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This is one of the foundational topic in Chemistry that one must take proper note of in order to understand other topics in Chemistry. As a beginner in Chemistry, you must acquaint yourself with this topic. Now, the Greek philosopher named Democritus(400 B.C) was the first to show that matter is made up of small particles called atoms. Now, picture a solid such as an Iron nail that is cut into small pieces until a piece can no longer be divided, i.e to obtain powdered iron fillings.

An atom is the smallest particle of an element, which takes part in a chemical reaction.

Atomic Theory Of Dalton

There are various postulates of Daltons Atomic Theory and the following assumptions were made:

  • Matter is made up of small or tiny indivisible particles called atoms
  • All atoms of a particular element are exactly alike in size, shape,and mass and are different from atoms of other elements.
  • In a chemical reaction, whole atoms(never fractions) combine, separate,or change places
  • When atoms combine with one another, they do so in small whole number ratio.

Composition Of The Atom

An atom has two main parts named the nucleus(center) and the shells which surround the nucleus at definite distances

An atom is composed of three subatomic particles namely Protons, Electrons,and Neutrons(P.E.N). The protons and neutrons are found in the nucleus while the electrons are in constant and continuous motion in the shells around the nucleus.

Relative Masses & Electoral Charges Of The Sub-atomic Particles

The mass of a proton is approximately equal to that of a neutron, while the mass of an electron is 1/1840 of that of a proton. This shows that an electron is extremely small;It contributes the least to the mass of an atom. In fact,the mass of an atom is concentrated in its nucleus.

ParticleChargeRelative Mass

Nuclear Particles

Nuclear Particles(Nucleons) are the protons and neutrons in the nucleus of an atom. They determine the atomic and mass numbers of an atom.

The atomic number of an element which is usually denoted by the letter Z is the number of protons in the nucleus of its atom. It is always a whole number and the number of protons in an atom is called nuclear charge. No two elements have the same atomic number.

Mass number of an element is the sum of the number of protons and neutrons in the nucleus of its atom.It is denoted by the symbol A.

The nuclear symbol of an element X is written as AZX where A stands for the mass number(protons +neutrons) and Z stands for the atomic number(number of protons).

The difference between the mass number and atomic number (A-Z) gives the number of neutrons.
Nuclear SymbolMass No. AAtomic No. ZProtonsElctronsNeutrons

After going through this table, you would be able to understand this topic clearly. You can get more questions related to this topic and use them for beteer understanding.

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