Back to Back Issues Page
Blowin' in the Wind, Issue #045 Electron Cloud Model Atom - August 1, 2007
August 02, 2007
Hello ,

Classic Electron Cloud Model Atom

Let's take a break from the weather and look at something else. The electron cloud model atom refers to the quantum-mechanical model for the structure of an atom. Some of the abstract concepts of quantum theory are quite difficult to fathom (kinda like Paris Hilton's behavior) and visualize by the human mind. So here's a bit of basics for the novice: An atom is the smallest particle of an element (one of the 100+ fundamental substances from which all matter is made) that retains the chemical properties of the element -- dismembering it further will turn it into something else and lose the characteristics of the element. Atoms form the basic building blocks of all matter and measure about a 100-millionth of a centimeter in diameter.

What is inside an atom?

Creating a dramatic change from the 19th century belief that an atom is an indivisible solid entity, experiments revealed we can divide atoms and they have an internal structure, with the bulk of one being empty space. It consists of a small, dense, positively charged core called the nucleus that occupies a minuscule volume. But it has almost all the mass of the atom. Thousands of times smaller, negatively charged particles, known as electrons, "spin" about in orbits around the nucleus -- maybe like how planets revolve around the sun, but with electrostatic force between the oppositely charged nucleus and electrons in play rather than the gravitational force of our solar system. The nucleus contains positively charged particles (protons) and neutral particles (neutrons). And what differentiates an atom of one element from that of another is the difference in the number of protons (which usually equals the number of electrons to give a neutral electric charge).

What is an electron cloud?

Although the above simplistic planetary model of the atom can successfully explain the behavior of a simple atom like hydrogen, which has just one proton, one electron and an atomic mass of 1, it fails to do the same for larger atoms with more protons, electrons and neutrons. This happens because the classical laws of mechanics that describe the motion of large objects fail for subatomic particles. Electrons do not necessarily move smoothly from one position to another. They can magically jump from one position to the next instead. That's where quantum mechanics steps in and the electron cloud model atom emerges. And also where the expression "quantum leap" comes into popular language.

According to quantum theory, the infinitesimally small size of electrons makes it inherently impossible for them to have a definite well-defined form and a spatial position characteristic of a normal object. In other words, it is impossible to pinpoint, and hence measure at any given point in time where exactly an electron is located inside the atom. One can only talk of the probability of an electron being at a given location. This gives to the electron's position a diffuse cloud-like appearance rather than a well-defined form characteristic of a familiar particle. The mathematical probability distribution map of the electron yields what we call the electron cloud, the thicker regions of the cloud indicating the regions where the electron is most likely to exist and the lighter regions the probability of its presence tapers off.


A question that may bug a layman is whether the electron cloud is a real, observable entity. Actually this mathematical concept, devised by physicists to correlate their observations of the behavior of an atom, is just an extremely complex, multidimensional mathematical abstraction. A model. Any visualization of the electron cloud by the human mind can only represent at best an extremely simplistic and vague picture of the reality. And that is not likely to ever change.

Get a jump start on your seasonal shopping for that tough to excite science guy in your group. This could be fun!
Back to Back Issues Page