Modern Atomic Theory | Discovery, History, Examples

Discovery of Modern Atomic Thoery

Modern Atomic Theory : With the development of a scientific atomic theory, the general philosophical problems gradually disappeared into the background. All attention is focused on the explanation of concrete phenomena.

modern atomic theory

The properties of the atoms are determined indirect relationship with the phenomena to be explained.

For this reason, the chemical atomic theory of the 19th century supposed that each identified chemical element has its specific atoms. And that each chemical compound has its molecules (fixed combinations of atoms). What particles act as unchanged and undivided units depends upon what kind of process is involved.

Some phenomena, such as evaporation, are explained by a process in which the molecules remain unchanged and identical. In chemical reactions the molecules lose their identity. Their structures are broken up, and the composing atoms, while retaining their own identity, are rearranged into new molecules. With nuclear reactions, a new level is reached.

The atoms themselves are no longer considered indivisible: more elementary particles than the atoms appear in the explanations of nuclear reactions. Atomic theory is the scientific theory that matter is composed of particles called atoms.

Atomic theory traces its origins to an ancient philosophical tradition known as atomism. According to this idea, if one were to take a lump of matter and cut it into ever smaller pieces, one would eventually reach a point where the pieces could not be further cut into anything smaller. Ancient Greek philosophers called these hypothetical ultimate particles of matter atoms, a word which meant “uncut.”

In the early 1800s, John Dalton noticed that chemical substances seemed to combine and break down into other substances by weight in proportions. Each chemical element is ultimately made up of tiny indivisible particles of consistent weight.

Shortly after 1850, certain physicists developed the kinetic theory of gases and of heat. Which mathematically modeled the behavior of gases by assuming that they were made of particles. In the early 20th century, Albert Einstein and Jean Perrin proved that the action of water molecules causes Brownian motion (the erratic motion of pollen grains in water); this third line of evidence silenced remaining doubts among scientists as to whether atoms and molecules were real.

Throughout the nineteenth century, some scientists had cautioned that the evidence for atoms was indirect. And therefore atoms might not actually be real but only seem to be real.

By the early 20th century, scientists had developed fairly detailed and precise models for the structure of matter. Which led to more rigorously defined classifications for the tiny invisible particles that make up ordinary matter.

An atom is now defined as the basic particle that composes a chemical element. Around the turn of the 20th century, physicists discovered that the particles that chemists called “atoms” are, in fact, agglomerations of even smaller particles (subatomic particles), but scientists kept the name out of the convention. The term elementary particle is now used to refer to actually indivisible particles.


The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India. The word “atom,” meaning “uncuttable,” was coined by the Pre-Socratic Greek philosophers Leucippus and his pupil Democritus (c.460–c.370 BC).

Democritus taught that atoms were infinite in number, uncreated, and eternal and that the qualities of an object result from the kind of atoms that compose it. Democritus’s atomism was refined and elaborated by the later Greek philosopher Epicurus (341–270 BC) and by the Roman Epicurean poet Lucretius (c.99–c.55 BC).

During the Early Middle Ages, atomism was mostly forgotten in western Europe. During the 12th century, it became known again in western Europe through references to it in the newly-rediscovered writings of Aristotle.

In the 14th century, the rediscovery of major works describing atomist teachings, including Lucretius’s De Rerum Natura and Diogenes Laertius’s Lives and Opinions of Eminent Philosophers, led to increased scholarly attention on the subject.

Nonetheless, because atomism was associated with the philosophy of Epicureanism. Which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable by most European philosophers.

The French Catholic priest Pierre Gassendi (1592–1655) revived Epicurean atomism with modifications, arguing that God created atoms and, though extremely numerous, are not infinite.

He was the first person who used the term “molecule” to describe the aggregation of atoms. Gassendi’s modified theory of atoms was popularized in France by the physician François Bernier (1620–1688) and in England by the natural philosopher Walter Charleton (1619–1707).

The chemist Robert Boyle (1627–1691) and the physicist Isaac Newton (1642–1727) both defended atomism and, by the end of the 17th century, it had become accepted by portions of the scientific community.

Postulates of Modern Atomic Theory

postulates of model atomic theory
  • Atom is no longer indivisible. It has been established that the atom is no longer indivisible, as suggested by Dalton. It is composed of sub-atomic particles, which are electrons, protons, and neutrons (even more particles were found to be present later on). However, the atom is still the smallest portion of matter which can take part in chemical combinations.
  • All atoms of an element may not be similar. The atoms of the same element may have different atomic masses. For example, the atoms of chlorine element have been found to possess atomic masses of 35 and 37. These are called isotopes.
  • Atoms of different elements may have the same atomic masses. Interestingly, the atomic mass of calcium (Ca) and argon (Ar), which represent different elements, is 40. These are called isobars.
  • Atoms may not always combine in simple whole-number ratios. Contrary to Dalton’s atomic theory, atoms do not combine in simple whole-number ratios in many cases. For example, in sucrose (C12H22O11), the elements carbon, hydrogen, and oxygen are present in the ratio of 12:22:11, and the ratio is not a simple whole-number ratio.
  • Atom is no longer indestructible. In many nuclear reactions, a certain mass of the nucleus is converted into energy in the form of alpha, beta, and gamma rays. Thus, an atom is no longer indestructible, as suggested by Dalton.

Modern Atomic Theory Models

modern atomic theory models

In 1913, Neil Bohr, a student of Rutherford’s, developed a new atom model. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system and is known as the planetary model. The following four principles can summarize the Bohr model:

  • Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called “stationary” orbits.
  • Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.
  • Energy is absorbed when an electron jumps from a lower orbit to a higher one, and energy is emitted when an electron falls from a higher orbit to a lower orbit.
  • The energy and frequency of light emitted or absorbed can be calculated using the difference between the two orbital energies.

Key Points of the Theory

  • Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms.
  • Atoms themselves are composed of protons, neutrons, and electrons.
  • Each element has its atomic number equal to the number of protons in its nucleus.
  • Isotopes of an element contain different numbers of neutrons.
  • Elements are represented by an atomic symbol.
  • The periodic table is a chart that organizes all the elements.

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