Saturday, October 15, 2016

Atomic Theory

Understanding what things were made of goes back far into history. The ancient Greeks had ideas and debated as long ago as 2500 years.
Democritus believed that all matter consisted of extremely small particles that could not be divided. He called these particles atoms from the greek word transliterated as atomos, which means "uncut" or "indivisible." He thought there were different types of atoms with specific sets of properties. The atoms in liquids, for example, were round and smooth, but the atoms in solids were rough and prickly.

Aristotle did not think there was a limit to the number of times matter could be divided. He proposed that all matter was made of four elements: earth, fire, air, and water. For centuries, most people accepted Aristotle's views on the structure of matter.

But by the 1800s, scientists had enough data from experiments to support a more substantial atomic model of matter. John Dalton, born in 1766, developed one early model.

Dalton's Theory

Dalton noticed that all compounds have something in common. No matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same. Dalton's theory was developed based on this observation.

Dalton proposed the theory that all matter is made up of individual particles called atoms which cannot be divided.
  • All elements are composed of atoms.
  • All atoms of the same element have the same mass, and atoms of different elements have different masses.
  • Compounds contain atoms of more than one element.
  • In a particular compound, atoms of different elements always combine in the same ratios.
According to Dalton's theory, atom were pictured as solid spheres, each one a tiny, solid sphere with a different mass. His theory satisfied what had, up to that point, been observed and was widely accepted. While incomplete, much of Dalton's theory is still useful in modeling how elements combine to form different compounds. 

However, in time, scientists found that not all of Dalton's ideas about atoms were completely correct. His views were not discarded, but instead, they were revised to take into account new discoveries.

Thompson's Model

A scientist, J. J. Thompson (1856-1940) studied atoms by putting a gas between metal plates and applying an electrical charge. No matter what material was used for the charged metal plates, a beam would appear in the gas, and the beam always behaved in the same way.

Thompson was able to conclude that the beam was made of negatively charged particles that had a mass of about 1/2000 that of a hydrogen atom, the lightest of all atoms. Thompson's experiments provided the first evidence that atoms are made of even smaller particles. 

Thompson's model of the atom resulted in what was called the "plum pudding" model of the atom: since an atom is neutrally charged, yet contains some negatively charged particles, it must also contain positively charged particles, and these particles are mixed together and spread throughout the atom.

As with Dalton's theory, Thompson's model fit what had been observed. Scientists briefly used Thompson's model to guide their investigations, but in 1909, a new discovery led to a even more useful model of the atom.

Rutherford's Atomic Theory

After discovering radioactive alpha particles, Rutherford wondered if they would pass through thin sheets of metal, like gold. Based on Thompson's "plum pudding" model, Rutherford believed that the mass and charge of the particles that make up an atom would, at any given point, be unable to stop the alpha particle.

What he discovered was that, although most of the alpha particles passed through without deviation of course, some of them were turned sharply, and some even bounced off the gold foil and reflected back the way they had come. For this to happen, the particles that make up an atom could not be evenly distributed. Thompson's model would have to be adjusted.

According to Rutherford's model, all of an atom's positive charge is concentrated in its nucleus, and it was the collision with the nucleus that caused the alpha particles to deflect and rebound. According to Rutherford, an atom has a dense, positively charged nucleus and electrons move randomly in the space around the nucleus.

Rutherford's model extended what was previously understood by identifying that atoms have a central, relatively dense (compared to the entire volume of an atom) nucleus around which electrons move, but other observations led to continued development. By 1913, Niels Bohr had provided additional insight into how atoms were constructed.

Bohr's Model

Bohr, who had worked with Rutherford, extended Rutherford's insight by examining more closely the electrons. In Bohr's model, electrons move with constant speed in fixed orbits (rather than randomly) around the nucleus—like planets around a sun. Each electron in an atom has a specific amount of energy. Electrons must orbit the nucleus in one of several fixed, specific orbits, and each orbit represents a specific energy level. The first orbital represents the lowest electron energy level, and the other orbitals represent progressively higher and higher energy levels.

It can be compared to stairs. Electrons can exist on any of the different stairs, and they can move between stairs, but they cannot exist between different stairs. Higher stairs represent higher energy levels.

Electrons can be, by increasing their energy, jump to higher levels, or if they give off energy, move to lower levels. An electron in an atom can move from one energy level to another when the atom gains or loses energy.

Bohr's model provided a great deal of insight in to how elements combine. Picturing the atom as a nucleus around which electrons orbit at different specific energy levels opened a vast amount of understanding. However, further discoveries called for additional refinement.

Electron Cloud Model

Evidence following Bohr's work led to the understanding that the electrons do not orbit the nucleus like a planet. While they do exist at specific energy levels and occupy orbitals, their position in the orbital is never 100% certain. They are somewhere in the orbital, but exactly where cannot be known specifically. Each orbital can be, therefore, conceived as an electron cloud. The concept can be considered analogous to the blur of an object spun at the end of a string. At any given moment, it is somewhere in the path, but the eye cannot pinpoint it.

Along the way, the model of the atom changed with each new discovery. The discovery of neutrons as part of the nucleus led to the understanding that the atom is made of positively charged protons, negatively charged electrons, and neutrally charged neutrons and that each element is composed of a specific combination of those sub-atomic particles. 

Ultimately, a model emerged that is very useful in explaining how chemical reactions take place (as well as in explaining many other aspects of chemistry and physics). A major part of this is understanding energy levels and orbitals for the electrons.

Atomic Orbitals

For any element, all electrons must exist in a set, specific orbital—though exactly where in that orbital is a matter of probability, not certainty. Various orbitals have different energy levels and can hold only a certain number of electrons:

Energy Level Number of Orbitals Maximum Electrons
1 1 2
2 4 8
3 9 18
4 16 32

This information directly leads to understanding of chemical reactions. 


Essentials of Atomic Theory


Definitions and content from:

Physical Science Concepts in Action, Pearson

Image from:
Physical Science Concepts in Action, Pearson, p 100