Atomic Structure - Chemistry Notes | Study General Science(Prelims) by IRS Divey Sethi - UPSC

Structure of an Atom:

1. Neutron 
2. Proton 
3. Electron

Atom

• The electron is a fundamental particle of an atom which carries a unit negative charge.

It was discovered by J.J. Thomson in 1886

• It is a fundamental particle of an atom carrying a unit positive charge.
• It was discovered by Rutherford and Goldstein in 1886.

• It is a fundamental particle of an atom carrying no charge.
• It was discovered by chadwick in 1932.

Neutrons

• Due to the presence of equal number of negative electrons and positive protons the atom as a whole is electrically neutral.
• Based on the above findings, one can say that the atom has two major divisions.
  • The first is the centre of an atom, called its nucleus. The protons and neutrons are located in the small nucleus at the centre of the atom. Due to the presence of protons the nucleus is positively charged.
  • The second are electrons, which revolve around the nucleus in different shells (or orbits). The space around the nucleus in which the electrons revolve, determines the size of the atom.

Structure of an Atom

1. They travel in straight lines and cast shadow of solid objects placed in their path.

Cathode Rays travel in straight lines

1. They cause mechanical motion. e.g., they consists of material particles.

Cathode Rays can cause motion

1. They carry negative charge.

Cathode Rays carry negative charge

• These rays are deflected when magnetic field is applied on them.
• They produce heating effect.
• They cause ionization of gas through which they pass.
• They produce green fluorescence on the glass walls of the discharge, tube as well as on certain other substance such as zinc sulphide.
• They effect the photographic plates.
• They have penetrating power

• They travel in straight lines, however, their speed in much less than that of cathode rays.
• They are made up of material particle.
• They are positively charged.
• They deflected in electric and magnetic field.
• The nature of the anode rays depends upon the gas taken in the discharge tube. Different gases give different types of positive rays, which contain particles having different masses and different charges. Therefore the e/m ratio is not constant for positive ray particles obtained from different gases.

• The nuclei of atoms are made up of protons and neutrons.
• These two components of the nucleus are referred to as nucleons.
• The electrons occupy the space outside the nucleus.
• Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons.
• This number is the atomic number given by the symbol Z.

• The atomic number is defined as the total number of protons present in the nucleus of an atom.
• Atomic number of an element = total number of protons in the nucleus = total number of electronic present outside the nucleus. Z=p=e

• As atoms are electrically neutral, an atom contains as many electrons as it has protons.
• The total number of protons and neutrons present in one atom of an element is known as its mass number.
• Mass number = number of protons + number of neutrons
• It can also be said that: Mass number = atomic number + number of neutrons

Mass number of carbon atomValency

• The outermost shell of an atom is called its Valence shell.

Valence electrons

Valence shells

• It is the decisive shell during a chemical reaction.
• The electrons of only this outermost shell are involved during chemical combinations; electrons are either given out from the outermost shell, or accepted into the outermost shell, or shared with the electrons in the outermost shell of another element.

Valence electrons of different elements

 Significance of Valence Electrons

• Valence electrons of an atom are responsible for chemical reactions as they take part in them.
• Elements having same number of valence electrons in their atoms possess similar chemical properties. All alkali metals have one valence electron in their atom. Thus, their chemical properties are similar.
• The number of the valence shell in an atom determines its position in the Periodic Table i.e. the period to which the element belongs.
• Elements having 1, 2 or 3 electrons in the valence shell are metals. Exception is H and He. Elements having 4 to 7 electrons in their valence shell are non-metals.

• Atoms having same atomic number but different atomic masses .
• In other words they have same number of protons and electrons but different number of neutrons.

Isotopes of Chlorine

Isotopes of hydrogen

Isotopes of hydrogen

• All isotopes of an element have the same number of valence electrons thus have identical chemical properties.
• The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei.
• The densities, melting points and boiling points etc., are slightly different.

• The atoms of different elements, which have the same mass number but different atomic numbers are called isobars.
• These have different number of protons but equal sum of number of protons and neutrons.

• Their atomic masses are nearly equal.
• They possess different chemical and physical properties.

• The atoms of different elements, which have the same number of neutrons but different atomic numbers, are called isotones.

• Atoms of radioactive element having same atomic number and same mass number but different radioactive properties.

• Molecules of different substances which contain the same number of atoms and the same total number of electrons which leads to similarity in their physical properties, e.g., co₂ and N₂o.