Atomic Structure: Structure of Atom – Electrons, Protons, and Neutrons (For CBSE, ICSE, IAS, NET, NRA 2022)

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Title: Atomic Structure

  • Atomic structure refers to the structure of atom comprising of a nucleus (center) in which the protons (positively charged) and neutrons (neutral) are present.
  • Negatively charged particles called electrons revolve around the center of the nucleus.
  • History of atomic structure and quantum mechanics dates to the times of Democritus, the man who first proposed that matter is composed of atoms.
  • The advances in atomic structure and quantum mechanics have led to the discovery of other fundamental particles.

Structure of Atom – Electrons, Protons, and Neutrons

The Structure of Atom
  • The study about the structure of atom gives a great insight into the entire class of chemical reactions, bonds, and their physical properties.
  • Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge) .
  • The first scientific theory of atomic structure was proposed by John Dalton in 1800՚s.
  • The outermost regions of the atom are called electron shells and contain the electrons (negatively charged) .

What is Atomic Structure?

  • The atomic structure of an element refers to the constitution of its nucleus and the arrangement of the electrons around it. Primarily, the atomic structure of matter is made up of protons, electrons, and neutrons.
  • Protons and neutrons make up the nucleus of the atom, which is surrounded by the electrons belonging to the atom.
  • An atomic number of an element describes the total number of protons in its nucleus.
  • Neutral atoms have equal numbers of protons and electrons.
  • Though, atoms may gain or lose electrons in order to increase their stability, and the resulting charged entity is called an ion.
  • Atoms of different elements have different atomic structures because they contain different numbers of protons and electrons. This is the reason for the unique characteristics of different elements.

Atomic Models

  • In the 18th and 19th centuries, many scientists attempted to explain the structure of the atom with the help of atomic models.
  • Each of these models had their own merits and demerits and were pivotal to the development of the modern atomic model.
  • Most notable contributions to the field were by the scientists John Dalton, J. J. Thomson, Ernest Rutherford, and Niels Bohr.
  • Their ideas on the structure of the atom are discussed in this subsection.
  • Dalton model (Billiard ball model) : English chemist John Dalton suggested that all matter is made up of atoms, which were indivisible and indestructible. He also stated that all the atoms of an element were the same, but the atoms of different elements differ in size and mass
  • Thomson model (Plum pudding model) : The role of the vacuum pump is to maintain “partial vacuum” inside the glass chamber. A high voltage power supply is connected using electrodes i.e.. , cathode and Anode is fitted inside the glass tube
  • Lewis model (Cubical atom model)
  • Nagaoka model (Saturnian model)
  • Rutherford model (Planetary model) : Rutherford՚s nuclear model. Rutherford overturned Thomson՚s model in 1911 with his famous gold-foil experiment, in which he demonstrated that the atom has a tiny, massive nucleus.
  • Bohr model (Rutherford – Bohr model) : In atomic physics, the Bohr model or Rutherford – Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons — like the structure of the Solar System, but with attraction provided by electrostatic forces in place of gravity.
  • Bohr – Sommerfeld model (Refined Bohr model) : The most prominent refinement to the Bohr model was the Sommerfeld model, which is sometimes called the Bohr-Sommerfeld model. In this model, electrons travel in elliptical orbits around the nucleus rather than in circular orbits
  • Gryzinski model (Free-fall model)

Atomic Structure of Isotopes

  • Nucleons are the components of the nucleus of an atom. A nucleon can either be a proton or a neutron.
  • Each element has a unique number of protons in it, which is described by its unique atomic number.
  • Though, several atomic structures of an element can exist, which differ in the total number of nucleons.
  • These variants of elements having a different nucleon number (also known as the mass number) are called isotopes of the element.
  • So, the isotopes of an element have the same number of protons but differ in the number of neutrons.
  • The atomic structure of an isotope is described with the help of the chemical symbol of the element, the atomic number of the element, and the mass number of the isotope.
  • For example, there exist three known naturally occurring isotopes of hydrogen, namely, protium, deuterium, and tritium. The atomic structures of these hydrogen isotopes are illustrated below.

Atomic Structures of Some Elements

The structure of atom of an element can be simply represented via the total number of protons, electrons, and neutrons present in it. The atomic structures of a few elements are illustrated below.

Hydrogen

Structure of Hydrogen atom: This implies that it contains one proton, one electron, and no neutrons (total number of neutrons = mass number – atomic number)

Carbon

Structure of Carbon atom: The electrons are distributed into two shells and the outermost shell (valence shell) has four electrons. The tetravalency of carbon enables it to form a variety of chemical bonds with various elements.

Dual Nature of Matter

  • Electrons, which were treated to be particles, the evidence of photoelectric effect, shows they also have wave nature.
  • This was proved by Thomas young with the help of his double slit experiment.
  • De-Broglie concluded that since nature is symmetrical, so should be light or any other matter wave.