An atom has two components – one is the nucleus and the other one is the outer shells or energy levels. Nuclei contain protons (+ve charge) and neutrons (no charge) whereas outer shells have electrons (-ve charge). JJ Thomson, E. Goldstein, Chadwick are to be given credit for the discovery of the subatomic particles. They, including many others, took their respective approaches towards defining the structure of an atom.

Subatomic Particles of  an Atom

1. Electron: It was discovered by JJ Thomson through Cathode ray experiment. He took the CRT (Cathode Ray Tubes ) and placed two electric plates around the CRT. The cathode ray got deflected from the negatively charged plate towards the one which is positively charged. 
2. Proton: It was discovered by Goldstein through the Anode ray experiment.
3. Neutron: It was discovered by James Chadwick through the bombardment of alpha particles on thin Beryllium sheets.

Specific Charge= Charge (q/e)Mass (m)

There are two different types of atoms :-

1. Isotopes: Isotopes are atoms that have the same number of protons but differ in the number of neutrons. So, they have different weights or atomic mass but have the identity of the same element.                                                     For example: An atom of the element Hydrogen has 1 proton. So, any atom that has one proton is hydrogen but if it has a variable number of neutrons that could be a different weight to that atom but it is still the same identity.
2. Ions: Ions are an atom or group of atoms that have the same identity but differ either by having a net positive or negative charge. You can determine this by looking at the balance of electrons and protons. If an atom has one more electron than the number of protons, it will have a net negative charge and if an atom has one more proton than the number of electrons, it will have a net positive charge.

Atomic Number, Atomic Mass and Atomic Symbols

Atomic Number is a specific number for each element. It is the same for all atoms of an element and is equal to the number of protons present in the nucleus of an atom.

Atomic Mass is the average mass of all the atoms present in an element. It is the weighted average of all the isotopes forming out of an element.

Atomic Symbols are a way to specify any atom via the isotope or an ion with a high degree of specificity. The general format for an atomic symbol is : abX 

X is the chemical element. On the left hand side there is a subscript ‘a’ and superscript ‘b’. b is the atomic mass units and a is the atomic number ( number of protons)

For example:- Chlorine has the symbol Cl and Its atomic number is 17. It has two isotopes – one is Chlorine 35 and Chlorine 37. So, Its atomic symbol will be 

                             1735Cl   or  1737Cl  

Dalton Atomic Theory

This theory is based on law of mass conservation and law of definite proportions according to which an atom is indivisible i.e. it is an ultimate particle which cannot be further subdivided.

The salient features of this theory are :-

• Each element is composed of extremely minute particles referred to as atoms.
• Atoms of the same element are similar while atoms of different elements are not similar.
• Atoms are indestructible that means they can neither be created nor be destroyed.

The drawbacks of this theory are:-

• Discovery of the subatomic particles
• Discovery of the isotopes

Examples of Atoms

An element consists of a lot of particles known as atoms. Here I have included the following 4 atoms of the respective elements of the periodic table :-

• Hydrogen: 

• Carbon:

• Copper:

• Gold:

Conclusion

The atomic structure initially was assumed to be an indestructible matter which cannot be divided further. And now we have a modern view of the atomic structure which comes out to possess totally different characteristics. Many scientists have been able to obtain computer-enhanced images of the outer structure of an atom by the use of STM (Scanning Tunneling Microscope) and AFM (Atomic Force Microscope).