Download 1.3 History of the Atom Notes

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Science 9: A Timeline of the Atom
Scientist and
approximate
date
Democritus
300 BC
Name of model, sketch and
main idea of theory
Importance and
improvement on previous
model
Shortcomings –
problems or why was
it changed
Atoms were the smallest pieces
of matter
Talks about atom as the smallest
particle of matter
“Atomos” in Ancient Greek
means “that which cannot be
further broken down into
smaller pieces
Defines the atom as an indivisible
particle
Does not talk about
SUBatomic particles
(electrons, protons,
neutrons)
Solid sphere model
Explains a lot of chemical
properties such as how atoms
combine to form molecules
Atoms seen has solid,
indestructible spheres (like
billiards balls)
John Dalton
1800s
Plum pudding model
J.J. Thompson
1850s
Atoms are solid spheres made
up of a positive core with tiny
negative particles embedded in
it
Does not give a scientific
view of the atom, only a
conceptual definition
Confirms the basic laws of
chemistry: Conservation of Mass
and Definite Proportions
Introduces the concept of the
nucleus
Infers on the existence of protons
and electrons
Does not include the
existence of a nucleus
Still Does not talk about
subatomic particles
Does not explain the
existence of electrons
outside the nucleus
Does not explain the role
of electrons in bonding
Gold Foil Experiment: proved
that the nucleus is positive and
the electrons are outside the
nucleus
Ernest Rutherford
1905
Neils Bohr
1920
First real modern view of the
atom
Does not place electrons
in definite energy shells
around the nucleus
Explained that the nucleus is a
positive core and electrons spin
around it
Doesn’t include neutrons
in the nucleus
Electrons in definite energy
shells around the nucleus
Explains the role of valence
electrons in bonding
Does not explain the
shapes of molecules
Used atomic spectra to prove
that electrons are placed in
orbitals (shells) around the
nucleus
Definite energy shells
Planetary Model: electrons
move around a nucleus
The Greeks and the Alchemists
Democritus is credited with first creating the theory that it is impossible to divide things ad infinitum, meaning
that there is a point where it is impossible to divide an object anymore. Democritus also believes that
everything, including the physical and spiritual, are made out of atoms. The word "atom" actually comes from
the Greek word atomon which means "that which cannot be divided.” Democritus was one in a growing number
of people that did not believe the thought that everything was made up of Earth, Wind, Water, and Fire.
Aristotle had the exact opposite theory that Democritus had. Aristotle believed that it was possible to divide an
object forever. Aristotle was also a firm believer in the theory that everything was made up of only four
different components: Earth, Wind, Water, and Fire. Even then, this theory was starting to lose followers and
sink the pit of theories that did not last.
The rise of Alchemy brought on a new type of process to create scientific theories. Before the Alcehmists, the
way people devised theories was to sit under large tress and talk about it. It was the Alchemists that actually
started to perform experiments and actually prove these theories that were only in the mind before then. The
Alchemists’ main goal was to change lead, one of the plainest metals, into gold, as a symbolic gesture of being
able to turn the worst kind of man into the perfect model of a human being.
Alchemy: Alchemy is an ancient practice shrouded in mystery and secrecy. Its practitioners mainly sought to turn lead into
gold. Alchemy was rooted in a complex spiritual worldview in which everything around us contains a sort of universal spirit, and
metals were believed not only to be alive but also to grow inside the Earth.
John Dalton
Our modern understanding of matter can be traced back to the atomic theory of John Dalton (1766-1844), an
English Chemist. Dalton was interested in the properties of gases, particularly the experiments of Robert Boyle
(1627-1691). Dalton thought that all gases consisted of tiny particles and he contributed to the theory of gases
by founding a law which we know today as Dalton's law of partial pressures. Dalton went further and asserted
that all matter and not just gases must consist of small particles. Relating his atomic theory to Democritus, he
used the name "atom" to name these tiny particles of matter.
Dalton's atomic theory held that all substances are composed of atoms in different proportions. All the atoms of
one element, said Dalton, were exactly identical and the atoms of each element were different to the atoms of
every other element. Furthermore, the atoms differed from each other only in mass. Now this last feature was
something that could be measured experimentally from the masses of the chemical elements known at that time.
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms are indivisible
and indestructible.
2) All atoms of a given element are identical in mass
and properties
3) Compounds are formed by a combination of two
or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's
theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical
reactions. Also, there are different kinds of atoms (differing by their masses) within an element that are known
as "isotopes", but isotopes of an element have the same chemical properties.
Dalton's theory quickly became the theoretical foundation in chemistry.
J.J Thomson
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 wherein 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.
It’s interesting to note that this model was sometimes visualized as having a cloud of positive charge, a striking
contrast to the most recent atomic model which describes the positive nucleus to be surrounded by an electron
cloud.
Ernest Rutherford
By 1911 the components of the atom had been discovered. The atom consisted of subatomic particles called
protons and electrons. However, it was not clear
how these
protons and electrons were arranged within the
atom. J.J.
Thomson suggested the “plum pudding” model.
In this model,
the electrons and protons are uniformly mixed
throughout the
atom:
Rutherford tested Thomson’s hypothesis by devising his “gold foil” experiment. Rutherford reasoned that if
Thomson’s model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot
high velocity alpha particles (positive particles – can think of them as “bullets”) at an atom, then there would be
very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most
alpha particles went right through the gold foil but to his amazement, a few alpha particles rebounded almost
directly backwards.
These deflections were not consistent with Thomson’s model. Rutherford was forced to discard the “plum
pudding” model and reasoned that they only way
the alpha particles
could be deflected backwards was if most of the
mass in an atom was
concentrated in a nucleus. He thus developed the
planetary model of
the atom which put all the protons in the nucleus
and the electrons
orbited around the nucleus like planets around the
sun.
Neils Bohr
Niels Bohr, a Danish scientist, explained this line spectrum while developing a model for the atom:

The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus
(think of planets orbiting around the sun).

Bohr used the term energy levels (or shells) to describe these orbits of differing energy. He said that the
energy of an electron is quantized, meaning electrons can have one energy level or another but nothing in
between.

The energy level an electron normally occupies is called its ground state. But it can move to a higherenergy, less-stable level, or shell, by absorbing energy. This higher-energy, less-stable state is called the
electron’s excited state.

After it’s done being excited, the electron can return to its original ground state by releasing the energy
it has absorbed, as shown in the diagram below.

Sometimes the energy released by electrons occupies the portion of the electromagnetic spectrum (the
range of wavelengths of energy) that humans detect as visible light. Slight variations in the amount of the
energy are seen as light of different colors.
Ground and excited states in the Bohr model.
Bohr found that the closer an electron is to the nucleus, the less energy it needs, but the farther away it is, the
more energy it needs. So Bohr numbered the electron’s energy levels. The higher the energy-level number, the
farther away the electron is from the nucleus — and the higher the energy.
Bohr also found that the various energy levels can hold differing numbers of electrons: energy level 1 may hold
up to 2 electrons, energy level 2 may hold up to 8 electrons, and so on.