models of the atom · dalton’s atomic theory in the early 1800s, the english chemist john dalton...
TRANSCRIPT
AIM: Models of the
Atom
DO NOW:
HW: ON A SEPARATE SHEET OF PAPER MAKE A TIMELINE OF THE HISTORY AND DEVELOPMENT OF THE
ATOM. INCLUDE:
1. DALTON’S MODEL, J. J. THOMPSON’S MODEL, RUTHERFORD MODEL, BOHR MODEL, ELECTRON
CLOUD MODEL
2. YEAR AND SHORT DESCRIPTION
3. IMAGE OR DRAWING OF MODEL
Democritus’s Atomic Philosophy
Proposed an Atomic Theory which states
that all atoms are small, hard, indivisible
and indestructible particles made of a
single material formed into different
shapes and sizes.
He named the smallest piece of matter
“atomos,” meaning “not to be cut.”
Dalton’s Atomic Theory
In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
Dalton’s Atomic Theory
Based on Dalton’s Atomic Theory (5
postulates), most scientists in the 1800s
believed that the atom was like a tiny solid
ball that could not be broken up into parts.
Dalton was credited for the three Atomic Laws
that were proven after his time.
Dalton’s Atomic Laws
1. Law of Conservation of Mass
Matter cannot be created or destroyed in any
physical or chemical process, just transferred.
2. Law of Constant Composition
When atoms combine to form molecules, the
ratio of atoms is constant.
Example – H2O will always have 2 times as
many Hydrogen atoms as Oxygen.
Dalton’s Atomic Laws
3. Law of Multiple Proportions – if two elements can
combine to form more than one compound, then the
ratio of the second element combined with a certain
mass of the first element is always a ratio of small whole
numbers.
Formula
Ratio of N:O
Example:
CO vs. CO2
J. J. Thomson’s Plum Pudding
Model
Used cathode rays to prove that
Dalton’s Solid-ball model could be
broken into smaller particles
Thomson is credited with discovering
electrons
J. J. Thomson’s Plum Pudding
A cathode ray is a tube that has a
piece of metal, called an electrode, at
each end. Each electrode is connected to a power
source (battery).
When the power is turned on, the electrodes
become charged and produce a stream of charged
particles. They travel from cathode, across the tube to
the anode.
J. J. Thomson’s Plum Pudding
Model
Stated that the atom is neutral
In 1897, he proposed the Plum Pudding
Model which states that atoms mostly
consist of positively charged material with
negatively charged particles (electrons)
located throughout the positive material
Rutherford Model
Rutherford’s experiment Involved firing a stream of tiny positively charged particles (alpha particles) at a thin sheet of gold foil (2000 atoms thick)
Most of the positively charged particles passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged particles, however,
did bounce away from the gold sheet as if they had hit
something solid. He knew that positive charges repel
positive charges.
Rutherford Model
Gold Foil Experiment and suggested the following characteristics of the atom:
It consists of a small core, or nucleus, that contains most of the mass of the atom
This nucleus is made up of particles called protons, which have a positive charge
The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space
Bohr Model In 1913, the Danish scientist Niels Bohr proposed an
improvement. In his model, he placed each electron in
a specific energy level.
According to Bohr’s atomic model, electrons move in
definite orbits around the nucleus, much like planets
circle the sun. These orbits, or energy levels, are located
at certain distances from the nucleus.
Additionally, the electrons can jump from a path in one
level to a path in another level (depending on their
energy)
Bohr’s ModelHe proposed the following
Protons and neutrons are in the
nucleus
Electrons can only be certain
distances from the nucleus
The electrons orbit the nucleus at fixed energy levels
The electrons must absorb or emit a fixed amount of
energy to travel between these energy levels
Electron Cloud Model (Wave
Mechanical Model)
In 1926, Erwin Schrodinger further explained the nature of electrons in an atom by stating that the
exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds;
electron clouds are places where the electrons are likely to be found
Electron Cloud Model (Wave
Mechanical Model) Depending on the electron’s energy they are locked into a
certain area in the cloud.
Electrons with the lowest energy are found in the energy level closest to the nucleus
Electrons with the highest energy are found in the outermost
energy levels, farther from the nucleus.
Today’s atomic model is based on the principles of wavemechanics.
Summary
Who is the father of atomic theory?
Dalton
What was the first model of the atom?
Dalton’s Tiny Ball Model
What are Dalton’s 3 Laws?
Law of Conservation of Mass, Law of Constant
Composition, Law of Multiple Porportion
Summary
How were Thomson’s and Dalton’s model
different?
Dalton’s model was 1 sphere that cannot be
divided, Thomson had the plum pudding where
electrons are randomly spread throughout a
positively charged sphere.
What did Thomson find out?
Atoms have electrons, they have a negative
charge
Summary
What were Rutherford’s conclusions from the
Gold Foil Experiment?
Atom has a positively charged nucleus
electrons are outside,
atoms are mostly empty
Nucleus contains most of the mass.
Summary Bohr Model of the Atom
Electrons go around the nucleus in concentric
circular orbits
Electron Cloud Model
The electron does not move in a definite path.
An electron is in an orbital which is the most
probable location where the electron is
The most dense part of the cloud is the most
probable place to find the electron