Introduction:

                    We know that atom is the smallest particle also the fundamental particle which cannot be broken further. Though it is the smallest particle which cannot be seen with our naked eye, it is the most useful and most dangerous particle. The collision of two atoms travelling at a speed of light in the opposite direction results in a huge destruction.

We can even look at the sample of the destruction. The HIROSHIMA and the NAGASAKI are still the victims of the atomic bomb. Very soon we are going to witness the artificial BIG-BANG theory using the protons to know the mystery of the universe.

In this way, the atoms are useful as well as dangerous. So let us see how this small particle called the ATOM works….

It has been said that during the 20th century, man harnessed the power of the atom. We made atomic bombs and generated electricity by nuclear power. We even split the atom into smaller pieces called sub atomic particles.

 

But what exactly is an atom? What is it made of? What does it look like? The pursuit of the structure of the atom has married many areas of chemistry and physics in perhaps one of the greatest contributions of modern science.
In this article, we will follow this fascinating story of how discoveries in various fields of science resulted in our modern view of the atom. We will look at the consequences of knowing the atom’s structure and how this structure will lead to new technologies.

WHAT IS AN ATOM?

The modern view of an atom has come from many fields of chemistry and physics. The idea of an atom came from ancient Greek science/philosophy and from the results of 18th and 19th century chemistry:

  • concept of the atom
  • measurements of atomic mass
  • repeating or periodic relationship between the elements

 

Concept of the Atom:

From the ancient Greeks through today, we have pondered what ordinary matter is made of. To understand the problem, here is a simple demonstration from a book entitled “The Extraordinary Chemistry of Ordinary Things, 3rd Edition” by Carl H. Snyder:

1. Take a pile of paper clips (all of the same size and colour).

2. Divide the pile into two equal piles.

3. Divide each of the smaller piles into two equal piles.

4. Repeat step 3 until you are down to a pile containing only one paper clip. That one paper clip still does the job of a paper clip (i.e., hold loose papers together).

5. Now, take a pair of scissors and cut that one paper clip in half. Can half of the paper clip do the same job as the single paper clip?

If you do the same thing with any element, you will reach an indivisible part that has the same properties of the element, like the single paper clip. This indivisible part is called an atom.

The idea of the atom was first devised by Democritus in 530 B.C. In 1808, an English school teacher and scientist named John Dalton proposed the modern atomic theory. Modern atomic theory simply states the following:

  • Every element is made of atoms – piles of paper clips.
  • All atoms of any element are the same – all the paper clips in the pile are the same size and colour.
  • Atoms of different elements are different (size, properties) – like different sizes and colours of paper clips.
  • Atoms of different elements can combine to form compounds – you can link different sizes and colours of paper clips together to make new structures.
  • In chemical reactions, atoms are not made, destroyed, or changed – no new paper clips appear, no paper clips get lost and no paper clips change from one size/colour to another.
  • In any compound, the numbers and kinds of atoms remain the same – the total number and types of paper clips that you start with are the same as when you finish.

Dalton’s atomic theory formed the groundwork of chemistry at that time. Dalton envisioned atoms as tiny spheres with hooks on them. With these hooks, one atom could combine with another in definite proportions. But some elements could combine to make different compounds (e.g., hydrogen + oxygen could make water or hydrogen peroxide). So, he could not say anything about the numbers of each atom in the molecules of specific substances. Did water have one oxygen with one hydrogen or one oxygen with two hydrogens? This point was resolved when chemists figured out how to weigh atoms.

Important terms:

  • atom – smallest piece of an element that keeps its chemical properties
  • compound – substance that can be broken into elements by chemical reactions
  • electron – particle orbiting the nucleus of an atom with a negative charge (mass = 9.10 x 10-28 grams)
  • element – substance that cannot be broken down by chemical reactions
  • ion – electrically charged atom (i.e., excess positive or negative charge)
  • molecule – smallest piece of a compound that keeps its chemical properties (made of two or more atoms)
  • neutron – particle in the nucleus of an atom with no charge (mass = 1.675 x 10-24 grams)
  • nucleus – dense, central core of an atom (made of protons and neutrons)
  • proton – particle in the nucleus of an atom with a positive charge (mass = 1.673 x 10-24 grams)

 

HOW MUCH DO ATOMS WEIGH?

The ability to weigh atoms came about by an observation from an Italian chemist named Amadeo Avogadro. Avogadro was working with gases (nitrogen, hydrogen, oxygen, chlorine) and noticed that when temperature and pressure was the same, these gases combined in definite volume ratios. For example:

  • One litre of nitrogen combined with three litres of hydrogen to form ammonia (NH3)
  • One litre of hydrogen combined with one litre of chlorine to make hydrogen chloride (HCl)

 

Avogadro said that at the same temperature and pressure, equal volumes of the gases had the same number of molecules. So, by weighing the volumes of gases, he could determine the ratios of atomic masses. For example, a litre of oxygen weighed 16 times more than a litre of hydrogen, so an atom of oxygen must be 16 times the mass of an atom of hydrogen. Work of this type resulted in a relative mass scale for elements in which all of the elements related to carbon (chosen as the standard -12). Once the relative mass scale was made, later experiments were able to relate the mass in grams of a substance to the number of atoms and an atomic mass unit (amu) was found; 1 amu or Dalton is equal to 1.66 x 10-24 grams.

At this time, chemists knew the atomic masses of elements and their chemical properties, and an astonishing phenomenon jumped out at them!

The Properties of Elements Showed a Repeating Pattern:

 At the time that atomic masses had been discovered, a Russian chemist named Dimitri Mendeleev was writing a textbook. For his book, he began to organize elements in terms of their properties by placing the elements and their newly discovered atomic masses in cards. He arranged the elements by increasing atomic mass and noticed that elements with similar properties appeared at regular intervals or periods. Mendeleev’s table had two problems:

  • There were some gaps in his “periodic table.”
  • When grouped by properties, most elements had increasing atomic masses, but some were out of order.

To explain the gaps, Mendeleev said that the gaps were due to undiscovered elements. In fact, his table successfully predicted the existence of gallium and germanium, which were discovered later. However, Mendeleev was never able to explain why some of the elements were out of order or why the elements should show this periodic behaviour. This would have to wait until we knew about the structure of the atom.

 

The Structure of the Atom: Early 20th Century Science:

To know the structure of the atom, we must know the following:

  • What are the parts of the atom?
  • How are these parts arranged?

Near the end of the 19th century, the atom was thought to be nothing more than a tiny indivisible sphere (Dalton’s view). However, a series of discoveries in the fields of chemistry, electricity and magnetism, radioactivity, and quantum mechanics in the late 19th and early 20th centuries changed all of that. Here is what these fields contributed:

  • The parts of the atom:
  • chemistry and electromagnetism —> electron (first subatomic particle)
  • radioactivity —> nucleus
    • proton
    • neutron
  • How the atom is arranged – quantum mechanics puts it all together:
  • atomic spectra —> Bohr model of the atom
  • wave-particle duality —> Quantum model of the atom

Chemistry and Electromagnetism: Discovering the Electron:

In the late 19th century, chemists and physicists were studying the relationship between electricity and matter. They were placing high voltage electric currents through glass tubes filled with low-pressure gas (mercury, neon, xenon) much like neon lights. Electric current was carried from one electrode (cathode) through the gas to the other electrode (anode) by a beam called cathode rays. In 1897, a British physicist, J. J. Thomson did a series of experiments with the following results:

  • He found that if the tube was placed within an electric or magnetic field, then the cathode rays could be deflected or moved (this is how the the cathode ray tube (CRT) on your television works).
  • By applying an electric field alone, a magnetic field alone, or both in combination, Thomson could measure the ratio of the electric charge to the mass of the cathode rays.
  • He found the same charge to mass ratio of cathode rays was seen regardless of what material was inside the tube or what the cathode was made of.

Thomson concluded the following:

  • Cathode rays were made of tiny, negatively charged particles, which he called electrons.
  • The electrons had to come from inside the atoms of the gas or metal electrode.
  • Because the charge to mass ratio was the same for any substance, the electrons were a basic part of all atoms.
  • Because the charge to mass ratio of the electron was very high, the electron must be very small.

Later, an American Physicist named Robert Millikan measured the electrical charge of an electron. With these two numbers (charge, charge to mass ratio), physicists calculated the mass of the electron as 9.10 x 10-28 grams. For comparison, a U.S. penny has a mass of 2.5 grams; so, 2.7 x 1027 or 2.7 billion billion billion electrons would weigh as much as a penny!

Two other conclusions came from the discovery of the electron:

  • Because the electron was negatively charged and atoms are electrically neutral, there must be a positive charge somewhere in the atom.
Because electrons are so much smaller than atoms, there must be other, more massive particles in the atom.

From these results, Thomson proposed a model of the atom that was like a watermelon. The red part was the positive charge and the seeds were the electrons.

Thus the atom model was proposed… the details about the atom will be shown in next article friends.

courtesy: howstuffworks

Posted by

Raviteja ( MGIT ECE 3rd year)

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