Science Class 9: Structure of the Atom

The elucidation of the structure of atoms is based on a series of experiments which studied static electricity and the condition under which electricity is conducted by different substances. 

Constituents of Atom:

An atom is further divisible and consists of charged particles. Following are three fundamental particles:

• Electron: Electrons are negatively charged particles revolving around the nucleus in orbits of fixed energy. It was discovered by JJ Thomson.
• J.J. Thomson was granted the Nobel prize in Physics in 1906 for his work on the revelation of electrons.

• Proton: Protons are positively charged particles residing in the nucleus. It was observed by Goldstein in 1896. 
• Neutron: These are neutral particles (no charge) residing in the nucleus of all atoms except Hydrogen. It was identified by James Chadwick in 1932.

Table: Comparison of Electron, Proton and Neutron

The Structure of an Atom

The revelation of electrons and protons prompted the disappointment of Dalton’s case that the molecule was indissoluble and indestructible. Later, many scientists proposed various atomic models to define the arrangement of electrons and protons in an atom. 

Thomson’s model of an atom (1904):

• The Plum pudding model represented an attempt to consolidate the known properties of atoms at the time:
• 1) Electrons are negatively-charged particles
• 2) Atoms are neutrally-charged.

• According to this model, an atom consists of a sphere of positive matter within which electrostatic forces determine the positioning of the negatively charged corpuscles. This fact explains the overall neutral charge of the atom, Thomson proposed that the corpuscles were distributed in a uniform sea of positive charge, in which they are scattered like “the plums in a pudding”.

Drawbacks of J.J. Thomson’s model of the atom 

1.Thomson’s model did not explain the results of the particle scattering experiment administered by Rutherford. The bulk of the particle passes through the foil and deviates from different angles. 

2.There were no experimental pieces of evidence provided by him. This was one among the main drawbacks of his model.

3. He did not explain the steadiness of atoms.

Rutherford’s model of an atom (1911):

Master Rutherford besieged the gold foil with a 1-molecule bar with the assistance of high energy alpha particles(α-molecule is a decidedly charged helium particle, He2 +).

Observations:

• Most-particles pass through a straight sheet.
•  Some had lost their way near the centre.
•  A Few were reflected.

Conclusion:

• Atoms are spherical and most of the atoms are empty.
• Mass and positive charge of the atom are located at the centre of the atom i.e., nucleus.
• Around the nucleus electrons revolve in circular motion.
• The gravitational force between the positively charged nucleus and the negatively charged electron is balanced by the centrifugal force.

Proposed Nuclear Model of Atom: 

• Positively charged centre of  an atom is called the nucleus of an atom. Hence all the mass of an atom is assumed to reside in the nucleus.
• Electrons rotate in roundabout ways around the core.
• The size of the core is a lot more modest than the size of the molecule.

Drawbacks: 

It couldn’t clarify the soundness of a molecule.

• The spinning electron would lose energy lastly should fall into the core.
• Thus, the molecule ought to be profoundly shaky and henceforth matter would not exist. However, iotas are very steady.

Bohr model of an atom (1913): 

In 1913, Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and if electrons jump to a lower-energy orbit, the difference is sent out as radiation. Bohr’s model explained why atoms only emit light of fixed wavelengths, and later incorporated the theories on light quanta.

Niels Bohr proposed the following principles for the atom model:

Electrons are distributed in various different shells having  discrete energy and revolve around the nucleus.

If the shell of the atom is complete then the atom will be stable and less reactive.

• Only specific orbits within the atom are allowed, called isolated orbits of electrons.
• Electrons don’t emanate any energy, when they are rotating in discrete circles.
• These orbits or shells are known as energy levels and are either designated as K, L, M, N etc. or also can be numbered as n = 1, 2, 3, 4, etc.

Distribution of electrons in different orbits (shell):

• The greatest number of electrons that a shell can oblige in various shells is given by the equation 2n2, where ‘n’ is the circle number or energy level list, 1,2, 3, etc.
• The maximum number of electrons that a shell can accommodate in different shells is as follows:

• First Orbit or K-Shell (n = 1) = 2n2 = 2 x 12 = 2 electrons
• Second Orbit or L-Shell (n = 2) = 2n2 = 2 x 22 = 8 electrons
• Third Orbit or M-Shell (n = 3) = 2n2 = 2 x 32 = 18 electrons
• Fourth Orbit or N-Shell (n = 4) = 2n2 = 2 x 42 = 32 electron

• Outermost orbit  can accommodate a maximum number of electrons that is   8.

• Electrons are not kept in a  given shell until the inner shell is filled. That is, the shells are filled in a phased manner.

DRAWBACKS OF BOHR’S MODEL:

1.It was primarily for hydrogen atom

2. It couldn’t elaborate spectra of multi-electron atoms

Wave nature of electron was not justified by the model (inconsistent with the de Broglie’s hypothesis of dual nature of matter)

3. It didn’t illustrate molecules making process of chemical reactions
4. It violated Heisenberg’s Principle(Δx× Δp ≥nh/(2π)) which said that it was impossible to evaluate the precise position and momentum of electrons (and other microscopic particles) simultaneously.Only their probability could be estimated.
5. Zeeman effect(spectral lines variation due to external magnetic field) and Stark Effect(spectral lines variation due to external electric field) couldn’t be described by the model

Conclusion:

The current article emphasises the structure of atoms and its findings. Three theories are explained in detail in the present article. While JJ Thomson discovered the electrons, Bohr’s atomic model made the modification and explained Thomson’s model further. Rutherford on the other hand reported different findings on the atomic model. Each theory has its own merits and demerits which is explained in detail.