The Atom Lesson 4 Quantum Numbers and Electron

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 2 n 2 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 the same. Energy as the Level number of the. Have main. Shapes level (up to four Can sublevels). 4 1 2 3 4 -7 s 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. The maximum electrons per level is obtained by adding the maximum number of electrons in each sublevel. 5

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 2 p sublevel contains 2 electrons, it is written 2 p 2. 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 6 2 p 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 1 A and 2 A have the s-orbital filled. • Groups 3 A - 8 A have the p-orbital filled. • Groups 3 B - 2 B 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 Start 1 s 2 s 3 s 4 s 5 s 6 s 7 s 2 p 3 p 4 p 5 p 6 p 3 d 4 d 4 f 5 d 5 f 6 d

Writing electron configurations Write configurations for O, Ni, Br, Sr O= 1 s 2 2 p 4 Ni= 1 s 2 2 p 6 3 s 2 3 p 6 4 s 2 3 d 8 Br= 1 s 2 2 p 6 3 s 2 3 p 6 4 s 2 3 d 10 4 p 5 Sr= 1 s 2 2 p 6 3 s 2 3 p 6 4 s 2 3 d 10 4 p 6 5 s 2 13

Electron Configurations Exceptions to the AUFBAU principle Chromium prefers a half full d as opposed to a full 4 s, thus 4 s 13 d 5 Copper prefers a full 3 d as opposed to a full 4 s, thus 4 s 13 d 10 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 d-sublevel is more stable than a half full 4 s

Valence Electrons = electrons in an 15 atom’s 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” electrons to chemists.

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 (group 18) 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 = 1 s 2 2 p 6 3 s 2 3 p 6 4 s 1 Ar= 1 s 2 2 p 6 3 s 2 3 p 6 Write configuration for K using shorthand K= [Ar] 4 s 1 17

Shorthand practice Write the shorthand electron configuration of: P [Ne] 3 s 2 3 p 3 Br [Ar]4 s 23 d 104 p 5 Ca [Ar] 4 s 2 V [Ar]4 s 23 d 3 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) Nitrogen Diagram Hund’s Rule: Give Orbital each orbital (of equal energy) one electron get two 1 s 2 s before 2 px 2 pany 2 p y z electrons 19

Hund’s 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= 1 s 2 2 s 2 3 p 6 2 p 6 3 s 2 4 s 2 3 d 6 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]2 s 22 p 2 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