Atomic Theory and Structure Flashcards
Week 3
Greek philosopher that proposed the idea that all matter is made up of atoms
Democritus - 5th Century B.C.
comes from the Greek word “atomos” which means indivisible.
Atom
Since the atom is too small to be seen even with the most powerful microscopes
Scientists’ reliance on models
structure or behavior of atoms cannot be clearly seen
Even the world’s best microscope
British schoolmaster and chemist formulated a precise definition of the indivisible building blocks of matter called “atomos”
John Dalton (18th Century)
composed of extremely small particles
Elements
small particles
atoms
are identical
Atom of elements
is different from the atom of all the other elements
Atom of one element
are composed of atoms of more than one element. The relative number of atoms of each
element in a given compound is always the same.
Compounds
only involve the rearrangement of atoms.
Chemical reactions
Atoms are
not created or destroyed in chemical reactions.
The Classical or Dalton’s Atomic Theory
- All matter is made up of tiny, indestructible unit particles called atoms.
- The atoms of a given element are all alike.
- During chemical reactions, atoms may combine or a combination of atoms may break down, but the atoms themselves are unchanged.
- When atoms combine to form molecules, they unite in small whole numbered ratios as 1:1, 1:2, 1:3, 2:
when two elements combine to form more than one compound, the mass of one element, which combines with a fixed mass of the other element, will always be ratios of whole numbers
Law of Multiple Proportions
in a chemical reaction mass is neither created nor destroyed
Law of Conservation of Mass
British scientist who observed many properties of rays, including the fact that the nature of the rays is the same regardless of the identity of the cathode material. He concluded that cathode rays are not waves but are particles with mass and this led to the discovery of electron
Joseph John Thomson ( mid 1800’s)
a pioneer in modern physics, discovered the electron in 1895, revolutionizing existing theories of atomic structure.
SIR JOSEPH JOHN THOMSON
recognized for his investigations into the conduction of electricity through gases and physics
Thomas
which earned him the 1906 Nobel Prize for
Gases (Thomson)
and for his work on the mathematics of the
electromagnetic- field theory proposed by James Maxwell.
Physics (Thomson)
measured mass/charge of e-
JJ Thomson (1906 Nobel Prize in Physics)
a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface
Cathode Ray Tube
positive charge spread over the entire sphere
Thomson’s Model of the Atom
pictured the atom as consisting of small, negatively charged particles of electricity, or electrons, embedded in a heavier, larger sphere of positive electrical charge, like plums in a pudding
Joseph John Thomson (20th Century)
first direct and compelling measurement of the electric charge of a single electron
Millikan’s Experiment
- atoms positive charge is concentrated in the nucleus
- proton (p) has opposite (+) charge of electron
- mass of p is 1840 x mass of e- (1.67 x 10-24 g)
Ernest Rutherford’s Experimental Design
- experimented with a thin gold sheet, bombarded with alpha particles leading to the
discovery of the nucleus - a British physicist from New Zealand, devised (1911) a solar-system model of the atom
in which electrons orbit a small, heavy, positively charged central core, or nucleus.
ERNEST RUTHERFORD (1908 Nobel Prize
in Chemistry)
described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated.
Rutherford’s Model of the Atom
developed the theory that an electron in an atom could gain or lose energy by absorbing or emitting a quantity of energy in which events, it jumps to a higher or lower orbit. As a result, the atom was visualized as a nucleus surrounded by electrons in different orbits
Niels Bohr (1913)
Remained relatively unchanged for over 2,000 years. In the late 18th century new discoveries were made that led to a better understanding of atoms and chemistry.
Many scientists since that time
have contributed new evidence
for the Atomic - Molecular
Theory..
The concept of atoms as proposed by
Democritus
- is one of many scientists that have given us a better understanding of Atoms.
- In 1913 the Danish physicist NIELS BOHR revised the planetary model by assuming that electrons can occupy only certain orbits
at specific distances from the nucleus. - Electrons can jump from alow-energy orbit near the nucleus to orbits of higher energy by absorbing energy (green trails).
- When the electrons return to a lower energy level (purple trails), they release the excess energy in the form of radiation of a characteristic wavelength, such as visible light.
NIELS BOHR
reveals the configuration of the atom’s 11 electrons. The two inner orbits are completely filled with two and eight electrons, respectively. Only the eleventh electron, which occupies the unfilled outer orbit, takes part in
chemical bonding.
A solar-system model of the sodium atom
the electrons are pictured as occupying shells, or layers of space, that are centered on a positively charged nucleus
In a modern representation of the sodium atom
Emphasizes proton, neutron and electron distribution; does not accurately depict current
accepted model of atomic structure.
Solar system depiction of atomic
structure.
are depicted as clouds of negative charge surrounding the nucleus. The density of the small dots is related to the probability of finding an electron at a particular
location.
Electrons
electrons orbit the nucleus in stable orbits. Although not a completely accurate model, it
can be used to explain absorption and emission.
- Electrons move from low energy to higher energy orbits by absorbing energy.
- Electrons move from high energy to lower energy orbits by emitting energy.
- Lower energy orbits are closer to the nucleus due to electrostatics.
Bohr model
