Exploring the Atom: The Tangled Tale of the Tiny
The word “atom” comes from the Greek “atomos,” meaning “indivisible.” This idea has been blown apart much like atoms have, but the name still sticks. Atoms are the smallest unit of any element and combine to build everything around us. We take the modern model of the atom for granted, but the atom traveled a long, messy road to get to this point.
The concept of the atom was thought up in both ancient Greece and India. The philosopher Democritus came up with the term “atomos” because he thought they were solid microscopic particles. They differed not in composition but size and shape. That view held sway for centuries until John Dalton put forth his views in the 1800s. They were fairly similar to those of Democritus, except that Dalton said that atoms of different elements have different masses. There were flickers of modern atomic science in some of the other postulates. A reaction was the rearrangement of atoms, and compounds were formed by joining two or more different atoms.
The model of the atom was rocked by J.J. Thomson’s work with cathode ray tubes. In 1897, he discovered the electron, which he called a “corpuscle.” In his Nobel lecture, he said that the beam in the cathode ray tube was made of negatively-charged particles because it was deflected by both electrical and magnetic fields. He also showed that it was part of many things under many conditions, leading him to say that it was part of the atom. He determined the electron’s mass and charge, as well.
His model of the atom, however, was not yet the modern model. He said that the atom was like a plum pudding. The negatively-charged particles were scattered in a pudding of positively-charged material, so that the atom had an overall neutral charge.
The Next Advance
Ernest Rutherford, a student of Thomson, made the next advance in the atomic model in 1911. He suggested an experiment for a graduate student that involved counting large-angle deflection of radioactive particles. A source emitted the particles in a stream aimed at a very thin (a few atoms thick) sheet of gold. A curved screen was set up behind and around the gold sheet; when the particles hit the screen, they would emit light.
Rutherford thought the possibility of large-angle deflection impossible, so he was thunderstruck when, a few days later, the student’s advisor came to him and said they saw large-angle deflection, some even straight back to the source. Rutherford remarked that it was “as if you fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.”
This led to Rutherford proposing that the majority of the atom’s positively-charged mass was concentrated in a small spot. This became known as the nucleus, while most of the rest of the atom was empty space. Rutherford proposed that electrons orbited the nucleus much like planets orbit the sun. Ruthrford’s model was improved by Niels Bohr, who said that electrons could orbit only at certain distances from the nucleus.
The Latest Model
Although many people think of the Rutherford/Bohr model when they think of the atom, it's not the most-current model. There were some issues with Bohr’s model that were resolved by quantum mechanics. Louis De Broglie suggested that electrons could behave like waves. Because waves don’t have a precise location, electrons were not traveling in well-defined orbits around the nucleus. The quantum mechanical model speaks of the electron cloud, befitting the fuzziness of the electron’s location.
The Atom: Explained
There are three basic parts of the atom: proton, electron, and neutron. The positively-charged proton gives the atom its identity as an element; the number of protons in an atom is called its atomic number. The atomic number is seen on the periodic table, often above the element’s symbol. For example, carbon has a six over it, which means that there are six protons in a carbon atom’s nucleus.
Neutrons are also found in the nucleus. They have no charge and are about as heavy as a proton and are roughly 1800 times heavier than an electron. They add mass to the nucleus and keep down the repulsion forces between the protons. The sum of the number of neutrons and number of protons is the atom’s mass, which is often listed below the element’s symbol. Note that the number below the symbol is not a whole number but a weighted average, so it is safe to round up or down to the nearest whole number. Atoms with the same number of protons but different numbers of neutrons are called “isotopes.”
Rutherford likened the electron in the atom to a “fly in a cathedral.” They are the smallest subatomic particles. They zip around the nucleus at breakneck speeds and relatively huge distances. They have the same charge as the proton but with an opposite sign, so an atom with neutral charge has the same number of electrons as protons. If not, the atom is called an “ion.” Electrons are found in different shells at different distances from the nucleus to minimize the repulsive force between them. Electrons can move between shells based on emission or absorption of energy. The electrons in the outermost shell are involved in bonding reactions with other atoms.
There are sub-sub-atomic particles that make up the proton and neutron. These are called “quarks.” They come in six flavors: up, down, top, bottom, charm, and strange. Strange names, indeed. Quarks have fractional charges and smaller masses. These masses can’t be measured very well because quarks have yet to be isolated. Quarks combine in groups of three to make protons and neutrons. A proton is made of two up quarks and a down quark. A neutron is made from two down quarks and an up one. It's fun to remember that a neutron has no charge and so it's a DUD (down, up, down).
A lot goes on in an atom. In fact, atoms vibrate with energy. They put that energy to use in bonding with other to form the molecules that make up us and everything around us.
The view that we now have of atoms has inched along glacially and arrived at today’s model, and an appreciation has come with this new understanding. Even one hundred years ago, no one would have thought particles smaller than protons exist. The atom is small, yet mighty, and still capable of surprising us.
"Discovery of the Electron: J. J. Thomson." Classic Chemistry. N.p., 11 Nov 2010. Web. 28 Feb 2013.
Mokeur. "History of the Atom from Bohr to Schrödinger." N.p., 23 Mar 2012. Web. 28 Feb 2013.
Nave, R. "Quarks." Hyperphysics. N.p., 04 Oct 2010. Web. 28 Feb 2013.
"The Nucleus." Cambridge Physics. Cambridge Univeristy, 28 Feb 2013. Web. 28 Feb 2013.