ATOMIC THEORY
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THE GREEK CONCEPT OF ATOMOS: THE INDIVISIBLE
ATOM Around 440 BC, Leucippus originated the atom
concept.His pupil, Democritus (c460-371 BC) extended it There are five major points to their atomic idea.
All matter is composed of atoms, which are too small to be seen. These atoms CANNOT be further split into smaller portions.
There is a void, which is empty space between atoms. Atoms are completely solid. Atoms are homogeneous, with no internal structure. Atoms can differ in size, shape, and weight
Spoke openly against the concept— the atom concept diminished
Only a few scholars gave it much thought. The Catholic Church accepted Aristotle's position
— equated atomistic ideas with Godlessness It was not until 1660 that Pierre Gassendi succeeded
in separating the two not until 1803 that John Dalton put the atom on
a solid scientific basis.
ARISTOTLE (384-322 BC)
• In 1808, John Dalton developed an atomic theory.
• Dalton believed that a few kinds of atoms made up all matter.
• According to Dalton, elements are composed of only one kind of atom and compounds are made from two or more kinds of atoms.
JOHN DALTON’S ATOMIC THEORY
1. All matter is composed of t iny particles called atoms.
2. All atoms of a given element have identical chemical properties that are characteristic of that element.
3. Atoms form chemical compounds by combining in whole-number ratios.
4. Atoms can change how they are combined, but they are neither created nor destroyed in chemical reactions.
5. Atoms of a given element are identical in their physical and chemical properties.
JOHN DALTON 1803-1807MODERN ATOMIC THEORY
John Dalton’s Atomic Theory
Almost right. A good start.
Structure of the atom after Dalton (ca. 1810)
very small
Atoms are neither created nor destroyed during physical or chemical processes. In a closed system
LAW OF CONSERVATION OF ATOMS & MASS
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LAW OF CONSERVATION OF ATOMS & MASS
Mass is neither created nor destroyed during physical or chemical processes.
LAW OF CONSERVATION OF ATOMS & MASS
States that two samples of a given compound are made of the same elements in exactly the same proportions by mass regardless of the sizes or sources of the samples.
So for example, every molecule of ethylene glycol is made of the same number and types of atoms.
THE LAW OF DEFINITE PROPORTIONS
A molecule of ethylene glycol has the formula C2H6O2, so the law of definite proportions tells you that all other molecules of ethylene glycol have the same formula.
So where we have 1mg or 1000kg of Ethylene glycol the ratio is the same
THE LAW OF DEFINITE PROPORTIONS
51.56% oxygen, 38.70% carbon, and 9.74% hydrogen
• The law of multiple proportions states that when two elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers.
THE LAW OF DEFINITE PROPORTIONS
Example: Nitrogen Oxides I & II
Nitrogen Oxide I : 46.68% Nitrogen and 53.32% OxygenNitrogen Oxide II : 30.45% Nitrogen and 69.55% Oxygen
in 100 g of each Compound: g O = 53.32 g & 69.55 g
g N = 46.68 g & 30.45 g g O /g N = 1.142 & 2.284Cmpd II 2.284 2
Cmpd I 1.142 1=
Cmpd I Cmpd II
If elements A and B react to form two compounds, the diff erent masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers:
LAW OF MULTIPLE PROPORTIONS
Proved that an atom can be divided into smaller parts
While experimenting with cathode-ray tubes, discovered corpuscles, which were later called electrons
Stated that the atom is neutral In 1897, 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
Won a Nobel Prize
Image taken from: www.wired.com/.../news/2008/04/dayintech_0430
J.J. Thomson (1856 – 1940)
J.J. Thomson (1897): Cathode Rays
Atoms subjected to high voltages give off cathode rays.
J.J. Thomson: Cathode Rays
Cathode rays can be deflected by a magnetic field.
Cathode rays are negatively charged particles (electrons).
Electrons are in atoms.
J.J. Thomson – The Electron
Structure of the atom after Thomson (ca. 1900)
“Plum pudding” model: Negative
electrons are embedded in a
positively charged mass.
Positively charged mass
Electrons (-)
Unlike electrical charges attract, and
that is what holds the atom together.
In 1909, performed the Gold Foil Experiment and suggested the following characteristics of the atom:o It consists of a small core, or
nucleus, that contains most of the mass of the atom
o This nucleus is made up of particles called protons, which have a positive charge
o The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space
Did extensive work on radioactivity (alpha & beta particles, gamma rays/waves) and was referred to as the “Father of Nuclear Physics”
Won a Nobel Prize Was a student of J.J. Thomson Was on the New Zealand $100 bill
Image taken from: http://www.scientific-web.com/en/Physics/Biographies/ErnestRutherford.html
Ernest Rutherford (1871 – 1937)
Ernest Rutherford (1910) Scattering experiment: firing alpha particles at a gold foil
The Nuclear Atom
Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged! Thompson’s model could not be correct.
Ernest Rutherford The Nucleus and the Proton
Structure of the atom after Rutherford (1910)
The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus.
Electrons (-) are now outside the nucleus.
The positively charged particles in
the nucleus are protons.
In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths. Additionally, the electrons can jump from a path in one level to a path in another level (depending on their energy)
Won a Nobel Prize Worked with Ernest
Rutherford
Image taken from: commons.wikimedia.org/wiki/File:Niels_Bohr.jpg
Niels Bohr (1885 – 1962)
3 p+
4 n02e– 1e–
Li shorthand
Bohr - Rutherford diagrams
Putting all this together, we get B-R diagrams To draw them you must know the # of protons, neutrons, and
electrons (2,8,8,2 fil l ing order) Draw protons (p+), (n0) in circle (i.e. “nucleus”) Draw electrons around in shells
2 p+
2 n0
He
3 p+
4 n0
Li
Draw Be, B, Al and shorthand diagrams for O, Na
11 p+12 n°
2e– 8e– 1e–
Na
8 p+8 n°
2e– 6e–
O
4 p+5 n°
Be
5 p+6 n°
B
13 p+14 n°
Al
In 1926, he 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
Did extensive work on the Wave formula Schrodinger equation
Won a Nobel Prize
Image taken from: nobelprize.org/.../1933/schrodinger-bio.html
Erwin Schrodinger (1887-1961)
Realized that the atomic mass of most elements was double the number of protons discovery of the neutron in 1932
Worked on the Manhattan Project
Worked with Ernest Rutherford
Won a Nobel Prize Image taken from:
www.wired.com/.../news/2009/02/dayintech_0227
James Chadwick (1891 – 1974)
James Chadwick – The Neutron
Structure of the atom after Chadwick (1932)
In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons.
Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics)
The positively charged particles in
the nucleus are protons.
nn+
Structure of the Atom
proton (+)
neutron
nucleus - responsible for the mass of the atom, positively charged
electrons - responsible for the volume and size of the atom, negatively charged
10-14 m
10-10 m
Subatomic Particles
Protons and electrons are the only particles that have a charge.
Protons and neutrons have essentially the same mass.
The mass of an electron is so small we ignore it.
Atomic Facts
Feature Size Mass1 amu = 1 atomic mass unit = 1.66054 x 10-24 g
Electrons are outside the nucleus in quantized energy states called orbitals.
proton (+) 10-15 m 1.0073 amu
neutron (0)
10-15 m 1.0087 amu
electron (-)
10-18 m ???
0.0006 amu
+ nn