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SSC Chemistry Structure of Atom Structure of Atom

Structure of Atom

Category : SSC

  • Law of conservation of mass: This law was stated by Lavoisier in 1744. It states that "In all physical and chemical changes, the total mass of readouts is equal to total mass of products."
  • Law of constant proportions (or constant composition): This law was first stated by Proust in 1797. According to the law "a chemical compound is always found to be made up of the same elements combined together in the same proportions by weight" e.g. the ratio of hydrogen and oxygen in pure water is always 1: 8 by weight. This law is also called law of definite proportions.
  • Law of multiple proportions: This law was given by John Dalton (1803) and states that "when two elements combine to form two or more compounds, the different mass of one of the elements and the fixed mass of the one with which it combines always form a whole number ratio". This law explains the concept of formation of more than one compound by two elements.
  • Dalton's Atomic theory: Postulates of Dalton s Atomic Theory

(i) Matter is made up of extremely small indivisible particles called atoms.

(ii) Atoms of the same substance are identical in all respects i.e., they possess same size, shape, mass, chemical properties etc.

(iii) Atoms of different substances are different in all respects i.e., they possess different size, shape, mass etc.

(iv) Atom is the smallest particle that takes part in a chemical reaction.

(v) Atoms of different elements may combine with each other in a fixed simple, whole number ratio to form compound atoms.

(vi) Atoms can neither be created nor destroyed i.e., atoms are indestructible.

  • Atom: It is the smallest particle of an element which can take part in a chemical change. It may or may not be capable of independent existence.
  • Symbol: The abbreviation used for lengthy names of elements are termed as their symbols. The symbol of an element is the first letter or the first and another letter of English name or Latin name of the element. While writing a symbol, the first letter is always capital and the second is always small.
  • Molecule: It is the smallest particle of an element or compound that is capable of independent existence and shows all the properties of that substance. [The molecules of an element is made up of only one and same type of atoms, while the molecule of a compound is made up of dissimilar atoms]
  • Atomicity: The number of atoms present in a molecule of an element or a compound is known as its atomicity. e.g. the atomicity of oxygen is 2 while atomicity ozone is 3.
  • Ion: It is an electrically charged atom or group of atom. It is formed by the loss or gain of electrons by an atom. Ions are of two types :
  • Cation: It is positively charged ion and is formed by the loss of electron from an atom e.g. \[{{H}^{+}},\]\[N{{a}^{+}},C{{a}^{2+}},A{{l}^{3+}}\]\[,NH_{4}^{+}\], etc.
  • Anion: It is negatively charged ion and is formed by the gain of electrons by an atom, e.g. \[C{{l}^{-}},{{O}^{2-}},{{C}^{-}},{{F}^{-}},CO_{4}^{3-}PO_{4}^{3}\] etc.
  • Valency: The combining power (or capacity,) of an element is its valency.
  • Formula of simple and molecular compounds Binary compounds are those compounds which are made up of two different elements e.g. NaCI, KBr, CaO etc.

Following rules are to be followed for writing the formula.

(i) The valencies or charges on the ions must be balanced.

(ii) For a compound made up of a metal and a non-metal the symbol of metal is written first.

(iii) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

  • Sub-Atomic Particles

(i) Electrons: Electron was discovered in cathode ray experiment.

(ii) The term electron was coined by G.J. Stoney

(iii) Protons were discovered in anode ray experiment. Anode rays are also called positive rays or canal rays. Protons was discovered by Wilhelm Wien in 1902. It was identified by J.J. Thomson.

(iv) Neutron was discovered by James Chadwick in 1932.

  • Valency: The electrons present in the outermost shell of an atom are known as valence electrons. These electrons determine the valency of an atom. Valency is equal to the number of valence electrons. In case the number of valence electrons is close to its full capacity.

            Then,     Valency = 8 - valence electrons

            If outermost shell is completely filled then valency is zero. Valency is the combining capacity of an atom.

  • Atomic number (Z): Atomic number of an element is equal to the number of protons present in the nucleus of an atom.

Atomic number (Z) = number of protons = number of electrons.

  • Mass number (A): It refers to the total number of neutrons and protons (i.e., sum of protons and neutrons) called collectively as nucleus, present in an atom.

Mass number (A) = number of protons + number of neutron

  • Isotopes: Atoms of the same element having same atomic number but different mass numbers are known as Isotopes e.g. \[_{17}^{35}Cl\]and\[_{17}^{36}Cl\],\[_{1}^{1}H\]and\[_{1}^{2}H\],\[_{6}^{12}C\]and\[_{6}^{14}C\]etc.
  • Applications of Isotopes: Isotopes are used in various fields. For example.

(i) Isotope of uranium is used as a fuel in nuclear reactor

(ii) Isotope of cobalt is used in treatment of cancer

(iii) Isotope of iodine is used in treatment of goitre.

  • Isobars: Atoms of different elements having same mass numbers are known as Isobars, e.g. K-40 and Ar-40
  • The discovery of cathode rays was done by J.J. Thomson an English physicist.
  • Properties of cathode rays:
  • The cathode rays are constituted by fast moving electrons.
  • These rays travel in a straight line.
  • These rays possess mechanical energy.
  • These rays produce heat when focussed on metals.
  • These rays produce flourescenes when focussed on metals.
  • They affects the photographic plate.
  • They are deflected by electric and magnetic field.
  • They ionize the gases through which they pass.
  • They travel in a straight line.
  • They cm produce mechanical effects.
  • Anode rays are positively charged.
  • The nature of anode rays depends upon the gas taken in the discharge tube.
  • The mass of anode rays particles is almost equal to the mass of an atom from which it is formed.
  • Sub-atomic Particles:

Electron, proton and neutron are subatomic particles. The credit for discovery of these particles goes to Electron — J.J. Thomson and Proton — E. Goldstein Another subatomic particle which is neutral and has a mass approx. equal to that of a proton was called neutron and was discovered by Chadwick. The neutron is a neutral particle found in the nucleus of an atoms. Atom of all elements contain neutron (except hydrogen atom which does not contain neutron). The relative mass of neutron is 1 amu and it carries no charge (i.e., it is neutral)


Properties of atomic particles (Comparative)





(i) Symbol

e or \[{{e}^{-}}\]



(ii) Nature

Negatively charged

Positively charged

Neutral(no charge)

(iii) (a) Charge

      (b) Unit charge

(a) \[-1.6\times {{10}^{-19}}C\]

(b) -1

(a) \[+1.6\times {{10}^{-19}}C\]

(b) +1



(iv) Mass (a) amu

(b) kg

(a) 0.0005486 amu

(b) \[9.1\times {{10}^{-31}}kg\]

(a) 1.00753 amu

(b) \[1.67265\times {{10}^{-27}}kg\]

(a) 1.00893 amu

(b) \[1.67495\times {{10}^{-27}}kg\]

(v) Location

Extra nuclear space



(vi) Notation




(vii) relative mass





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