If you still think of atom as electrons revolving around the nucleus just as planets are revolving around the sun, then sorry, but you are wrong. The atom looks entirely different from the old models that we study in high school. So, here we are to explore the modern model of atom.
THE HISTORY OF ATOMIC MODELS:
The concept of atom dates back to the early Greeks (400 B.C), but we are going to discuss the relatively modern ideas.
Below is the picture of one of the earliest modern models of atom, The plum pudding model. It was proposed by J.J Thomson in 1904. This model said that the atoms of the elements consist of a number of negatively electrified corpuscles (electrons) enclosed in a sphere of uniform positive electrification.
This model was soon proved wrong by Earnest Rutherford. He bombarded very thin sheets of gold foil with fast moving alpha particles. Alpha particles, a type of natural radioactive particle, are positively charged particles with a mass about four times that of a hydrogen atom. According to plum pudding model, almost all alpha particles should have passed undeflected. But the experiment showed that some particles were deflected at a relatively larger angle. So, he proposed a new model, Rutherford atomic model /nuclear model. According to this model, almost all positive charge and mass of the atom is concentrated in a central region called nucleus. Electrons revolve around the nucleus and make most of the volume. (Neutrons were not yet discovered).
This model had flaws. One of them was that revolving electrons should give continuous spectrum, but line spectrum was obtained. Another was that, the electrons would continuously loose energy, so their orbit should become smaller and smaller, until they fall into the nucleus. To answer these, Neil Bohr proposed the upgraded version, the Bohr Atomic Model.
According to bohr, electrons revolve in fixed and definite circular orbits, or energy level. As long as electron remains in an energy level, no energy is emitted. However, when an electron makes a transition from one energy level to another, photon of fixed energy is released or absorbed.
THE ELECTRON CLOUD MODEL
Bohr model was accepted for quite a long time. But it all changed with the introduction of quantum mechanics. During the 20th century many scientists continued to refine the atomic model. Through the work of Schrodinger and Heisenberg our perception of electrons changed forever. These two men discovered that the idea of electrons moving in clean, circular orbits was false. In fact, Heisenberg discovered it is impossible to know the location and velocity of an electron at any given time. In other words, electrons are completely unpredictable. Schrodinger used this knowledge to show that electrons actually exist in electron clouds. These clouds have various energy levels like Bohr.
This led to Electron Cloud or Quantum mechanical model of atom.
THE MODEL AND ITS POSTULATES:
- According to the modern atomic model, an atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.
- According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun therefore, no circular orbit exist around the nucleus as claimed in Bohr’s theory.
- According to Heisenberg’s uncertainty principle it is impossible to chase an electron around the nucleus. However, electrons are not randomly whirling in an atom but are confined/locked to certain regions. Erwin Schrodinger Wave functions predict this region in space around a nucleus where these electrons are probably found at most called orbital.
- Atomic orbital can be visualized as “fuzzy clouds”.
The higher the density of the “cloud,” the higher the probability of finding an electron at that point.
- These clouds have no definite boundary but usually the region where an electron will spend 90 percent of its time is depicted by drawing an encircle.