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Timeline of Atomic Structure

Scientists have contributed to the development of atomic structure. The timeline of this structure shows the discovery of atomic particles, and their arrangement.
Ningthoujam Sandhyarani Feb 6, 2019
Atoms are the building blocks of all substances. These fundamental units are made up of three particles: neutrons (no charge), protons (positively charged), and electrons (negatively charged). The number of protons present in an atom is called the atomic number, while atomic mass is the number of protons plus the average number of neutrons.
Each of the elements found in the nature differ in the number of these three atomic particles. An element has a fixed number of protons, but can have a varied number of neutrons. Atoms with different number of neutrons are referred to as isotopes.

Atomic Structure

The protons and neutrons are present at the atomic center, which is called the nucleus, while the electrons revolve around this center in a specific path called orbitals, shells, or energy levels. An atom can have many shells, depending upon its atomic number. Each shell allows the presence of a definite number of electrons.
For example, two electrons in the first shell, eight electrons in the second and third shell, and so on. The mass of an atom is mostly concentrated in the nucleus.
In a stable atom that has a zero charge, protons and electrons are equal in number. An atom becomes charged after donating or accepting electrons. The charge is either positive, which develops after donating electrons, or negative, which develops after accepting the electrons.
In other words, a positively charged atom consists of more protons than electrons; whereas a negatively charged one shows the opposite characteristic.

Atomic Structure Timeline

The idea of atoms was in existence since the Greek era. It was mentioned by Democritus (460 B.C.) as atoma, a Greek word meaning indivisible.
However, the actual credit for the discovery of atoms goes to John Dalton who proposed the atomic theory, and suggested that all the matter is made up of spherical solid particles called atoms. After this discovery, many improvements have been made regarding atoms and their structure.

1869: Mendeleev Periodic Table

Dmitri Mendeleev arranged the elements with similar properties in seven groups.
He also proposed the periodic law, stating that the properties of elements differ based on their atomic weights.

1898: Discovery of Electron

Joseph John Thompson claimed that atoms could eject small negatively charged particles, which were named as electrons.

1904: Inert Gases

Abegg stated that the stability and chemical inactivity of inert gases were due to their stable electronic configuration.

1911: Atomic Nucleus

Ernest Rutherford suggested that the atomic nucleus was a small, highly condensed, and positively charged structure. He was the first to assume the position of electrons around the nucleus.

1914: Charge in Nucleus

H.G.J Moseley discovered the positive charge of the atomic nucleus. According to him, the atomic number of an element was equal to the number of protons.

1919: Isotopes

Aston discovered the existence of atoms with different number of neutrons for the first time by using mass spectrograph. He named them as isotopes.

1922: Orbitals

Niels Bohr described atomic structure and the presence of orbital shells. The atomic model consisted of successive shells, in which the electrons were arranged.

1923: Dual Nature of Atoms

De Broglie discovered the dual nature of electrons. He proposed that these units could behave as waves, or also as particles.

1927: Atomic Formula

Heisenberg came up with the representation of atoms by means of a formula. He founded the principle of indeterminacy, stating that the position and velocity of a particle could not be determined at the same time.

1930: Atom Model

Schrodinger observed electrons as continuous clouds. He introduced wave mechanics in the atomic model.

1932: Discovery of Neutrons

James Chadwick discovered neutrons by using alpha particles. He suggested that the mass of neutrons was similar to that of protons.
Following these findings and discoveries, various studies were performed based on atoms, chemical reactions, and the mechanism of chain reactions. The discovery of atom is indeed a milestone in science, which helps in studying elements, compounds, and their properties.