STRUCTURE OF THE ATOM | Study

Charged Particles in Matter

• Whenever we rub two objects together, they become electrically charged. 
• This is because atoms contain charged particles in them. 
• Therefore, atoms can be divided further into particles i.e proton, electron and neutron.

Atoms consist of an equal number of  protons and electrons.

Protons exist in the interiors of the atom and electrons exist in the exteriors of the atom. Therefore, electrons can be removed from an atom.

Since electrons exist in the exteriors of the atom they can be removed from an atom.

Dalton’s Atomic Theory

The postulates of the atomic theory by John Dalton

• The matter is made up of tiny particles called Atoms that cannot be divided.
• Atoms are never formed or destroyed during a chemical reaction. 
• Atoms of an element exhibit the same nature. 
• Atoms of the same element have equal size, mass and they exhibit similar chemical properties.
• Atoms of different elements exhibit variant chemical properties. 
• Atoms form compounds by combining in a ratio of whole numbers.
• A compound contains molecules in which a constant number and types of atoms are present.

Failure of Dalton’s Atomic Theory

Dalton suggested that atoms can neither be created nor destroyed and are indivisible. 

But the discovery of electrons and protons in atoms disproved this aspect of Dalton’s theory.

Thomson’s Model of an Atom

According to J.J. Thomson, the structure of an atom can be compared to Christmas pudding.

According to this model the electrons are present inside a positive sphere.

An atom is composed of a positively charged sphere in which electrons are embedded.

Atoms are neutral as the positive and negative charges are equal in number.

Rutherford’s Model of an Atom

Rutherford’s Experiment

Rutherford experimented by passing alpha rays through a thin gold foil.

He expected that the gold atoms would deflect the Alpha particles.

Based on his experiment Rutherford gave the nuclear model of an atom as the following.

Rutherford’s Atomic Model

Rutherford’s Atomic Model is known as Planetary Atomic Model and Nuclear Atomic Model.

According to Rutherford’s Atomic Model:

• Atoms contain a lot of unoccupied space
• The center of the atom is highly positive , Rutherford named it as nucleus
• The atom contains an equal amount of positive and negative charges.

Nucleus of Atom

The nucleus is located at the center of the atom.

All the mass of the atom is because of the nucleus.

The electrons revolve around the nucleus in circular parts which called Orbits

The size of an atomic nucleus is much smaller than its atom.

Drawbacks of the Nuclear Atomic Model

The Rutherford’s Atomic Model failed to explain how an atom remains stable despite having positive and negative charges present in it. 

Maxwell’s theory of radiation if any charged particle moves in a circular motion it radiates energy. 

So, if electrons move in a circular motion around the nucleus they should radiate some energy as a result this decreases at the speed of the electrons. As a result, they would fall into the nucleus and the nucleus should collapse because of its high positive charge. 

But it is not happening because the matter is not collapsing. 

Nucleons:  The subatomic particles present in the nucleus are collectively called Nucleons. Protons and Neutrons are nucleons.

Bohr’s Model of an Atom

Bohr Atomic Model states as the following:

• Electrons revolve around the nucleus in particular circular paths, called orbits.
• The electrons do not emit any energy while moving in their orbits.
• The orbits are also called Energy Levels.
• Energy Levels or Orbits are represented by using letters or numbers as shown in the figure.

Neutron:

J. Chadwick discovered Neutron, a subatomic particle of an atom. 

Neutron carries no charge. 

Subatomic Particles of Atom

Electronic Configuration: The distribution of electrons in different shells or orbits is called Electronic Configuration. 

• If Orbit number = n
• Then number of electrons present in an Orbit = 2n²
• So, for n =1
• Maximum electrons present in shell – K = 2 * (1)² = 2
• The outermost shell can contain at most 8 electrons.
• The shells in an atom are filled in sequence.
• Thus, until the inner shells of an atom are filled completely the outer shells cannot contain any electrons.

Valency

• Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.
• The atoms which have completely filled the outermost shell are not very active chemically.
• The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.
• For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.
• What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?

Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.

Representation Element:

Atomic Number of an Element

Atomic Number (Z) = Number of protons in an atom

Mass Number of an Element

Mass Number = Number of protons + Number of neutrons

Isotopes

• The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.
• Isotopes are pure substances.
• Isotopes have a similar chemical nature.
• Isotopes have distinct physical characteristics.

Use of Isotopes:

1. The fuel of Nuclear Reactor – Isotope of Uranium

2. Treatment of Cancer – Isotope of Cobalt

3. Treatment of Goiter – Isotope of Iodine

Example: Consider two atomic species namely U and V. Are they isotopes?

From the above example, we can infer that U and V are isotopes because their atomic number is the same.

Isobars

The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars.