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

Video

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