The matter could not be divided into smaller and smaller pieces forever, eventually, the smallest possible piece would be obtained.
This piece would be invisible.
He named the smallest piece of matter "atomos" meaning "not to be cut".
To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.
Atoms were infinite in number, always moving and capable of joining together.
This theory was ignored and forgotten for more than 2000 years!
In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.
His Theory -
He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.
Atoms of the same element are exactly alike.
Atoms of different elements are different.
Compounds are formed by the joining of atoms of two or more elements.
This theory became one of the foundations of modern chemistry.
In 1932, James Chadwick conducted an investigation involving radiation and discovered the presence of the neutron.
The Wave Model -
Today’s atomic model is based on the principles of wave mechanics.
According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.
In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
According to the modern atomic model, an atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.
Electron Cloud -
A space in which electrons are likely to be found.
Electrons whirl about the nucleus billions of times in one second
They are not moving around in random patterns.
Location of electrons depends upon how much energy the electron has.
Depending on their energy they are locked into a certain area in the cloud.
Electrons with the lowest energy are found in the energy level closest to the nucleus
Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.
In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.
Thomson's Model -
He proposed a model of the atom that is sometimes called the “Plum Pudding” model.
Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.
Thomson studied the passage of an electric current through a gas.
As the current passed through the gas, it gave off rays of negatively charged particles.
Thomson concluded that the negative charges came from within the atom.
A particle smaller than an atom had to exist.
The atom was divisible!
Thomson called the negatively charged “corpuscles,” today known as electrons.
Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.
But he could never find them.
In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unravellingRutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)
The mysteries of the atomic structure -
Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.
Rutherford Experiment -
This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.
Rutherford concluded that an atom had a small, dense, positively charged centre that repelled his positively charged “bullets.”
He called the centre of the atom the “nucleus”
The nucleus is tiny compared to the atom as a whole.
Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.
In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.
According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.