atomic theory

Atomic Theory : 

Atomic Theory By: Sakshi Sood

What is atom ? : 

What is atom ? The atom is the smallest particle into which an element can be divided an still be the same substance .The existence of atoms in not a new idea; the atomic theory has been around for more than 2,000 years.

Democritus : 

Democritus A Greek philosopher named Democritus proposed that you would eventually end up with an “uncutable” particle In fact atom means “indivisible”. Democritus proposed that all atoms are small, hard particles made of a single material formed into different shapes and sizes. Atoms are always moving and they form different materials by joining together

ARISTOTLE : 

ARISTOTLE A Greek philosopher disagreed with Democritus. He said that you would never end up with an indivisible particle. Although his ideas were proved to be incorrect, he had such an influence that Democritus’s ideas were largely ignored

DALTON CREATES AN ATOMIC THEORY BASED ON EXPERIMENTS...!! : 

DALTON CREATES AN ATOMIC THEORY BASED ON EXPERIMENTS...!! John Dalton, a British chemist and school teacher, wanted to know why elements combined in specific proportions based on mass to form compounds. He performed experiments with different substances. His results demonstrated that elements combine in specific proportions because they are made of individual atoms.

Postulates of Dalton’s Atomic Theory : 

Postulates of Dalton’s Atomic Theory All matter, whether an element, a compound or a mixture is made up of extremely small particles called atoms. Atoms of the same element are identical in all respects, i.e., size, shape, mass, properties. Atoms of different elements have different sizes and masses and also posses different properties. Atoms of the same element or different elements combine together to form molecules or compounds. When atoms of different elements combine together to form compounds, they do so in a simple whole number ratio. Atoms of two different elements may combine in different ratios to form more than one compound.

Drawbacks of Dalton’s Atomic Theory : 

Drawbacks of Dalton’s Atomic Theory Atom is no longer considered as the smallest indivisible particle. Atoms of the same element may have different masses. Atoms of different elements may have same masses. Substances made up of the same kind of atoms may have different properties. The ratio in which the different atoms combine to form compound may be fixed and integral but may not be simple.

THOMSON FINDS ELECTRONS IN THE ATOM : 

THOMSON FINDS ELECTRONS IN THE ATOM In 1987, a British scientist named J.J. Thomson made a discovery that identified an error in Dalton’s theory. Thomson discovered that there are small particles inside the atom.. Therefore, atoms can be divided into even smaller particles.

THOMSON’S CATHODE-RAY TUBE EXPERIMENT : 

THOMSON’S CATHODE-RAY TUBE EXPERIMENT Thomson experimented with a cathode-ray tube. He discovered that a positively charged plate attracts the beam. Thomson concluded that the beam was made of particles with a negative electric charge.

NEGATIVE CORPUSCLES : 

NEGATIVE CORPUSCLES Thomson concluded that corpuscles are present in every type of atom and they are all identical. The negatively charged particles are now called ELECTRONS Because atoms have no overall charge, he realized that positive charges must be present to balance the negative charges of the electrons, but he didn’t know the location of atoms, so he proposed the “plum pudding model” where the atom is mostly positively charged material with the electrons attached to it.

RUTHERFORD OPENS AN ATOMIC SHOOTING GALLERY : 

RUTHERFORD OPENS AN ATOMIC SHOOTING GALLERY In 1909, a former student of Thomson’s decided to test Thomson’s theory. He designed an experiment to investigate the structure of the atom

Slide 12: 

Most of the particles passed straight through the gold foil. An element such as radium produced the particles. The lead stopped all positive particles except for a small stream aimed at the gold foil. Very few particles bounced back. Zinc sulfide Some particles were slightly deflected

RUTHERFORD PRESENTS A NEW ATOMIC THEORY : 

RUTHERFORD PRESENTS A NEW ATOMIC THEORY To explain the deflection of the particles, Rutherford proposed that in the center of the atom is a tiny, extremely dense, positively charged region called the NUCLEUS. Positively charged particles that passed close by the nucleus were pushed away by the positive charges in the nucleus. A particle that headed straight for a nucleus would be pushed almost straight back in the direction from which it came. He could calculate the diameter of the nucleus that was 100,000 times smaller than the diameter of the gold atom

Drawback of Rutherford’s Model of Atom : 

Drawback of Rutherford’s Model of Atom