Dalton's Model of an Atom
Based on all his observations, Dalton proposed his model of an atom. It is often referred to as the billiard ball model. He defined an atom to be a ball-like structure, as the concepts of atomic nucleus and electrons were unknown at the time.Schrödinger's cat is a famous hypothetical experiment designed to point out a flaw in the Copenhagen interpretation of superposition as it applies to quantum theory. A living cat is placed into a steel chamber along with a hammer, a vial of hydrocyanic acid and a very small amount of radioactive substance.
The word quantum derives from the Latin, meaning "how great" or "how much". The discovery that particles are discrete packets of energy with wave-like properties led to the branch of physics dealing with atomic and subatomic systems which is today called quantum mechanics.
Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta.
[/caption]The most widely accepted atom model is that of Niels Bohr. Bohr's model was first introduced in 1913.
Bohr thought that electrons orbited the nucleus in quantised orbits. Bohr built upon Rutherford's model of the atom. He believed that electrons moved around the nucleus in circular orbits with quantised potential and kinetic energies. So it was not possible for electrons to occupy just any energy level.
Bohr improved Rutherford's model by proposing that electrons travelled about the nucleus in orbits that had specific energy levels. They could jump from one level to another but could not be at any place in between, and they would absorb or emit specific amounts of energy (quanta) when they jumped between levels.
The main problem with Rutherford's model was that he couldn't explain why negatively charged electrons remain in orbit when they should instantly fall into the positively charged nucleus. This problem would be solved by Niels Bohr in 1913 (discussed in Chapter 10).
Niels Bohr improved Rutherford's model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus. The Dalton model has changed over time because of the discovery of subatomic particles .
Today, we know that the Bohr Model has some inaccuracies, but it's still used because of its simple approach to atomic theory. The Bohr model was also the first atomic model to incorporate quantum theory, meaning that it's the predecessor of today's more accurate quantum-mechanical models.
Unlike the plum pudding model, where those atoms simply floated in "soup," Rutherford believed they orbited the central nucleus just as planets orbit the sun. He proposed that this happened because the central nucleus contained positively charged protons that forced the negatively charged electrons to orbit around it.
The Heisenberg uncertainty principle states that the exact position and momentum of an electron cannot be simultaneously determined. This is because electrons simply don't have a definite position, and direction of motion, at the same time! We know the direction of motion.
What are important features of the quantum mechanical model of atom? The important features are: (i) The energy of electrons in atoms is quantized. (ii) The existence of quantized electronic energy levels is a direct result of the wave like properties of electrons.
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.
Rutherford's atomic model needed to be replaced because it could not explain the chemical properties of elements. What was the quantum mechanical model determine about electrons in atoms? The quantum mechanical model determines the allowed energies an electron can have.
The Bohr Model is very limited in terms of size. Poor spectral predictions are obtained when larger atoms are in question. It cannot predict the relative intensities of spectral lines. It does not explain the Zeeman Effect, when the spectral line is split into several components in the presence of a magnetic field.
Key points. Erwin Schrödinger proposed the quantum mechanical model of the atom, which treats electrons as matter waves. The Heisenberg uncertainty principle states that we can't know both the energy and position of an electron.
Niels Bohr proposed the Bohr Model of the Atom in 1915. The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the sun (except that the orbits are not planar).
The quantum mechanical model is important to understanding chemistry because it explains how electrons exist in atoms and how those electrons determine the chemical and physical properties of the elements.
