

The electrons will assume arrangements that have the lowest possible energies. Ground State Configuration – the lowest energy arrangement of the electrons for each element.

Atomic Orbitals There are four main atomic orbitals which describe the electron configuration of the elements: S orbital - spherical shape P orbital – dumbbell shape D orbital – clover shape F orbital – Too complex to discuss.

Periodic Table with Orbitals

s, p and d orbitals

Atomic Orbitals Energy levels of the three orbitals of interest: S orbital – lowest energy P orbital – slightly higher in energy D orbital – higher in energy than P orbital

Electron Configuration Rules The number of electrons in an atom is the same as the number of protons. So the periodic table will be of real value in determining electron configurations. To build up electron configurations for any particular atom, first energy levels of the orbitals are determined.

Electron Configuration Rules The electrons are added to the orbitals one by one according to three basic rules: Aufbau Principle – An electron occupies the lowest energy orbital that can receive it. The orbital with the lowest energy is the 1s orbital. The one electron of hydrogen goes in this orbital.

Electron Configuration Rules The 2s orbital is the next highest in energy, then the 2p orbitals. The numbers 1,2,3 etc. refer to the row of the periodic table the atom is located in. As can be seen on the diagram there is only 1-s orbital, 3-p orbitals and 5-d orbitals. These refer to their orientation in space.

Electron Configuration Rules Note on the energy level diagram that the 4s orbital is lower in energy than the 3d orbital. Therefore, the 4s orbital is filled before any electrons enter the 3d orbitals.

Aufbau Principle

Electron Configuration Rules 2) Pauli Exclusion Principle – no more than two electrons may be present in an orbital and their spins must be paired. This rule basically states no two atoms can have the same electron configurations.

Electron Configuration Rules 3) Hund’s Rule – orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron. The spins of these electrons must be opposite. This rule is because similarly charged electrons want to be as far away as possible. 2p orbital

Energy level diagram for oxygen

Electronic Configurations Electronic configurations are important in chemistry: To predict what type of bonding will occur with a particular element and which electrons are being used in the bonding. Helps explain the properties of elements.

Electronic Configurations While energy level diagrams are very useful they are bulky to work with. Electron configuration notations are simpler and give the same information.

Electronic Configurations Electron configuration notations eliminate the lines and arrows of the diagrams. Instead the number of electrons in an energy level is shown by adding a superscript to the energy level designation. Example: hydrogen - 1S1

Electronic Configurations Example: hydrogen - 1S1 The large 1 indicates hydrogen is in the first row of the periodic table. The S indicates the electron is in the s orbital. The superscript 1 indicates that there is one electron in the 1S orbital.

Electronic Configurations Example: helium - 1S2 The superscript 2 indicates that there are two electrons electron in the 1S orbital. Problem: Give the electron configuration of boron and explain how the electrons are arranged.

Elements of the Second Period In the first period elements, hydrogen and helium, electrons occupy the first energy level – 1s. After the 1s orbital is filled, the next electron occupies the 2s orbital – Aufbau principle. Lithium has an electron configuration of 1s22s1

Aufbau Principle

Elements of the Second Period Inner Shell Electrons – The electrons which are in the levels above the element. In the case of lithium that is the 1s level. Also referred to as the core electrons.

Elements of the Second Period

Elements of the Second Period When you get to neon (Ne) all the 2s and 2p orbitals are full. Octet Rule – when all of the sublevels (s and p orbitals) of the highest occupied level is filled with eight electrons. All the elements in the last column of the periodic table obey the octet rule.

Noble Gases Neon is a member of the Group 18 elements (last column). These elements include neon, argon, krypton, xenon and radon). These elements are known as the noble gases.

Elements of the Third Period

Elements of the Third Period To simplify sodium’s notation, the symbol for neon, enclosed in brackets, is used to represent the complete neon configuration. [Ne] = 1s22s22p6 So the electron configuration for sodium can be written: [Ne]3s1 This is the noble gas configuration

Noble Gas Configuration So the electron configuration for sodium can be written: [Ne]3s1 Valence electrons – the electrons located on the same row as the element. For sodium this would be 3s1 The inner shell electrons would be all electrons up to and including Ne.

Elements of the Fourth Period With the 4s level full (calcium), the 4p and 3d sublevels are next available. Referring to the Aufbau diagram of energy levels, the 3d sublevel is lower in energy than the 4p sublevel. There are five 3d orbitals that hold a total of 10 electrons. Elements range from Sc to Zn.

Elements of the Fourth Period

Elements of the Fourth Period

Elements of the Fifth Period Elements in the fifth period start with the 5s orbital. 5s 4d 5p

Periodic Table with Orbitals