# Unit 2 Chapter 4 Lecture 2 - Mrs. Horne's Science Site The Atom Lesson 4 Quantum Numbers and Electron Configurations 1 An atomic orbital is the region of space in which there is a high probability of finding an electron. Quantum Numbers Each electron must have a different location (atomic orbital) in the atom. The electrons are described by four quantum numbers.

**Principal **Angular momentum Magnetic Spin 2 Principal Quantum Number Energy Level where the electron is located Numbered 1 to 7 out from the Nucleus Each level holds a maximum of 2n2 number of electrons Principal level 1 holds 2 electrons Principal level 2 holds 8 electrons Principal level 3 holds 18 electrons

Principal level 4 holds 32 electrons These energy levels correspond to the periods on the periodic table 3 Angular Momentum Quantum Number Electrons also occupy energy sublevels within each level. These sublevels are given the designations s, p, d, and f. These designations are in reference to the sharp, principal, diffuse, and fine lines in emission spectra. Shapes of the atomic orbital include: spherical (s), dumbbell (p), double-dumbbell (d), quadruple-dumbbell (f)

The number of sublevels in each level is Energy Level Can Have Shapes the same as the number of the main level 1 sublevels). s (up to four 4 2 3 4-7 s, p s, p, d s, p, d, f

Electron Occupancy in Sublevels The maximum number of electrons in each of the energy sublevels depends on the sublevel: The s sublevel holds a maximum of 2 electrons. The p sublevel holds a maximum of 6 electrons. The d sublevel holds a maximum of 10

electrons. The f sublevel holds a maximum of 14 electrons. 5 The maximum electrons per level is obtained by adding the maximum number Aufbau Principle Gives the order in which atomic orbitals

are filled Electrons occupy the orbitals of lowest energy first The Periodic Table is a guide for the Aufbau Principle, going from left to right as you move down the periodic table Each element represents one electron, each period (row) represents one energy level. 6 Electron Configurations The electron configuration of an atom is a method of writing the location of electrons by sublevel.

The sublevel is written followed by a superscript with the number of electrons in the sublevel. If the 2p sublevel contains 2 electrons, it is written 2p2. The electron sublevels are arranged according to increasing energy. 7 Electron Configurations

Assigning the address of all electrons in atom Use only principle and angular momentum quantum numbers 2p 6 8 Principal Energy level Orbital Shape Number of electrons in that shape

Electron Configurations and the Periodic Table The periodic table can be used as a guide for electron configurations. The period number is the value of n. Groups 1A and 2A have the s-orbital filled. Groups 3A - 8A have the p-orbital filled. Groups 3B - 2B have the d-orbital filled. The lanthanides and actinides have the f-orbital filled. Blocks and Sublevels We can use the periodic table to predict which sublevel is being filled by a particular element.

Diagonal Method Orbital Filling Order Try using the diagram to the right. Using diagonal arrows, it allows you to determine the order in which sub orbitals fill with electrons 1s 2s

3s 4s 5s 6s 7s Start 2p 3p 4p 5p 6p 3d 4d 4f

5d 5f 6d Writing electron configurations Write configurations for O, Ni, Br, Sr O= 1s2 2s2 2p4 Ni= 1s2 2s2 2p6 3s2 3p6 4s2 3d8 Br= 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 Sr= 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 13 Electron Configurations

Exceptions to the AUFBAU principle Chromium prefers a half full d as opposed to a full 4s, thus 4s13d5 Copper prefers a full 3d as opposed to a full 4s, thus 4s13d10 This half filled, or filled d orbital, is used most of the time to explain this, but other transition metals do not follow this trend. AUFBAU exceptions of chromium and copper as a half full sublevel are more stable than a full 4 s sublevel, or for copper that a full dsublevel is more stable than a half full 4s Valence Electrons Valence Electrons = electrons in an 15

atoms outermost principal energy level (furthest from nucleus). When an atom undergoes a chemical reaction, only the outermost electrons are involved. These electrons are generally further from the nucleus are of the highest energy and determine the chemical properties of an element-they are the most important Shorthand e- configurations Since the valence electrons are the important electrons, we use a shorthand system to show an

elements valence electrons All noble gases (group18) have 8 valence electrons (except for helium) and therefore have a very stable configuration (most atoms want 8 valence electrons) 16 Electron Configuration Shorthand Write configurations for K and Ar K = 1s2 2s2 2p6 3s2 3p6 4s1 Ar= 1s2 2s2 2p6 3s2 3p6 Write configuration for K using

shorthand K= [Ar] 4s1 17 Shorthand practice Write the shorthand electron configuration of: P [Ne] 3s2 3p3 Br [Ar]4s23d104p5 Ca

[Ar] 4s2 V [Ar]4s23d3 18 Orbital Diagrams An orbital is the region of space where there is a high probability of finding an atom. The higher the energy of an orbital, the larger its size. Each atomic orbital has a box (2 electrons per box)

Hunds Rule: Give eachDiagram orbital (of Nitrogen Orbital equal energy)1sone before 2s electron 2px 2py 2p z any get two electrons 19 Hunds Rule

Within a sublevel, place one e- per orbital before pairing them. Empty Bus Seat Rule WRONG 20 RIGHT Electron Diagram Problem Write the orbital diagram and determine the number of unpaired electrons for iron. Fe= 1s2 2s2 2p6

3p6 4s2 3s2 3d6 4 unpaired electrons 21 Electron Dot Structures Because valence electrons are so important in the formation of bonds chemists represent them visually using another

shorthand method An electron dot structure consists of an atoms symbol surrounded by dots that represent the atoms valence electrons. Example : Carbon [He]2s22p2 has 4 valence electrons 22 C Rules for adding the dots Place valence electrons one at a time on all four sides of the symbol, then (if needed) pair each electron up until all have been used.

Ar Exception: Helium has a full valence shell with 2 electrons He 23 Practice Draw the electron dot structures for: Sr Sr F F Na Na S

S 24 Practice Draw the electron dot structures for: Si Si Al Al P P Xe Xe 25