(ATPT) Structure of an Atom Lesson

Structure of an Atom

In the previous module, you learned about mixtures and, specifically, the properties of solutions. Moving forward in this course we are going to be focusing on pure substances. Any material that is not a mixture, is called a pure substance. Pure substances include elements and compounds.

An element is a chemical substance that can't be divided or changed into other chemical substances by any ordinary chemical means. The smallest unit of an element is the atom. There are 116 officially named elements and about 118 known elements. Most of these are natural, but some are man-made. The elements we know are represented in the periodic table, where each element is abbreviated to a chemical symbol

A compound is a chemical substance that forms when two or more different elements combine in a fixed ratio. Water (H2O), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride (NaCl) is a compound made up of one sodium atom for every chlorine atom. An important characteristic of a compound is that it has a chemical formula, which describes the ratio in which the atoms of each element in the compound occur. In the next module, we will be looking at chemical formulas in detail.

Atomic Structure imageThe concept of the atom was first proposed roughly two thousand years ago by the Greek philosopher Democritus. He argued that matter was finite and comprised of particles that are indivisible. Like all the philosophers of his time, Democritus based his argument on reason, not experimental data. In more recent years, the composition of matter has been studied further, and only within the last century was it determined that the atom is indeed divisible. Today, scientists believe that even some subatomic particles can theoretically be divided even further. The figure above illustrates our modern model of the atom. At the center is a nucleus containing protons and neutrons. Around the nucleus are much lighter particles called electrons. The atom is largely comprised of empty space. For a more detailed history of how our understanding of the atom has evolved, watch the video below.   You do not need to memorize the history of the atom.  By watching the video you can see how a lot of what we know about the structure of atoms has been developed over a long period of time. This is often how scientific knowledge develops, with one person building on the ideas of someone else. You will see how our modern understanding of the atom has evolved over time.

In summary of what you have seen in the video above, philosopher, Democritus, first talked about the idea of atoms in the fifth century BC. The Greek word atomos means indivisible because they believed that atoms could not be broken into smaller pieces. Nowadays, we know that atoms are made up of a positively charged nucleus in the center surrounded by negatively charged electrons. However, in the past, before the structure of the atom was properly understood, scientists came up with lots of different models or pictures to describe what atoms look like. Let's look at how the model of the atom has changed over time.

Dalton's Model of the Atom

image of dalton model (empty circle)John Dalton experimented and made observations on what the ancient Greeks (notably Democritus) had proposed long ago, that all matter is composed of small, indivisible (cannot be divided) objects. From his experiments and observations, Dalton proposed a new theory of the atom. This later became known as Dalton's atomic theory. The general tenets of this theory were as follows:

  1. All matter is composed of extremely small particles called atoms.
  2. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
  3. Atoms cannot be subdivided, created, or destroyed.
  4. Atoms of different elements can combine in simple whole-number ratios to form chemical compounds.
  5. In chemical reactions, atoms are combined, separated, or rearranged.

The scientific community has largely accepted Dalton's atomic theory, although a couple of modifications have been made since its conception. We know now that (1) an atom can be further subdivided, and (2) not all atoms of an element have identical masses.

J.J. Thomson's Model of the Atom

Thomson Model: image of atom filled with electrons and "soup" of positive chargeJ.J. Thomson used the cathode ray tube experiment to determine that the atom was not just a small ball, but was made up of smaller subatomic particles, specifically the electron. After the electron was discovered by J.J. Thomson in 1897, people realized that atoms were made up of even smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet and so the "plum pudding model" was put forward in 1904. In this model, the atom is made up of negative electrons that float in a "soup" of positive charge, much like plums in a pudding or raisins in a fruitcake. In 1906, Thomson was awarded the Nobel Prize for his work in this field. However, even with the Plum Pudding Model, there was still no understanding of how these electrons in the atom were arranged.

Rutherford's Model of the Atom

Rutherford Model: atom with electron orbiting the nucleus and nucleusBy 1900, it was known that the electron carried a negative charge. It was also known that the electron makes up an extremely small fraction of the mass of an atom. Ernest Rutherford set out to determine how the remainder of the mass and charge was distributed in the atom. Rutherford was a physicist from New Zealand who was working under the direction of J. J. Thomson.  

One of Rutherford's famous experiments was called the gold foil experiment (illustrated in the picture below).  In this experiment, Rutherford used a radioactive source to direct alpha particles (positively charged particles) toward a very thin sheet of gold foil. Surrounding the foil was a screen that fluoresced when struck by the alpha particles. He was able to observe that most alpha particles easily passed through the gold foil and struck the fluorescent screen behind the foil. However, there were some instances in which the alpha particles were deflected very strongly, often back toward the emission source. If the plum pudding model were correct, all of the alpha particles would be expected to pass through the gold foil with little or no deflection. The strong deflection experienced by a small portion of the alpha particles could be better explained by an atom that contained a very small, dense nucleus. Because some of the alpha particles emitted from the source were repelled by the nucleus, Rutherford concluded that the nucleus must be made up of these positively charged alpha particles, which he named protons. He proposed that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons.

Gold Foil Experiment

Bohr's Model of the Atom

Bohr Model: electron orbit around nucleusThere were, however, some problems with Rutherford's model: for example, it could not explain the very interesting observation that atoms only emit light at certain wavelengths or frequencies. Niels Bohr solved this problem by proposing that the electrons could only orbit the nucleus in certain special orbits at different energy levels around the nucleus. His model of the atom is known as a planetary model of the atom.  

Chadwick's Model of the Atom

Chadwick Model: proton, neutron, electron all orbiting nucleusRutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! To make sure that the atom stays electrically neutral, this particle would have to be neutral itself.

In 1932 James Chadwick, an English physicist who was mentored by Rutherford, discovered the neutron and measured its mass. His experiment consisted of bombarding beryllium atoms with alpha particles through a paraffin wax target and studying the effects. From his analysis, he concluded that the nucleus also contains a particle that has equal mass to the proton, but unlike the proton, is electrically neutral, hence the name neutron.

Chadwick's work resulted in a new understanding of the nucleus of the atom; it is comprised of both protons and neutrons. Because the masses of subatomic particles are so small, a new unit, called an atomic mass unit (amu), was defined. Protons and neutrons each have a mass of approximately one amu.

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