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Atomic Model Timeline

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Camille Jao

on 7 November 2014

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Transcript of Atomic Model Timeline

1897-1904 — Plum Pudding Model
J.J. Thomson, a scientist who discovered the electrons, proposed his model of an atom based on his discovery. Thomson believed that an atom is made up of negative electrons that are evenly spread in a positively charged sphere (just like the plums in a pudding).
1912 — Nuclear Model
1913 — Bohr's Model
Niels Bohr, a Danish scientist, expanded on Rutherford's idea and made more detailed observation on the behavior of electrons.
1930 — Quantum Model
The Quantum model is the latest model of atom. It is based on probability of where electrons could be. This model uses complex shapes of orbitals (or electron clouds) to determined possible areas with electrons.
1803 — Dalton's Model
John Dalton, a English chemist, was the first scientist that made conclusions about the physical feature of an atom. Based on an ancient greek philosopher Democritus' idea about how matters are made up of tiny spheres, Dalton proposed his model of the tiny spheres which he called "atoms."
all matters are made up of atoms, which are indivisible
all atoms of a given element are identical
different elements have different atoms
atoms cannot be created or destroyed
atoms are rearranged during a chemical reaction
Dalton's Atomic Theory
Was Dalton Right?
Dalton's theory was not completely correct. Later experiments found out that atoms of the same element can differ by their number of neutrons, which were not discovered at Dalton's time yet. Also, atoms can be destroyed by nuclear reactions. However, his ideas were a big step to the exploration of atoms.
Dalton believed that the tiny building blocks of all matters are represent by spheres.
How Did Thomson Discovered the Electrons?
Improvement & Weakness
Thomson's discovery of the electrons proved one of Dalton's atomic theory incorrect
atoms can be divided into smaller particles
Thomson got awarded the Noble Prize for his work
However, his model did not explain how electrons are arranged in an atom.
Ernest Rutherford carried out a whole new idea of how an atom looks like with his Nuclear Model. After experimenting with radiation that was discovered by previous scientists, Rutherford discovered the nucleus in an atom.
Rutherford first discovered the 3 types of radiation:
alpha particles - positively charged particles
beta particles - electrons
gamma rays - high energy light
Gold Foil Experiment
Rutherford then used alpha particles to shoot at a piece of thin gold foil. According to the Plum Pudding model, the alpha particles should go directly through the gold foil. This is because if electrons are evenly spread within the atom, the entire atom would be neutral. Since alpha particles are positive, they would pass through the gold atoms.
However...
What actually happened was some alpha particles did pass through while some other deflected.
Some alpha particles deflected because they hit the
nucleus
!
Since the alpha particles are positive, the nucleus must be positive so they'll repeal. There Rutherford concluded that an atom has positive particles centered in a dense nucleus with electrons surrounding it.
Improvement
Rutherford improved the plum pudding model by discovering the nucleus and how electrons are orbiting around it
Rutherford discovered that the nucleus is dense with positively charged particles, which is later on called protons
Weakness
Scientists passed electricity through glass tubes that contain oxygen. They added a positive and a negative plate on the side of the glass tube. When they turned the power on, the beam deflected to the positive plate, which indicated that it has a negative charge.
Scientists were unsure what exactly that beam is. It could either be wave or particles. Then, they did the same experiment without the charge plates. This time, they added an object in the middle of the beam path.
Since only particles could not pass through an object, scientists conclude these rays as particles.
Scientists realized that those particles are found in all the gases they experimented with the cathode ray tube.

Thomson concluded that those particles, which are called
electrons
, are smaller parts of atoms.
Rutherford's model did not describe how electrons orbit around the nucleus
Basing on the Nuclear Model, Bohr discovered the energy level
n
that indicates which orbital the electrons are.
According to the Nuclear Model, electrons are moving freely within the vast atom. When energy is added to an atom, it would emit energy in the form of light, which create a continuous spectrum that represents all the frequencies of light.
However, that is not observed. Scientists realized that when an atom is energized, an emission spectrum produced, showing specific wavelengths and frequencies of light.
Based on the observation, Bohr concluded that electrons can only exist in specific orbits depending on the energy level.
n
stands for the energy level corresponding to each orbit. Electrons can jump between these orbits when energy is add to the atom.
Electrons would jump away from the nucleus when the atom absorb energy, then they would return to ground state by emitting energy.
ground state
When electrons return to the ground state, they emit energy by the form of light. Since they can only move between particular orbits, only certain colors in a spectrum will be produced.
Due the different number of electrons in each type of atom, a unique emission spectrum is produced by each element.
Improvement & Weakness
Bohr's model explained why an emission spectrum is produced by an energized atom
improved the nuclear model and solved the problems of how atoms can only emit light at certain wavelengths and frequencies
discovered the existence of
n
(energy level) that refers to the specific orbits
the model contradicts with the fact that atoms can only emit light at certain wavelengths and frequencies
Bohr's model only works for hydrogen that had only one electron
Uncertainty Principle
: it is impossible to know the exact position & momentum of an electron.
Since we cannot know where exactly the electrons are, we need to predict possible areas.
Quantum numbers are used to specify the possible location of electrons in an atom.
Erwin Schrödinger, an Austrian physicist, used math equations to describe the probability of finding electrons.
Using the 4 quantum numbers, we can predict where the electrons are likely to appear. With those numbers, we can form electron configuration that indicates the possible location of electrons for each element.

Hydrogen: 1s (
n
= 1;
l
= s)
Oxygen: 1s 2s 2p (
n
= 1,
l
= s) (
n
= 2,
l
= s;
l
= p,
m
= x y z)
1
2
2
4
l
Improvement
a more realistic model describing the position of electrons
improved Bohr's model of neat orbits with definite electron paths that is incorrect
created 4 quantum number that can be used to find areas with possible electrons
developed the electron configuration that is used to indicate the arrangement of electrons
Evolution of the Atom
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