Atomic Theory Part 2

Where we left off… What we know (as of Rutherford/Chadwick): Electrons are negative and have almost no mass and are distributed around the nucleus Protons are positive & neutrons have no charge. Both are much heavier and are found in the nucleus

Where we left off… After discovering the nucleus, Rutherford used existing ideas about the atom and proposed an atomic model in which the electrons move around the nucleus like the planets move around the sun. Rutherford’s atomic model explained only a few simple properties of atoms. It could not explain the chemical properties of elements. Neils Bohr, a student of Rutherford, developed a model that could better explain the behavior of the atom.

Neils Bohr In 1913 Bohr changed Rutherford’s model to incorporate new knowledge about the atom and it’s ability to absorb or emit light. Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Electrons orbit at certain fixed levels of energy, called energy levels When electrons gain or release energy they can move up and down the energy levels The amount of energy needed to move up or down an energy level is known as a quantum.

Neils Bohr You can think of energy levels like a ladder… A person can climb up and down the ladder using the rungs, just like an electron can move up and down energy levels. A person can’t stand between rungs, and an electron can’t be between energy levels.

Neils Bohr Unlike a regular ladder, energy levels are NOT equally spaced! The lower energy levels at the bottom and the higher energy levels are at the top It takes a lot more energy to move between the low energy levels than it does to move between the higher energy levels

Neils Bohr’s Model Neils Bohr’s model made a lot of improvement on the atom but we’re still not quite there... Later on, calculations showed that the electron didn’t move in a nice orbit like Bohr thought.

Erwin Schrodinger In 1926 Schrodinger took those calculations and came up and solved a mathematical equation to describe the movement of electrons This mathematical equation led to the creation of the quantum mechanical model The quantum mechanical model still restricts the energy of electrons to certain levels. The quantum mechanical model does not specify an exact path the electron takes around the nucleus.

Quantum mechanical model The quantum mechanical model determines: The possible energy levels an electron can have How likely it is to find the electron in various locations around the nucleus of an atom. It is similar to a ceiling fan – you have a general idea of where the blade is but you don’t know where it will be at any exact moment

Quantum mechanical model Protons and neutrons are still in the dense nucleus Electrons are still confined to energy levels Electrons do not have an exact position or an orbit within the atom Electrons are in the electron cloud, which is the location they are likely to be found in 90% of the time. The darker the area the more likely the electrons will be there.