[Chemistry Class Notes] on Structure of an Atom Pdf for Exam

Atoms are the basic components that a chemical element is made up of. They obtain subatomic particles in their nucleus and in the electron cloud that surrounds the nucleus. These subatomic particles include protons and neutrons, which are located in the nucleus, and electrons, which are located in the cloud, or field that surrounds the nucleus. Protons have a positive electrical charge, neutrons are uncharged, and electrons are negatively charged. Protons and electrons are the same in quantity, which is why the atoms are particles that are electrically neutral.

Structure of an Atom 

The model of an electric carbon, which consists of 6 protons, 6 electrons and in most cases, 6 neutrons, has electrons in simple rings or shells surrounding the nucleus. Each cell represents a different energy level, and it can hold only a certain maximum number of electrons. 

In atoms with more than one energy level or shell, 8 electrons are required in its outermost shell for stability. So, neon, which has 8 electrons in its outermost shell, is stable, and oxygen, which has only 6 electrons in its outermost shell, is chemically reactive. 

Atoms that do have fewer than 8 electrons in their outermost shells, will try to share, gain, or lose electrons to become stable. This is called the octet rule, but there are some exceptions, such as we saw with hydrogen and helium which only have one energy level or shell. 

What is Dalton’s Atomic Theory?

The English Chemist John Dalton proposed the theory that every matter is composed of atoms which are indivisible and indestructible. The postulates of Dalton’s theory are given in the following:

• All matter is composed of atoms.

• Atoms are indivisible and indestructible.

• Each and every atom comprises its own constant mass that differs from element to element.

• During a chemical reaction, atoms get rearranged. 

• There is neither creation nor the destruction of atoms, the only transformation from one form to another can be possible. The Laws of chemical reactions were successfully explained by Dalton’s atomic theory which includes the Law of conservation of mass, Law of constant properties, Law of multiple proportions, and Law of reciprocal proportions. 

Drawbacks of Dalton’s Atomic Theory

• The existence of isotopes was not explained by the theory.

• An appropriate explanation of the structure of the atom was missing.

• Later on, the discovery of the presence of particles inside the atom proved that the atoms are divisible. 

What was Thomson’s Atomic Model?

The English chemist Sir Joseph John Thomson was better known for his discovery of electrons for which he got the Nobel Prize. The following experiment was performed by him to invent electron:

Cathode Ray Experiment

A tube of glass with two openings, one for the vaccum pump and the other for the intake through which a gas is pumped in is present is taken. A partial vacuum is maintained by the vacuum pump inside the glass chamber. A high voltage power supply is linked with the help of electrodes i.e. Cathode and Anode are placed inside the glass tube. 

Observations

• There were rays transmitted from the cathode towards the anode when a high voltage power supply is connected. The ‘fluorescent spots’ present on the ZnS screen confirms the above-mentioned fact. These rays were termed as ‘Cathode Rays’. 

• The cathode rays get deviated towards the positive electrode when an external electric field is projected. But they are in a straight line in the absence of an electric field.

• They seem to rotate when rotor blades are fixed in the path of the cathode rays. The fact of composing cathode rays by particles of a certain mass has been proved by the fact. 

• Compiling all the pieces of evidence, Thomson concluded that cathode rays are composed of negatively charged particles called ‘electrons’. 

• The charge to mass ratio (e/m) was found by applying the electric and magnetic field upon the cathode ray (electrons). (e/m) for electron:1.758820 x 10¹¹ e/bg Mullikin found the charge of the electron with the help of this ratio through an oil drop experiment. Charge of e- =1.6 x 10^-16  C and Mass of e- = 9.1093 x 10^-31 kg

Thomson described the structure of the atom as a positively charged sphere into which negatively charged electrons were entrenched on the basis of his experiment. The popular name of Thomson’s model is “plum pudding model” because it can be seen as a plum pudding dish where the pudding means the positively charged atom and the plum pieces stand for the electrons. 

Drawbacks of Thomson’s Model

Rutherford invented another subatomic particle called ‘nucleus’ by modifying the atomic structure of J.J. Thomson. He has performed the Alpha Ray Scattering Experiment:

Construction

• A very thin gold foil comprising 1000 atoms thick is taken.

• The gold foil is supposed to be bombarded by Alpha rays (doubly charged Helium He2+).

• ZnS screen is located behind the gold foil. 

Rutherford’s Atomic Structure 

Around the year 1911, British physicist Ernest Rutherford, carried out the famous Gold foil experiment to understand the structure of an atom. In the experiment, he used a source of alpha particles, an atom, and a detector. 

His first conclusion was that most of the space inside the atom is empty. This is because more of the alpha particles could easily pass through the atom and hit the detector straight. Now if most of the space inside the atom, then the positive charge could not be found. That is explained in the second conclusion. According to Rutherford, the positive charge is compactly packed in a tiny entity that occupies an extremely small space inside the atom. 

This is how the structure of an atom has been postulated over the years. The advent of new theories and explanations resulted in the present model of atoms we study. Understand how the scientists have postulated their theories and their drawbacks to realize the development of this scientific concept perfectly.  

Conclusions

• Rutherford concluded that a maximum portion of the atom is empty because most rays went through.

• Few rays got reflected as there might be a repulsion of its positive charge with some other positive charge inside the atom.

• It was observed that 1/1000th of rays got sharply deflected as a result of a very strong positive charge in the centre of the atom. This positive charge was termed as ‘nucleus’.