Electrons in Atoms Up until now, the model of the atom presented considered atoms as combinations of protons & neutrons that make-up the nucleus, which is surrounded by electrons. This model does not explain enough about their properties

The Development of Atomic Models

Dalton’s Atomic Theory: Atom is indivisible

Thomson: Discovered existence of electrons Thomson: Discovered existence of electrons. Plum Pudding Model: Solid sphere w/+ charge with negatively charged electrons embedded.

Rutherford: Discovered nucleus with gold foil experiment Rutherford: Discovered nucleus with gold foil experiment. Atom was made of dense nucleus with + charge. Nucleus was surrounded by empty space and electrons. PROBLEM: The trouble with Rutherford’s model is that opposites attract. Why didn’t the electrons collapse into the nucleus?

Bohr: Proposed that electrons were arranged in concentric paths (orbits) around the nucleus. Electrons had a fixed energy & do not lose energy and fall into nucleus. The energy level of the electron is the region in which the electron is found.

12.2 The Quantum Mechanical Model Schrodinger (1926): wrote a mathematical equation to describe an atom using all the information available to date. This modern theory called the Quantum Mechanical Theory describes the positions of the electrons mathematically. The theory is based on the probability of finding an electron in a certain area. If the area around the nucleus is described as a cloud, then the areas with highest probability have a denser cloud and the lowest less fuzzy.

Energy levels are like rungs on a ladder. Lowest rung has lowest energy/Highest rung has highest energy. As you climb each rung of the ladder the energy goes up. A person can’t stand between rungs/neither can an electron. To move from level to level, an electron must gain or release just the right amount of energy.

Quantum: The energy required to move an electron from its present energy level to a higher one. The amount of energy gained or lost is not always the same. The energy levels are not evenly spaced like the rungs in a ladder. The levels are closer together at the top.

n - 1 n - 2

12.3 Atomic Orbitals The Quantum Mechanical Model restricts the electrons to certain energy levels as does Bohr, but designates electrons by principle quantum numbers. (n) Each principle quantum number refers to a principle energy level in the atom. You may want to find electrons within the sublevels..but remember: The Quantum Theory describes “probability clouds”; these are called atomic orbitals. Atomic Orbitals are designated by letters. (Draw models) (w/periodic table) Principle Energy Level Number of Sublevels Type of Sublevel Orbitals Maximum # of Electrons in level N = 1 1 1s (1 orbital) 2 N = 2 2s (1 orbital) 2p (3 orbitals) 8 N = 3 3 3s), 3p, 3d (5 orbitals) 18 N = 4 4 4s,4p,4d, 4f (7 orbitals) 32

12.4 Electron Configurations In all natural phenomena, change proceeds toward the lowest possible energy state. High energy systems are unstable. In atoms, the electrons and nucleus interact to make the most stable arrangement possible. The way in which they (electrons) are arranged around the nuclei of atoms is called electron configuration. We use three rules to find electron configuration. The Aufbau Principle The Pauli Exclusion Hund’s Rule

Aufbau Principle 6s 6p 6d 6f 5s 5p 5d 5f 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s Electrons enter the lower energy levels first. All 3 p orbitals and all 5 p orbitals have same energy levels p is not equal to d 7s 7p 7d 7f 6s 6p 6d 6f 5s 5p 5d 5f 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s

Pauli Exclusion Principle An atomic orbital may have only 2 electrons and they must have opposite spins. Clockwise and Counterclockwise. ↑↓ Hund’s Rule When electrons occupy orbitals of equal energy, like 3p-5d-7f; One electron enters each orbital until all orbitals contain 1 electron of parallel spins, Then additional electrons are added.