Download According to Democritus, atoms were small

Democritus
This is the Greek philosopher Democritus who began the search for a description of
matter more than 2400 years ago. He asked: Could matter be divided into smaller and
smaller pieces forever, or was there a limit to the number of times a piece of matter
could be divided?
His theory: Matter could NOT be divided into smaller and smaller pieces forever;
eventually the smallest possible piece would be obtained. This piece would be
indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”
According to Democritus, atoms were small, hard, indestructible 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. He said, “The universe is
composed of two elements: the atoms and the void in which they exist and move.”
Dalton
John Dalton (1766-1844) was such a brilliant youth that he became an English school
teacher when barely 12 years old. Dalton performed experiments with various
chemicals that showed that matter, indeed, seem to consist of elementary lumpy
particles (atoms). Although he did not know about their structure, he knew that the
evidence pointed to something fundamental.
He proposed the Atomic theory of matter based on his experimental observations. To
Dalton :
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All matter is made of atoms. Atoms are indivisible and indestructible.
All atoms of an element are identical in size, mass and properties. Atoms of other
elements are different in size, mass and properties.
Compounds are formed by a combination of two or more different kinds of
atoms. These chemical compounds are represented in whole number ratios.
A chemical reaction, atoms are combined, separated or rearranged.
Atoms cannot be subdivided, created or destroyed.
Thomson
Thomson's atomic theory proposed a model of atom which is known as the “plum
pudding model”.
In the year 1897 Joseph John Thomson (1856–1940) totally changed the view of an atom
by discovering the electron. Thomson’s atomic theory suggested that part of the atom
was composed of negative electrons; however, the rest of the positively charge part of
the atom was not clearly defined.
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An atom consists of a sphere of positive charge with negatively charged electron
embedded in it. The electrons are free to rotate within the cloud of positive
substance.
The positive and the negative charges in an atom are equal in magnitude, due to
which an atom is electrically neutral. It has no overall negative or positive
charge.
Rutherford
100 years ago, on March 7, 1911, Ernest Rutherford (1871–1937) presented a paper on
atomic theory to the Manchester Literary and Philosophical Society. His new theory
rejected Thomson’s theory and said it was incorrect. Based on an experiment he had
performed, he realized that the atom must have almost all of its mass concentrated at its
center, in a nucleus, with the vast majority of the atom consisting mostly of empty
space.
In 1909, he had conducted an experiment with two other scientists, in which they
studied the deflection angles of "alpha particles" that they shot through a
microscopically thin layer of gold.
He showed that while the nucleus contains virtually all of the mass of the atom, it only
takes up one-billionth of the volume of the atom, a very tiny amount. Much smaller
particles - electrons - orbit the nucleus at a (relatively) great distance.
Rutherford theory has since become known as the planetary model because electrons
orbit the nucleus like planets around the Sun.
 The center of the atom is called the nucleus. The nucleus holds the positive
charge.
 The magnitude of the charge of the nucleus is proportional to the atom’s atomic
mass. The rest of the atomic mass is made up of neutrons.
 The electrons move in a cloud around the positively charge nucleus.