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Dalton's Atomic Theory
It was in the early 1800s that John Dalton, an observer of weather and discoverer of
color blindness among other things, came up with his atomic theory.
Main points of Dalton's Atomic Theory
=> Elements are made of tiny particles called atoms.
=> All matter is made of atoms. Atoms are indivisible and indestructible.
=> All atoms of a given element are identical in mass and properties
=> The atoms of a given element are different from those of any other element; the
atoms of different elements can be distinguished from one another by their respective
relative atomic weights.
=> Atoms of one element can combine with atoms of other elements to form chemical
compounds; a given compound always has the same relative numbers of types of atoms.
Compounds are formed by a combination of two or more different kinds of atoms.
=> Atoms cannot be created, divided into smaller particles, nor destroyed in the
chemical process; a chemical reaction simply changes the way atoms are grouped
together. A chemical reaction is a rearrangement of atoms.
PLANETARY MODEL
The planetary model may not be the ultimate, perfect model of the atom, but don't
underestimate its power. It already allows us to visualize correctly a great many
phenomena.
Rutherford naturally considered a planetary-model atom, the Rutherford model of 1911
- electrons orbiting a solar nucleus – however, said planetary-model atom has a
technical difficulty. The laws of classical mechanics, predict that the electron will
release electromagnetic radiation while orbiting a nucleus. Because the electron would
lose energy, it would gradually spiral inwards, collapsing into the nucleus. This atom
model is disastrous, because it predicts that all atoms are unstable.
Also, as the electron spirals inward, the emission would gradually increase in frequency
as the orbit got smaller and faster. This would produce a continuous smear, in
frequency, of electromagnetic radiation. However, late 19th century experiments with
electric discharges through various low-pressure gasses in evacuated glass tubes had
shown that atoms will only emit light (that is, electromagnetic radiation) at certain
discrete frequencies.
The Rutherford Bohr model of the hydrogen atom.
In 1913, Rutherford and Bohr pictured the arrangement of the atom's parts to look like
our solar system. At the center of every atom is a nucleus, which is comparable to the
sun in our solar system. Electrons move around the nucleus in "orbits" similar to the
way planets move around the sun.
Each orbit around the nucleus represents an energy level, and electrons cannot exist in
between orbits. Orbits closer to the nucleus have lower energy. If energy is added, an
electron can be "excited" to jump to a higher energy level--an orbit farther from the
nucleus. Eventually, though, the electron will return to its original state, and the atom
will give off energy equal to the difference between the two orbits.
Electron Cloud Model
electron orbitals
The electron cloud model is a model of the atom wherein electrons are no longer
depicted as particles moving around the nucleus in a fixed orbit. Instead, as a quantum
mechanically-influenced model, we shouldn't know exactly where they are, and hence
describe their probable location around the nucleus only as an arbitrary 'cloud'.
Plum Pudding Model
The Plum Pudding Model is an atom model proposed by JJ Thomson, the physicist who
discovered the electron. It is also known as the Chocolate Chip Cookie or Blueberry
Muffin Model. You can easily picture it by imagining the said goodies. For example,
you can imagine a plum pudding where in the pudding itself is positively charged and
the plums, dotting the dough, are the negatively charged electrons.
Thus, in contrast to today's atom that has a very dense and very small (compared to the
whole atom) positively charged nucleus, Thomson's had a more dispersed positive
charge. As a whole, the plum pudding representation only strived to explain why most
atoms were neutral.