1) The document traces the history of atomic theory from ancient Greece to modern times, starting with Democritus' idea of atoms that was rejected by Aristotle. 2) In the 1600s, chemistry emerged as a science, with Antoine Lavoisier distinguishing elements and compounds. John Dalton further developed atomic theory in 1803, proposing atoms of different elements have different properties. 3) Ernest Rutherford's 1909 gold foil experiment discovered the atomic nucleus, replacing the plum pudding model and showing atoms have mostly empty space. This led to models placing electrons in distinct orbits around the nucleus.

1. Webquest script
YOUR THEORY
Remember when you came up with the idea of indivisible particles of matter,
atoms? Well, your idea has changed science and the way people think of the
world in many ways.
However, for about 2000 years, no one really explored it further. Why? Aristotle,
also from Greece, decided that it had no merit—it was worthless to explore any
further. He controlled people’s opinions and therefore not until 1800 did anyone
look deeper into atoms.
The greek name “atomos” stuck forever on as the name of this tiny particle of
matter. However, different theories showed that the root of this name is
misleading—it is not actually indivisible!
CHEMISTRY ARISES
It was in the 1600s that chemistry was invented. Chemistry is the science of
chemical compounds, elements, and how they work together. In 1777, Antoine
Lavoisier was the first to talk about elements. Elements are basic substances that
cannot be broken down into new substances by any chemical means.
Compounds, however, can be broken down, forming two or more different
elements.
Lavoisier also came up with the law of conservation of mass, a very important
aspect in chemistry and science over all. It states that the mass of a closed system
remains constant over time. It says that atoms of an object cannot be destroyed
nor created.
This whole idea led to the new discovery in 1803, changing people’s minds from
alchemy to the chemistry we know today…
JOHN DALTON
In 1803, John Dalton, a great chemist from England, showed that elements were
made up of atoms, connecting to your idea. Dalton was born in 1766 , therefore
developing his idea when he was 37 years old while working as a teacher and
private tutor in Manchester. He was the first to propose an atomic theory to
explain atoms more, expanding on your discovery.
His atomic theory stated the following:
1. Matter is composed of indivisible particles
2. All atoms of a particular element are identical
3. Different elements are made of a different type of atom
4. Atoms combine in whole-number ratios only
5. In chemical reactions, atoms are just rearranged (not changed, destroyed
nor created)
John Dalton also came up with the Law of Definite Proportions. This law helped
understand the third point of his atomic theory. It stated that if two elements can
form more than one compound, then the ratios of masses of the second element
combined in fixed mass of the first element will be ratios of small whole
numbers. For example, when 10 g of lead and 1.56 g of sulfur are combined, they
create 11.56 g of lead sulfide, but when you add more sulfur or more lead, there
is still 11.56 g of lead sulfide with leftover of the element in abundance.It is

2. complicated to understand, yet it is a law that has been very important in the
chemistry world.
NEW, MINOR DISCOVERIES
After Dalton’s first discoveries about atom properties, many scientists around
the world came up with new ideas. In 1827, Robert Brown observed pollen
grains in water and how they moved (Brownian Motion).
THOMSON
In 1856, another scientist who would revolutionize chemistry was born—JJ
Thomson. The English chemist was the first to discover electrons. In an
experiment done in 1897 using cathode ray tubes and currents of electricity, he
saw rays and at first believed they were particles even smaller than atoms that
were inside the atoms. He called them “corpuscles”, which were very light,
negatively charged particles that made up the atom, later named “electrons”. He
saw that these particles were 1000 times smaller than the atom.
This new discovery proved that your idea of the atom being indivisible was
wrong. Atoms now have a building block, meaning that somehow they could be
broken down further.
After his new discovery, he came up with a new atomic theory in 1904—the
“plum pudding” theory. Weird name, no? Well, this name came from his idea that
the atom had to stay neutral somehow. His theory said that an atom has
electrons surrounded by a kind of “soup” of positive charge, or “pudding”. The
electrons are the plums.
MINOR DISCOVERIES
In 1900, Max Planck came up with the quantum theory, which would later
change the way of looking at matter. He showed that when if vibrate atoms
strong enough, you can measure their energy only in discrete units (energy
packets called quanta). Eight years later, the American Robert Millikan
discovered the electric charge of the electron.
RUTHERFORD
After the discovery of electrons, the New Zealander chemist Ernest Rutherford
decided to investigate more. In 1909, he set up the Gold Foil Experiment, also
known as the Geiger-Marsden experiment, along side two other scientists. The
experiment consisted of aiming a beam of alpha particles at an extremely thin
piece of gold foil (about 0.0000086 cm thick). The experimenter had to look for
where the alpha particles went on the screen behind with high deflection angles.
According to the plum pudding theory, the particles should have passed right
through the foil to the screen behind, but there were a few particles that changed
direction by a few angles or even 90 degrees. There was a small amount of alpha
particles that ended up hitting the gold foil and bouncing back.
This whole experiment led to the new discovery—the atomic nucleus. The
nucleus is a very small positively charged ball inside the atom that is surrounded
by the electrons. This disproved the plum pudding theory and led to even
another discovery—what made the nucleus positively charged?
Gradually, through many experiments of changing one element into another,
scientists learnt about the proton. The proton is a particle that makes up the

3. nucleus of the atom and unlike the electron, it is positively charged, allowing the
atom to be neutral.
Rutherford then created the Rutherford model of the atom. He also said that
most of the mass of the atom comes from the tiny little nucleus, and most of it is
just empty space.
DID YOU KNOW: nucleus makes up only one thousand million millionth of the
atomic volume, but has almost all the atom’s mass?
The proton’s mass is 1,836 times greater than the mass of the electron?
BOHR
In 1885, a Danish physicist would come and help us understand the structure of
the atom even better. In 1912, Niels Bohr added three parts to the description of
the atom. He noticed that the electrons were not scattered around the nucleus
nor in a spiral, but that electrons orbit around the nucleus in only specific
distances. He found that when electrons jumped from a higher-energy orbit to a
lower-energy orbit, the atom gives off energy, but when electrons went from a
lower-energy orbit to a higher-energy orbit, the atom absorbed energy.
COMPLEX EXPANSION
By the 1920s, scientists began to analyze the latest model of the atom and
noticed that there was a little bit of error. Bohr and Sommerfeld both worked to
find that the orbits around the nucleus have different shapes and tilts in relation
to magnetic field. After many research, experiments, and testing, scientists
discovered the different properties of these orbits. As a result, they came up with
the idea that there is an electron “cloud” around the nucleus, which shows the
probability of the location of the different electrons.
THE NEUTRON
There is yet another particle that is part of the atom, though. In 1931, James
Chadwick realized that the mass of the nucleus could not come only from the
proton. He called it the neutron—a particle with no charge, a little heavier than
the proton, and with the mass of 1840 electrons. Its name comes from its
neutrality, or no charge.
TODAY’S MODEL
The model found to be the most accurate nowadays is the Schrodinger equation,
where there is an electron cloud around the nucleus.
Remember Dalton’s first atomic theory? Well, new things have been found to
show that two parts of his theory were wrong. Firstly, atoms actually are
divisible—even protons and neutrons themselves! The very small particles that
make up protons and neutrons are known as quarks.
Another part of the theory said that all atoms of an element are identical—this is
somewhat incorrect. Isotopes are atoms of the same element, but have different
masses than the other atoms of that element. One can have more neutrons than
protons or less neutrons, but the amount of protons never changes.