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Atomic Structure: Definition, History & Timeline

Instructor: Anthony Grattini

Tony has a BA in Biology and has taught secondary Life, Earth, and Physical Science, as well as Honors & AP Chemistry.

From its philosophical roots to the modern-day scientific model, learn the basic history and structure of the atom, the basic building block of all matter.

Philosophical Beginning

Let's begin the discussion of atomic structure with a brief history of why and how human beings first began contemplating the atom. Many philosophers throughout antiquity passionately attempted to understand the universe through deep concentrated thought. The ancient Greek philosopher western society credits for formulating atomic theory was named Democritus, who lived around 460-370 BC. One thought exercise performed by Democritus was to imagine what the smallest piece of a material could be that still retains the properties of the original material. His idea was to find the smallest piece of matter that exists inside of all matter.

Imagine cutting a piece of cloth into tiny pieces, each piece can still be labeled a piece of cloth. Eventually the scissors would no longer be able to cut the smallest piece of cloth, and this hypothetically uncuttable piece was labeled the atom, from the Greek word atomos, which literally means uncuttable. Today's definition of the atom is the basic building block of an element that still retains all of the basic properties of that element.

Dalton's Atomic Theory

During the period 1803-1807, a chemical atomic theory was developed by an English schoolteacher named John Dalton. Reasoning from a large number of observations, Dalton made four statements concerning the atom. He stated that each element is made up of atoms. All atoms of an element are identical. Atoms are neither created nor destroyed in chemical reactions. And lastly, compounds are formed when atoms of more than one element combine.

These prior ideas, through time, evolve into the present atomic theory that scientists agree best explains the basic structure of the atom.

Basic Structure of the Atom

Carbon Atom

Today, scientists agree that the atom is the basic building block of an element, and it's sub-atomically composed of protons, neutrons, and electrons. All atoms have a nucleus, or center, composed of protons and neutrons. The protons are positively (+1) charged particles, each with a mass of 1.673 x 10 -27 kg. Each proton is made of smaller units known as quarks, two up quarks and one down quark, and all protons are held together by the strong nuclear force.

The strong nuclear force is the strongest of the four known fundamental forces in the entire universe (strong, weak, electromagnetic, and gravity). It holds together all of the positive protons in the nucleus in such a tiny space. Without the strong nuclear force, these positive protons would fly apart naturally due to like charges repelling each other, similar to what occurs when one tries to place the north ends of two magnets together. The neutron is important for the strong nuclear force.

Surrounding the positively charged nucleus of an atom is a cloud of negatively charged electrons. The electrons orbit the nucleus of an atom at about 2,200 km/second, forming electron clouds around the nucleus. Because these electrons are negatively charged, they're attracted to the positive nucleus of the atom. The closer these electrons are to the nucleus, the stronger the attraction.

The electrons have a relatively negligible mass, 9.109 x 10-31 kg, when compared to the protons and neutrons. To understand this relatively negligible mass when comparing protons to electrons, let's blow them up to a size we can understand. If a proton were to be the same weight as me, 180 lbs., the electron would only be 0.098 lbs. Uncharged atoms, neither positive nor negative, will have an equal number of protons and electrons. For instance, the atoms of the element carbon have 6 protons and 6 electrons.

Size of an Atom

Atoms are extremely small. Most atoms have diameters between 1 x 10-10 m and 5 x 10-10 m. One scientific unit used to simplify this extremely small distance is the Angstrom, which equals 10-10 m. Thus, atoms have diameters on the order of 1-5 Angstroms. The nucleus of the atom only comprises about 1/100,000 the entire size of the atom, yet 99.9% of the mass of the atom is found within this relatively tiny space. Because the tiny nucleus carries most of the mass of the atom in such a small volume, it is incredibly dense. A matchbox full of material of such density would weigh over 2.5 billion tons! Astrophysicists have suggested that the interior of a collapsed star may approach this density.

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