Early Greek Theories of Early Greek Theories of MatterMatter

Greek philosophers first proposed an Greek philosophers first proposed an atomic theory of matter in the 5atomic theory of matter in the 5thth

century B.C. They believed that all century B.C. They believed that all substances were composed of small, substances were composed of small,

indivisible particles called atoms They indivisible particles called atoms They were thought to be different sizes, have were thought to be different sizes, have

regular shapes and be in constant regular shapes and be in constant motion. Empty space was through to motion. Empty space was through to

exist between atoms. Aristotle severely exist between atoms. Aristotle severely criticized this theory arguing that atoms criticized this theory arguing that atoms

in continuous motions in a void is in continuous motions in a void is illogical (his opinion might have been illogical (his opinion might have been

colored by the fact that belief in a void colored by the fact that belief in a void at the time was considered to be at the time was considered to be

atheistic)atheistic)

Aristotle’s TheoryAristotle’s Theory

Aristotle developed a theory of Aristotle developed a theory of matter based on the idea that matter based on the idea that all matter is made up of four all matter is made up of four basic substances – earth, air, basic substances – earth, air,

fire and water. He believed that fire and water. He believed that each basic substance had each basic substance had

different combinations of four different combinations of four specific qualities – dry, hot, cold specific qualities – dry, hot, cold

and moist. His theory of the and moist. His theory of the structure of matter was the structure of matter was the prevailing model for almost prevailing model for almost 2000 years, including the 2000 years, including the

period of alchemy in the middle period of alchemy in the middle ages. ages.

EarthAir

Fire

Water

Hot

ColdWet

Dry

Dalton’s Atomic TheoryDalton’s Atomic Theory

Dalton introduced and atomic theory of Dalton introduced and atomic theory of matter in 1803, which came to replace matter in 1803, which came to replace

Aristotle’s model. His theory states:Aristotle’s model. His theory states:

• All matter is composed of tiny, indivisible All matter is composed of tiny, indivisible particles called atomsparticles called atoms

• Atoms of an element have identical Atoms of an element have identical propertiesproperties

• Atoms of different elements have different Atoms of different elements have different propertiesproperties

• Atoms of two or more elements can Atoms of two or more elements can combine in constant ratios to form new combine in constant ratios to form new

substancessubstances

Dalton’s Atomic TheoryDalton’s Atomic Theory

Dalton’s theory was successful in Dalton’s theory was successful in explaining the laws of conservation explaining the laws of conservation of mass, definite composition and of mass, definite composition and

multiple proportions. Since atoms are multiple proportions. Since atoms are indivisible, and are rearranged only indivisible, and are rearranged only when compounds are formed, you when compounds are formed, you

must end up with the same number must end up with the same number and kinds of atoms after a chemical and kinds of atoms after a chemical

reaction.reaction.

Development of Atomic TheoryDevelopment of Atomic Theory

In the late 1800’s, J.J. Thompson found In the late 1800’s, J.J. Thompson found evidence for the existence of negatively evidence for the existence of negatively charged particles that could be removed charged particles that could be removed from atoms. In 1987, he postulated the from atoms. In 1987, he postulated the

existence of electrons, subatomic particles existence of electrons, subatomic particles possessing a negative charge. With the possessing a negative charge. With the

new idea, Thompson developed a model of new idea, Thompson developed a model of the atom that has electrons evenly the atom that has electrons evenly

distributed inside the spherical positive distributed inside the spherical positive part of the atom. In 1904, H. Nagaoka part of the atom. In 1904, H. Nagaoka

represented the atom as a large, positively represented the atom as a large, positively charge sphere surrounded by a ring of charge sphere surrounded by a ring of

negative electrons.negative electrons.

Development of Atomic TheoryDevelopment of Atomic Theory

It was Ernest Rutherford who did work (at It was Ernest Rutherford who did work (at McGill University) to test the current McGill University) to test the current

atomic models. He shot alpha particles atomic models. He shot alpha particles through very thin pieces of gold foil. He through very thin pieces of gold foil. He

predicted that all the particles would travel predicted that all the particles would travel through the foil and although most did, a through the foil and although most did, a

small percentage were deflected. He small percentage were deflected. He deduced that an atom must contain a tiny, deduced that an atom must contain a tiny, positively charge core (the nucleus) which positively charge core (the nucleus) which

is surrounded yb a mostly empty space is surrounded yb a mostly empty space containing negative electrons.containing negative electrons.

Development of Atomic TheoryDevelopment of Atomic Theory

In 1914, Rutherford coined the word “proton” for In 1914, Rutherford coined the word “proton” for the smallest unit of positive charge. It was one of the smallest unit of positive charge. It was one of his students (H.G.J Moseley) who showed that the his students (H.G.J Moseley) who showed that the positive charge in the nucleus of atoms increases positive charge in the nucleus of atoms increases by one unit in progressing from each element to by one unit in progressing from each element to the next in the periodic table. This discovery led the next in the periodic table. This discovery led

Moseley to the concept of atomic number, Moseley to the concept of atomic number, defined as the number of protons in the nucleus defined as the number of protons in the nucleus of an atom. Originally the elements were listed of an atom. Originally the elements were listed

and numbered in order of increasing atomic and numbered in order of increasing atomic mass, this sequence now had an explanation, the mass, this sequence now had an explanation, the periodic table is a list of elements in order of the periodic table is a list of elements in order of the

number of protons in the nucleusnumber of protons in the nucleus

Development of Atomic TheoryDevelopment of Atomic Theory

The estimated mass of a proton and the number of The estimated mass of a proton and the number of protons could not account for all the atomic mass. protons could not account for all the atomic mass.

Rutherford predicted the existence of a neutral Rutherford predicted the existence of a neutral particle similar in mass to a proton. In 1932 particle similar in mass to a proton. In 1932

James Chadwick demonstrated that atomic nuclei James Chadwick demonstrated that atomic nuclei must contain heavy neutral particles as well as must contain heavy neutral particles as well as

positive particle in order to account for the positive particle in order to account for the atom’s mass. These were called neutrons. An atom’s mass. These were called neutrons. An atomatom, therefore is composed of a nucleus , therefore is composed of a nucleus

containing protons and neutrons, and a number containing protons and neutrons, and a number of electrons equal to the number of protons. An of electrons equal to the number of protons. An

atom is electrically neutralatom is electrically neutral

IsotopesIsotopes

Frederick Soddy, a colleague of Rutherford’s Frederick Soddy, a colleague of Rutherford’s at McGill, was the first to propose that the at McGill, was the first to propose that the number of neutrons can vary from atom to number of neutrons can vary from atom to atom, even in atoms of the same element. atom, even in atoms of the same element.

An An isotopeisotope is a form of an element in is a form of an element in which the atoms have the same number of which the atoms have the same number of protons as all other form of that element, protons as all other form of that element,

but a different number of neutrons.but a different number of neutrons.

CarbonCarbon

All carbon atoms (atomic number 6) have 6 All carbon atoms (atomic number 6) have 6 protons in the nucleus. The most common protons in the nucleus. The most common

form of carbon, carbon-12, also has 6 form of carbon, carbon-12, also has 6 neutronsneutrons

Carbon-14, however, is an isotope of carbon Carbon-14, however, is an isotope of carbon because it has 6 protons and 8 neutrons in because it has 6 protons and 8 neutrons in

the nucleus.the nucleus.

..

IsotopesIsotopes

Different isotopes of the same element Different isotopes of the same element have the same chemical properties have the same chemical properties

but different masses.but different masses.

All elements exist naturally as a All elements exist naturally as a mixture of isotopes. The mass mixture of isotopes. The mass number is based on the most number is based on the most

common isotopes.common isotopes.

Summary of the Rutherford Summary of the Rutherford ModelModel

An atom contains equal numbers of An atom contains equal numbers of negatively charged electrons and positively negatively charged electrons and positively charged protonscharged protons

Most of the mass of the atom and all of its Most of the mass of the atom and all of its positive charge is contained in a tiny core positive charge is contained in a tiny core called the nucleuscalled the nucleus

The nucleus contains protons and neutrons The nucleus contains protons and neutrons that have approximately the same mass that have approximately the same mass eacheach

The number of protons is the same as the The number of protons is the same as the atomic numberatomic number

The total number of protons and neutrons is The total number of protons and neutrons is the same as the mass numberthe same as the mass number