Structure of an Atom

Made by:

Aneesh Bapat VIII Dahlia

Roll no.36

Abhinav Vidyalaya English Medium High School

Alchemy

The ancient Arabs invented the science of alchemy.

This was the precursor to modern chemistry. In alchemy, it was believed that everything was

made up of the five ‘elements’- water, earth, fire, air and aether.

This theory, which was accepted in Europe for centuries was in contradiction of the atomic theory and is now known to be false.

But although it was widely accepted, a very small group of people had in fact proposed the atomic theory…

Ancient Atomic Theories

In ancient Greece, Democritus(400BC) and Lucretius (50 BC) had suspected that everything was made of atoms.

Lucretius has backed up his atomic theory in his book De Rerum Natura.

But his ‘atom’ was closer to the modern molecule than atom.

He claimed that there was a lot of space between atoms; that was what allowed fluids to change shape and metals to be molded into desired shapes.

The Ancient Indian Atomic Theory

Maharshi Kanad was once walking with his food, splitting it into morsels, when it occurred to him that at some tiniest size, it would be impossible to split it into further smaller bits.

This laid the foundation of his atomic theory, where atoms were called Peelavs or smallest of the small particles.

But both ancient theories were absurd in some ways, for example stating that atoms of metals were rigidly joined together by hooks or that chilli tasted hot because its atoms were spiky.

About John Dalton John Dalton was an

English chemist. He is regarded as the

father of the Atomic theory because he put forth the first complete atomic theory.

This is because his theory completely explained two discovered laws: The law of conservation of mass and the law of definite proportions

Dalton’s Theory

Dalton’s theory Consisted of the followingfour postulates: All matter is made of atoms Atoms are indivisible and cannot be

created or destroyed. Atoms of the given element are identical

in all aspects. Atoms combine in fixed ratios of whole

numbers to form compounds.

Prout’s Hypothesis

At the time when Dalton’s atomic model was prevalent, William Prout made a hypothesis based on his observations stating that only hydrogen was the fundamental particle or protyle and all other atoms were made out of jointed hydrogen atoms.

This theory, although it was incorrect, influenced the name of the proton.

About JJ Thomson

The Full name of JJ Thomson was Sir Joseph John Thomson.

He was an English physicist who got the Nobel Prize for his discovery of the electron in the year 1906.

He also discovered the positively charged and heavier than electron alpha particle which we now know to be a helium nucleus.

The Big Experiment

When J.J. Thomson was working with a cathode ray tube, he observed blips (blinking white spots) on the screen.

When he held the tube between one positively charged and one negatively charged metal plate, the blips deflected towards the positive plate.

This proved the existence of tiny negatively charged particles we now call electrons.

Thomson’s Model

After discovering the electron, Thomson revised Dalton’s theory and put forth his new theory.

In this theory, the atom was a positively charged sphere with electrons floating in it.

As this model looks like a pudding (atom) with plums (electrons) floating in it, it was called the plum pudding model.

About Rutherford Ernest Rutherford was born in New

Zealand but lived most of his life in UK. He did path breaking work in the field

of radioactivity and correctly identified and named alpha and beta forms of radiation. He also discovered the proton or positively charged subatomic particle.

But he is most commonly known for his atomic theory …

The Gold Foil Experiment

In order to test Thomson’s Atomic theory, Ernest Rutherford performed an experiment with gold foil which was used because it can be made into a super thin layer.

He bombarded the gold foil with the positive alpha particles.

Thomson had predicted that all would pass through. Most of the alpha particles did pass through indicating that an atom is mostly empty space.

But very few were deflected or bounced back, indicating that an atom had a positive nucleus.Thus, Thomson was proved wrong.

Rutherford’s Model

Rutherford Based his model on the deductions of the gold foil experiment.

He thought that an atom has a positively charged nucleus with negatively charged electrons revolving around it.

An atom is mostly empty space – If an atom were the size of a soccer field, the nucleus would be only as large as the ball.

Rutherford’s model is also called the planetary model

Limitations of Rutherford’s Model

One major shortcoming in Rutherford’s model is that the electron should describe a spiral before collapsing into the nucleus. But this does not happen.

Another major drawback is that it does not say anything about the actual positions of the electrons.

The Third Component of an Atom

The third component of an atom, the neutron was discovered in 1932 by James Chadwick.

James Chadwick was a student of Ernest Rutherford and he got the nobel prize for this discovery.

The actual construction of the experiment is complicated, but it is similar to the gold foil experiment.

Niels Bohr

Niels Bohr was a famous Danish atomic physicist most famous for his atomic model.

Now for an unconventional introduction, his university had failed him for answering the question ‘how to find out the height of a building using a barometer’ saying that ‘by taking a long enough string, going to the top of the building dangling down the barometer and adding the length of string used and the height of the barometer.’ When he protested, his headmaster asked him to answer a question on which his passing would depend. He gave four or five methods of answering the question!

Bohr’s Model

Niels Bohr improved the planetary model of the atom by saying that only orbits of certain radii were allowed. An electron cannot exist between two adjacent allowed orbits.

The radius of the orbit was dependant on the energy level of the electron it contains; closer orbits have higher energy electrons.

An electron must gain or lose energy as required to jump from one orbital to the other and a non-jumping electron does not give out energy.

This model was satisfactory in most ways but could only explain the hydrogen atom.

The Shell Model

After Bohr proposed his model, he revised it with the help of Erwin Schrodinger , Werner Heisenburg and Max Born.

This model says that electrons are located in spherical regions called shells around the nucleus, similar to Bohr’s orbits.

The four common shells are called K,L,M,N where K is the innermost and N the outermost.

These shells can hold 2, 8, 18 and 32 electrons respectively from innermost to outermost.

The formula for finding out the number of electrons for the nth shell is 2n2.

Stages in the Evolution of the Atomic Theory

Properties of Components of the Atom The proton, neutron and electron are the

components of the atom. An electron has unit negative charge and

negligible mass. A proton has unit positive charge and

mass nearly equal to a hydrogen atom (1proton +1electron) .This known as 1 atomic mass unit (1 amu).

A neutron also has a mass of 1 amu but lacks any charge. It is electrically neutral and so, called a neutron.

Atomic Number and Mass Number The atomic number is the number of protons in

an atom. This is also equivalent to the number of electrons in that atom. It is represented by the letter Z.It affects the chemical properties of the element.

The atomic mass number is counted in amu’s and is equal to the total number of protons and neutron in an atom. It is represented by letter A. It affects the physical properties of the element.

Isotopes

An isotope is an atom of the same element, but with a different atomic mass number.

It has the same chemical properties and the same atomic number.

The below diagram shows the isotopes of Hydrogen.( The black balls are neutrons.)

Isobars and Isotones

Isobars are different elements having an equal atomic mass (protons +neutrons)

Eg: Calcium and argon both have an atomic mass of 40 atomic mass units.

Isotones are also necessarily different elements (Can even be isotopes) but having the same number of neutrons.

E.g.: chlorine 37 and potassium 39 (The number indicates the atomic mass of the

element which is an isotope.)

Why an Atom Holds Together

Due to complicated interactions between quarks of protons and neutrons, (Quarks are particles that make up protons and neutrons.) a strong Adhesive force is generated.

This force, called the nuclear force, is stronger than the electric repulsive force of the protons and hence the atom is held together.

So we see that it is the neutron which is responsible for holding the atom together.