Quantum Mechanical Model of the Atom Orbitals and Electron Configuration Mrs. Hayes Chemistry

Quantum Mechanical Model

Orbitals and the Quantum Mechanical Model of the Atom Since we know that Bohr’s model of hydrogen will not work for all atoms, we need another way to predict where electrons are located. We use “orbitals” not “orbits” to make this prediction.

s, p, d orbital shapes

Quantum Mechanical Model

Energy Levels The energy levels in an atom are sort of like rungs of a ladder. The more energy an electron has, the farther away from the nucleus it usually will be. The energy levels are not evenly spaced. They get closer together as you travel farther away. To move from one “rung” to another requires a “quantum” of energy.

e- Movement is Based on 3 Factors Their energies. Their orientation in space. How wildly they swing about in their path (some swing back and forth like waving your arms instead of making a full circle).

Quantum Numbers Describe the locations of the e-’s around the nucleus. Quantum #’s are sort of like a home address for the electron.

Quantum Numbers Principal Q. #: Describes the distance that the electron is from the nucleus. The bigger the number, the farther away the electron is. Example: (1=closest, 2, 3, 4...farther away) Magnetic Q. #: tells how many orientations in 3-D there are about the nucleus for each orbital shape. s = 1 orientation p= 3 orientations... (x, y, and z) d= 5 orientations f= 7 orientations

Quantum Numbers Angular Momentum Q. #: Describes the shape of the electron’s path around the nucleus with a letter: (s, p, d, & f) These are sometimes called “sublevels”. Spin Q. #: describes how the electron in an orientation is spinning around the nucleus. Some like to imagine it spinning “clockwise” and “counterclockwise”. The spin is represented as an arrow in the direction of the spin.

Electron Configurations “Configuration” means the arrangement of the parts of something. Electron configuration is the arrangement of electrons in space around the nucleus.

Rules for e- configurations Rule #1 (Aufbau Principle): Electrons fill lowest energy orbitals first. Rule #2: Only 2 electrons can fit into each orientation. Rule #3 (Pauli Exclusion Principle): Electrons in the same orientation have opposite spin. Rule #4 (Hund’s Rule): “Monopoly rule”---> Every “□” in an orbital shape gets an electron before any orientation gets a second e-.

Electron Configurations The orientations are represented with a line or a box. Examples: ___ This means a spherical orbital at a distance of 1s, “1” (close) to the nucleus. This orbital is centered about the x, y, and z axis. □ □ □ This represents an ellipsoid orbital with its 4p with 3 possible orientations at a distance of “4 ”from the nucleus

Electron Configuration Pattern

What do the Numbers and Letters Represent? The large numbers represent the main energy levels and are determined by the electrons’ average distance from the nucleus. The letters are the sublevels in which the electrons are found (s, p, d, f) Each main energy level has a certain number of sublevels.

How to Use the Configuration We move across the periodic table from left to right just like we read a book. Let’s take the element Lithium (Li, 3) Lithium has 3 protons, so a neutral atom of Li will have 3 electrons We need space for 3 electrons: 1s2, 2s1 There are 2 electrons in the first sublevel, and one electron in the second sublevel. 2 + 1 = 3…that’s it!

Orbital Diagrams We can DRAW the Configuration, too. 2s orbital can hold 2 electrons, but we have only one to place there. 1s2 2s1 You must fill the first sublevel before going to the next one. Electrons have opposite “spins” in an orbital, so we have the arrows going in the opposite direction. Lithium

Let’s Try Fluorine… Fluorine has an atomic number of 9, so it has 9 electrons in a neutral atom. 1s2,2s2, 2p5 The 2p orbital can hold 6 electrons, but we only have five to place there. 1s2 2s2 2p5

What About Arsenic? Atomic number = 33, so 33 electrons Fill up the s orbitals first Fill up the sublevels until we get to 4p…then we have only three left there.

Hund’s Rule. Each orbital of equal energy must be occupied before a second electron can enter. All single electrons have the same spin. 4p3 3d10 4s2 3p6 3s2 2p6 Pauli says no two electrons in orbital can be alike, so opposite spins. 2s2 1s2 Aufbau principle

We Also Have Shorthand- The “Noble Gas Configuration” Use the nearest noble gas BEFORE the element to replace the sublevel orbitals in your configuration. Example for Magnesium: Neon, 3s2 Neon replaces 1s2, 2s2, and 2p6