Chemistry : Atom and Atomic Structure

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What is an Atom?

All substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element which can take part in chemical reactions and may or may not exist independently.

What is the Structure of an Atom?

The history of the discovery of the atomic structure is outlined below.

• John Dalton (1808) proposed the Atomic Theory. According to Dalton, matter is made up of tiny particles called atoms. The atom is the smallest particle of matter that takes part in a chemical reaction. Atoms are indivisible and cannot be created or destroyed. Further, atoms of the same element are identical in every respect.
• J. J. Thomson (1897) discovered electrons in Cathode Ray experiments. According to Thomson, atoms are divisible. Atoms contain very tiny negatively charged particles called electrons.
• E. Goldstein (1900) discovered protons in Anode Ray experiments. According to Goldstein, atoms contain positively charged particles called protons. Since atoms contain negatively charged particles, they must contain positively charged particles for them to be electrically neutral.
• E. Rutherford (1911) discovered the nucleus and provided the basis for the modern atomic structure through his alpha particle scattering experiment. According to Rutherford, the atoms is made of two parts: the nucleus and the extra-nuclear part. His experiments proved that the atom is largely empty and has a heavy positively-charged body at the center called the nucleus. The central nucleus is positively-charged and the negatively-charged electrons revolve around the nucleus.
• James Chadwick (1932) disovered neutrons. According to Chadwick, atoms contain neutral particles called neutrons in their nucleus along with the subatomic particles (i.e., electrons and protons).
• N. Bohr (1940) provided the modern concept of the atomic model. According to Bohr, the atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged) revolve around the nucleus in different imaginary paths called orbits or shells.

What is Atomic Number and Atomic Weight?

• Atomic number of an element is the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the number of protons equal the number of electrons in an atom.
• Atomic weight (or relative atomic mass) of an element is the number of times an atom of that element is heavier than an atom of hydrogen. The atomic weight of hydrogen is taken to be unity [1].
• Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom.

The elements are arranged according to increasing atomic numbers (along with their atomic mass) in a table called the Periodic Table