Chapter 13 Notes Electron Models

Evolution of Electron Models The first model of the electron was given by J.J. Thompson—the electron’s discoverer. His was the “plum pudding” model.

The Rutherford Model With Rutherford’s discovery of the nucleus of an atom, the atomic model changed.

The Bohr Model Niels Bohr introduced his model, which answered why electrons do not fall into the nucleus. He introduced the concept of energy levels, where the electrons orbited similar to the way the planets orbit the sun.

Bohr Model and Energy Levels In the Bohr model, electrons are in energy levels, or regions where they most probably are orbiting around the nucleus. The analogy is that energy levels are like the rungs of a ladder—you cannot be between rungs, just like an electron cannot be between energy levels. A quantum of energy is the amount of energy it takes to move from one energy level to the next.

Quantum Mechanical Model In 1926, Erwin Schrodinger used the new quantum theory to write and solve mathematical equations to describe electron location.

The Quantum Mechanical Model, cont. Today’s model comes from the solutions to Schrodinger’s equations. Previous models were based on physical models of the motion of large objects. This model does not predict the path of electrons, but estimates the probability of finding an electron in a certain position. There is no physical analogy for this model!

Where are the electrons? In an atom, principal energy levels (n) can hold electrons. These principal energy levels are assigned values in order of increasing energy (n=1,2,3,4...). Within each principal energy level, electrons occupy energy sublevels. There are as many sublevels as the number of the energy level (i.e., level 1 has 1 sublevel, level 2 has 2 sublevels, etc.)

Where are the electrons? There are four types of sublevels we will talk about—s,p,d and f. Inside the sublevel are atomic orbitals that hold the electrons. Every atomic orbital can hold two electrons. S has one orbital, P has three, D has five and F has seven. How many electrons can each one hold?

Orbital Shapes s orbital = s sublevel + + p x orbitalp y orbital p z orbital = p sublevel

Where are the electrons? So how many electrons can each energy level hold? –Level 1 has an s sublevel=2 e - –Level 2 has an s and a p sublevel=8e - –Level 3 has an s, p and d sublevel=18e - –Level 4 has an s, p, d and f sublevel=32e -

Electron Configuration

In the atom, electrons and the nucleus interact to make the most stable arrangement possible. The ways that electrons are arranged around the nucleus of an atom is called the electron configuration.

Aufbau Principal Electrons enter orbitals of the lowest energy first. Pauli’s Exclusion Principal An atomic orbital may describe at most two electrons. Hund’s Rule with parallelWhen electrons occupy orbitals of equal energy, one electron enters each orbital until all orbitals contain one electron with parallel spins.

EMR and Quantum Theory

How did scientist find out about energy levels and sublevels? When we previously found the electron configuration for elements, it was for electrons at ground state (the lowest energy possible).

How did scientist find out about energy levels and sublevels? As energy is added to atoms, they absorb the energy by electrons going from ground state to an excited state, where electrons are no longer in the lowest energy orbitals.

How did scientist find out about energy levels and sublevels? Electrons can then only go back to ground state by releasing the energy, usually in the form of light in discreet packets called photons.

How did scientist find out about energy levels and sublevels? If electrons could orbit anywhere, when they went from excited state to ground state they would emit light of all wavelengths. This doesn’t happen! Only certain wavelengths of light are emitted, which is different for each atom.

How did scientist find out about energy levels and sublevels? This is called an atomic emission spectra, and is different for every element!

How did scientist find out about energy levels and sublevels? A scientist named Max Planck studied the cooling of metal and how its color changes, and tied the idea of frequency of light to energy. The bands of light correspond to the specific energy levels.