Structure Of The Atoms

 Atom - Atom is the smallest particle of an element that take part in chemical reaction.

Atom contain subatomic particles electrons, protons and neutrons.

• Subatomic particles with negative charge are called electrons and denoted by e^-. It's charge is -1 or 1.602 x 10^-19. Electrons are identified by J.J Thomson.
• Subatomic particles with positive charge are called protons and denoted by p⁺. It's charge is  +1 or 1.602 x 10^-19. It is also known as canal ray. (Properties of Canal Ray: i. Canal rays are positively charged particles ii. fast-moving ray and deflected by the electric and magnetic field. iii.α-particles are much heavier than the protons.) 
• Another sub atomic particle neutron is denoted by n⁰. It's charge is 0. J.Chadwick discovered neutrons.In an atom nucleus consists of protons and neutrons. 

Atomic structure according to Thomson model:  

• Atom is a positively charged sphere in which electrons are embedded in it like watermelons.
• In an atom the value of positive charges is equal to the value of negative charges. So atoms are initially electrically neutral in charge.

Rutherford's Experiment: 

• Rutherford had chosen a very thin layer of gold foil for his alpha ray experiment as gold is the most malleable metal that can be easily converted into a thin sheet.
• In this experiment, fast moving alpha (α)-particles that are doubly charged helium ions of mass 4 u, were made to fall on a thin gold foil.
• In this experiment it was expected that the alpha particles would be deflected by subatomic particles of the gold atom, but larger deflections were not expected.

Observations of Rutherford E:

• Most of the fast moving α-particles passed through the gold foil without getting deflected.
• Conclusion- Most of the space inside the atom is empty
• Reason- Protons are positively charged particles present at the center of the atom and electrons revolve around the nucleus which are negatively charged particles. α-particles are positively charged particles and when these particles pass directly through the gold foil it can be attracted by electrons or deflected by protons but the large free space present in the atom and due to this some particles pass directly through gold foil.
• Few particles were deflected from their path by a small angle.
• Conclusion- The positive charge particles of the atom occupied very little space.
• Reason- At the center of the atom, a positively charged particle proton is present and when the positively charged alpha particle passes through a path near the center, the same positive charges cause a small deflection in the alpha particle.
• A very small number of α-particles were completely deflected (by 180 degree) or bounced back toward the source.
• Conclusion- All the positive charge and mass of the gold atom is concentrated in a very small space within the atom.
• Reason- α-particles are positively charged particles deflected by protons.

Atomic structure according to Rutherford:

• The positively charged particles protons and most of the mass of an atom was concentrated in a small volume called nucleus.
• The negatively charged particles electrons revolve around the nucleus of the atom in a well-defined circular path called orbits.
• The volume and size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model of the atom

According to Rutherford’s model of atom:

• The orbital revolution of the electron is not stable. 
• The motion of any particle in a circular path or orbit would undergo acceleration.During acceleration, charged particles radiate and lose energy and finally fall into the nucleus. Therefore the atoms are highly unstable and hence matter would not exist. But we know that atoms are quite stable.

BOHR’S MODEL OF ATOM

Neil's Bohr put forward the following postulates about the model of an atom:

• Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
• While revolving in discrete orbits the electrons do not radiate energy.
• These orbits or shells are called energy levels.

Electrons Distribution in Different Orbits or Shells:

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.

The following rules are followed for writing the number of electrons in different energy levels or shells:

• The maximum number of electrons present in a shell is given by the formula 2n² ,where ‘n’ is the orbit number or energy level index, 1,2,3,....Hence the maximum number of electrons in different shells are as follows:
• First orbit or K-shell ( first shell)will be = 2 × 1² = 2, 
• second orbit or L-shell will be = 2 × 2²= 8, 
• third orbit or M-shell will be = 2 × 3² = 18,
• fourth orbit or N-shell will be= 2 × 4²= 32, and so on.
• The maximum number of electrons can be present in the outermost orbit is 8.
• The shells are filled by the electrons in a step wise manner. Electrons are not accommodated in a given shell, unless the inner shells are filled. 

Electronic configuration

Arrangement of electrons around the nucleus of an atom is called electronic configuration.

Valency

The combining capacity of an atom tendency to react and form molecules with atoms of the same or different elements to attain a fully-filled outermost shell is called valency.

The electrons present in the outermost shell of an atom are known as the valence electrons.

An outermost-shell, which had eight electrons was said to possess an octet and they are little chemical activity as their combining capacity or valency is zero.

Combining capacity of an atom depends on losing or gaining electrons to attain octet. 

If an atom contains one, two or three electrons each in their outermost shell, they lose electrons and valencies are 1, 2 and 3 respectively. 

If an atom contains five, six or seven electrons each in their outermost shell, they gain electrons and valencies are 3 (8 - 5),  2 (8 - 6) and 1 (8 - 7)respectively.

Atomic Number

Total number of protons of an atom called its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number.

Total number of protons (positive)is equal to the total number of electrons (negative) before reaction in an atom. So initially all atoms are electrically neutral in nature 

Atomic number = Number of protons = Number of electrons

Mass Number

Mass number of an atom depends on the number of protons and neutrons present in the nucleus. 

Mass number of an atom is the total number of protons and neutrons present in the nucleus. Mass number denoted by A and number of neutrons denoted by N. 

Mass number (A) = No. of protons + No. of neutron.

The protons and neutrons together are called nucleon.

Isotopes

The atoms of the same element, having the same atomic number but different mass numbers are called isotopes. There are three isotopes of hydrogen atom, namely protium (¹H₁), deuterium  (²H₁)or tritium(³H₂).

Other examples are 

(i) Carbon, ¹²C₆ and ¹⁴C₆, 

(ii) Chlorine, ³⁵Cl₁₇ and ³⁷Cl₁₇, .

The chemical properties of isotopes are similar but their physical properties are different because chemical properties depends on atomic number and in isotopes atomic numbers are same.

Applications of Isotopes

The chemical properties of all the isotopes of an element are the same. But some isotopes are useful in various fields. 

Some of them are : 

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

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

(iii) Isotope of iodine is used in the treatment of goiter.

Isobar

Atoms of different elements with different atomic numbers, that have the same mass number, are called isobars.

Example: Calcium and Argon

⁴⁰Ca₂₀ , ⁴⁰Ar₁₈

Both calcium and argon have same mass number and different atomic number, therefore same in physical properties and different in chemical properties.