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The Quantum Mechanical Model: Definition & Overview

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  • 0:00 Definition of the Atom
  • 1:16 The Quantum Mechanical Model
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Discover the quantum mechanical model of the atom and how it is different than the orbital model. Learn about the quantum mechanical properties of electrons, then take a quiz to test what you know.

Definition of the Atom

Look around you. What do you see? Maybe you see a computer in front of you, a ceiling above you, the clothes you're wearing, or a tree outside a window. What do all of these things have in common? Not a lot, you might think. But, all matter in the universe, everything around you is made of the same basic stuff. Everything in the universe is made of the 118 elements discovered in chemistry—118 different types of atoms.

Atoms are the building blocks of the universe made up of protons and neutrons inside a central nucleus, surrounded by orbiting electrons. The model of the atom below is the one most people learn in chemistry classes at school, called the planetary model, which is a model of the atom that appears like a planetary system with the nucleus acting as the sun and the electrons acting as the planets.

It's a greatly simplified version of the model of what is now believed to be true, but the older model has been replaced more than once. It was replaced with a more detailed version first, called the Bohr model (named after physicist Neils Bohr), where electrons can only orbit with specific amounts of energy. And then later, it was replaced with the quantum mechanical model.

The Quantum Mechanical Model

The discovery that began quantum mechanics as a field of study was when physicists Albert Einstein and Max Planck proved that light and matter can behave both as particles and waves. This began the era of quantum mechanics, which is basically the physics of the very tiny. If particles like electrons can behave as waves, it means that they don't have an exact position the way we would imagine for a traditional particle. Quantum mechanics tells us that you can't precisely know both the position and velocity of an electron at the same time.

This means that we shouldn't imagine electrons as single objects going around the atom. Instead, all we know is the probability of finding an electron at a particular location. What we end up with is something called an electron cloud. An electron cloud is an area of space in which an electron is likely to be found. It's like a 3-D graph showing the probability of finding the electron at each location in space.

Quantum mechanics also tells us that a particle has certain numbers (called quantum numbers) that represent its properties. Just like how materials can be hard or soft, shiny or dull, particles have numbers to describe the properties. These include a particle's orbital quantum numbers, magnetic quantum number, and its spin. No two electrons in an atom can have exactly the same quantum numbers.

Orbital quantum numbers tell you what energy level the electron is in. In the Bohr model, this represents how high the orbit is above the nucleus; higher orbits have more energy. The first orbit is n=1, the second is n=2, and so on. The magnetic quantum number is just a number that represents which direction the electron is pointing. The other important quantum mechanical property, called spin, is related to the fact that electrons come in pairs. In each pair, one electron spins one way (with a spin of one half), and the other electron spins the other way (with a spin of negative one half). Two electrons with the same spin cannot exist as a pair. This might seem kind of random, but it has effects in terms of how magnetic material is. Materials that have unpaired electrons are more likely to be magnetic.

Although the quantum mechanical model of the atom is more abstract and complex, it's a much more accurate picture of how atoms work. Because of this, it allows us to make much better predictions about how particles will behave when we try to interact with them. We use quantum mechanics to create flatscreen TVs, camera sensors, and computers, so knowing about quantum mechanics is very useful.

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