Microsoft word - development of atomic theories

Development of atomic theory
Democritus (460BC - 370BC), a Greek philosopher, thought matter was composed
of very tiny, invisible and indivisible particles that he called “atoms”.
The later philosophers Plato and Aristotle strongly opposed Democritus’ views,
claiming that matter has a continuous structure. Their ideas prevailed until the 19th
century, when John Dalton, an English scientist, revived the atomic theory.
- Matter is made of very small indivisible particles called atoms that cannot be - All atoms of a particular element are identical. - Atoms of different elements differ in their masses. - Atoms can combine with one another to form compounds. - A chemical reaction is a change in the arrangement of a cluster of atoms. At the end of the 19th century an English scientist Joseph Thomson carried out an
experiment that allowed him to determine the mass and the charge of the particles
constituting a beam of cathode rays (cathode rays are emitted from a negatively
charged electrode when a high voltage is applied). Thomson named this particles
electrons.
With this discovery scientists realized that atoms are not indivisible. Moreover, at
that time scientists already knew that atoms are electrically neutral. Based on these
two data Thomson proposed a model for the atom in which negatively charged
electrons were embedded in a sphere uniformly filled with positive charge, like
raisins in plum-pudding. That is why nowadays we call it the plum-pudding atomic
model.
In 1910 the scientist Ernest Rutherford ran an experiment in which a very tiny gold
foil was bombarded with positively charged alpha particles emitted by a radioactive
element. According to Thomson’s model for the atom, all the alpha particles were
expected to pass through the foil without impediment. Most of the particles did
indeed pass through the gold foil but, to his astonishment, some of them were
reflected as though they had hit an unexpectedly massive positive object. Rutherford
realized that the positive charge and most of the mass of the atom are concentrated in
a very small volume: the nucleus. Soon after his experiment Rutherford proposed a
new model of the atom called the planetary model, with the nucleus at the centre
and electrons circling around in individual orbits.
In 1914 scientists understood that a particular type of rays, called anode rays, were
made up of subatomic particles with a positive charge which they named them
protons: the second subatomic particle had been discovered.
Only in 1932 did the English physicist J. Chadwick discover the third subatomic
particle: the neutron.
In an atom, protons (with a positive charge and a relative mass approximately = 1)
and neutrons (with no charge and a relative mass approximately = 1) are in the
nucleus and electrons (with a negative charge and a relative mass = 1/1837) move
around the nucleus.
All the atoms of the same element have the same number of protons (electrons are equal to protons) but they can have a different number of neutrons. The number of protons of an atom is called the atomic number and it is written
beside the symbol of the element at the bottom left of it (e.g. 17Cl).
The number of protons and neutrons of an atom is named the mass number and it is
written at the top left of the symbol (e.g. 3H).
The mass number minus the atomic number gives you the number of neutrons (e.g. 3 1H means 1 proton, 1 electron, and 2 neutrons). Atoms with the same atomic number and different mass number are called isotopes.
All the isotopes of the same element have the same chemical properties because they
have the same number of electrons but differ in the physical properties because they
have different masses.
In 1913 the physicist Niels Bohr, studying the electromagnetic waves proposed a
new model of the atom. According to this new model, electrons can move in orbits
with a specific distance from the nucleus because the energy they possess is not
continuous but quantized.
The distance from the nucleus (the radius of the orbit) can be determined by the equation r = 53n2(10-12 m) r = radius n = principal quantum number (1,2,3,…) Usually electrons move in orbits with the lowest energy (atom in the ground state)
but when an electron absorbs energy it can move in an orbit with a higher energy
(excited atom).
In a very short time the electron will then return to the previous orbit (fundamental
orbit) releasing the energy previously absorbed as electromagnetic waves.

Source: http://www.liceojoyce.it/materiali%20didattici/clil_scienze/development%20of%20atomic%20theories.pdf