Atomic Structure

The atomic structure of an element refers to the constitution of its nucleus and the arrangement of the electrons around it. Primarily, the atomic structure of matter is made up of protons, electrons and neutrons.

The protons and neutrons make up the nucleus of the atom, which is surrounded by the electrons belonging to the atom. The atomic number of an element describes the total number of protons in its nucleus.

Neutral atoms have equal numbers of protons and electrons. However, atoms may gain or lose electrons in order to increase their stability and the resulting charged entity is called an ion.

Atoms of different elements have different atomic structures because they contain different numbers of protons and electrons. This is the reason for the unique characteristics of different elements.Atomic structure refers to the structure of an atom comprising a nucleus (centre) in which the protons (positively charged) and neutrons (neutral) are present. The negatively charged particles called electrons revolve around the centre of the nucleus.

History of Atomic Structure

The history of atomic structure and quantum mechanics dates back to the times of Democritus, the man who first proposed that matter is composed of atoms. The study about the structure of an atom gives a great insight into the entire class of chemical reactions, bonds and their physical properties. The first scientific theory of atomic structure was proposed by John Dalton in the 1800s.

The discovery of subatomic particles has been the base for many other discoveries and inventions.

Atomic Models

In the 18th and 19th centuries, many scientists attempted to explain the structure of the atom with the help of atomic models. The most notable contributions to the field were by the scientists such as John Dalton, J.J. Thomson, Ernest Rutherford and Niels Bohr.

Dalton’s Atomic Theory

The English chemist John Dalton suggested that all matter is made up of atoms, which were indivisible and indestructible. He also stated that all the atoms of an element were exactly the same, but the atoms of different elements differ in size and mass.

Chemical reactions, according to Dalton’s atomic theory, involve a rearrangement of atoms to form products. According to the postulates proposed by Dalton, the atomic structure comprised atoms, the smallest particle responsible for the chemical reactions to occur.

The following are the postulates of his theory:

Every matter is made up of atoms.
Atoms are indivisible.
Specific elements have only one type of atoms in them.
Each atom has its own constant mass that varies from element to element.
Atoms undergo rearrangement during a chemical reaction.
Atoms can neither be created nor be destroyed but can be transformed from one form to another.

Dalton’s atomic theory successfully explained the Laws of chemical reactions, namely, the Law of conservation of mass, Law of constant properties, Law of multiple proportions and Law of reciprocal proportions.

Demerits of Dalton’s Atomic Theory

The theory was unable to explain the existence of isotopes.
Nothing about the structure of atom was appropriately explained.
Later, the scientists discovered particles inside the atom that proved, the atoms are divisible.

The discovery of particles inside atoms led to a better understanding of chemical species, these particles inside the atoms are called subatomic particles. The discovery of various subatomic particles is as follows:

Thomson Atomic Model

The English chemist Sir Joseph John Thomson put forth his model describing the atomic structure in the early 1900s.

He was later awarded the Nobel prize for the discovery of “electrons”. His work is based on an experiment called cathode ray experiment. The construction of working of the experiment is as follows:

Cathode Ray Experiment

It has a tube made of glass which has two openings, one for the vacuum pump and the other for the inlet through which a gas is pumped in.

The role of the vacuum pump is to maintain “partial vacuum” inside the glass chamber. A high voltage power supply is connected using electrodes i.e. cathode and Anode is fitted inside the glass tube.

Observations:

When a high voltage power supply is switched on, there were rays emerging from the cathode towards the anode. This was confirmed by the ‘Fluorescent spots’ on the ZnS screen used. These rays were called “Cathode Rays”.
When an external electric field is applied, the cathode rays get deflected towards the positive electrode, but in the absence of electric field, they travel in a straight line.
When rotor Blades are placed in the path of the cathode rays, they seem to rotate. This proves that the cathode rays are made up of particles of a certain mass, so that they have some energy.
With all this evidence, Thompson concluded that cathode rays are made of negatively charged particles called “electrons”.
On applying the electric and magnetic field upon the cathode rays (electrons), Thomson found the charge to mass ratio (e/m) of electrons. (e/m) for electron: 17588 × 10¹¹ e/bg.

From this ratio, the charge of the electron was found by Mullikin through oil drop experiment. [Charge of e^– = 1.6 × 10^-16 C and Mass of e^– = 9.1093 × 10^-31 kg].

Conclusions:

Based on conclusions from his cathode ray experiment, Thomson described the atomic structure as a positively charged sphere into which negatively charged electrons were embedded.

It is commonly referred to as the “plum pudding model” because it can be visualized as a plum pudding dish where the pudding describes the positively charged atom and the plum pieces describe the electrons.

Thomson’s atomic structure described atoms as electrically neutral, i.e. the positive and the negative charges were of equal magnitude.

Limitations of Thomson’s Atomic Structure: Thomson’s atomic model does not clearly explain the stability of an atom. Also, further discoveries of other subatomic particles, couldn’t be placed inside his atomic model.

Rutherford Atomic Theory

Rutherford, a student of J. J. Thomson modified the atomic structure with the discovery of another subatomic particle called “Nucleus”. His atomic model is based on the Alpha ray scattering experiment.

Alpha Ray Scattering Experiment

Construction:

A very thin gold foil of 1000 atoms thick is taken.
Alpha rays (doubly charged Helium He²⁺) were made to bombard the gold foil.
Zn S screen is placed behind the gold foil.

Observations:

Most of the rays just went through the gold foil making scintillations (bright spots) in the ZnS
A few rays got reflected after hitting the gold foil.
One in 1000 rays got reflected by an angle of 180° (retraced path) after hitting the gold foil.

Conclusions:

Since most rays passed through, Rutherford concluded that most of the space inside the atom is empty.
Few rays got reflected because of the repulsion of its positive with some other positive charge inside the atom.
1/1000th of rays got strongly deflected because of a very strong positive charge in the center of the atom. He called this strong positive charge as “nucleus”.
He said most of the charge and mass of the atom resides in the Nucleus

Rutherford’s Structure of Atom

Based on the above observations and conclusions, Rutherford proposed his own atomic structure which is as follows.

The nucleus is at the center of an atom, where most of the charge and mass are concentrated.
Atomic structure is spherical.
Electrons revolve around the nucleus in a circular orbit, similar to the way planets orbit the sun.

Limitations of Rutherford Atomic Model

If electrons have to revolve around the nucleus, they will spend energy and that too against the strong force of attraction from the nucleus, a lot of energy will be spent by the electrons and eventually, they will lose all their energy and will fall into the nucleus so the stability of atom is not explained.
If electrons continuously revolve around the ‘nucleus, the type of spectrum expected is a continuous spectrum. But in reality, what we see is a line spectrum.

Bohr’s Atomic Theory

Neils Bohr put forth his model of the atom in the year 1915. This is the most widely used atomic model to describe the atomic structure of an element which is based on Planck’s theory of quantization.

Postulates:

The electrons inside atoms are placed in discrete orbits called “stationery orbits”.
The energy levels of these shells can be represented via quantum numbers.
Electrons can jump to higher levels by absorbing energy and move to lower energy levels by losing or emitting its energy.
As longs as, an electron stays in its own stationery, there will be no absorption or emission of energy.
Electrons revolve around the nucleus in these stationery orbits only.
The energy of the stationary orbits is quantized.

Limitations of Bohr’s Atomic Theory:

Bohr’s atomic structure works only for single electron species such as H, He+, Li2+, Be3+, ….
When the emission spectrum of hydrogen was observed under a more accurate spectrometer, each line spectrum was seen to be a combination of no of smaller discrete lines.
Both Stark and Zeeman effects couldn’t be explain using Bohr’s theory.

Subatomic Particles

Protons

Protons are positively charged subatomic particles. The charge of a proton is 1e, which corresponds to approximately 1.602 × 10^-19
The mass of a proton is approximately 1.672 × 10^-24
Protons are over 1800 times heavier than electrons.
The total number of protons in the atoms of an element is always equal to the atomic number of the element.

Neutrons

The mass of a neutron is almost the same as that of a proton i.e. 1.674×10^-24
Neutrons are electrically neutral particles and carry no charge.
Different isotopes of an element have the same number of protons but vary in the number of neutrons present in their respective nuclei.

Electrons

The charge of an electron is -1e, which approximates to -1.602 × 10^-19
The mass of an electron is approximately 9.1 × 10^-31.
Due to the relatively negligible mass of electrons, they are ignored when calculating the mass of an atom.

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