Models of the Atom: A Historical perspective

Early Greek Theories Democritus 400 B.C. – His theory: thought matter could not be divided indefinitely There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut He named the smallest piece of matter “atomos,” meaning “not to be cut” He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? Democritus: you would eventually arrive at individual pieces that could not be cut. To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together.

Early greek theories Aristotle Very famous Greek philosopher Believed matter could be divided smaller and smaller pieces forever Believed that matter was made of a combination of four “elements”: earth, fire, water, air Aristotle was wrong. However, his theory persisted for 2000 years. He held a very strong influence on popular belief and his views on this were accepted for two thousand years.

What is an atom? PIECES OF CARBON An atom is the smallest particle that an element can be divided and still be that element. For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore. CARBON ATOM

John Dalton All matter is made of atoms 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios). All matter is made of atoms Atoms of the same element are identical Each element has different atoms Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. 2,000 years later, Dalton brings back Democritus’s idea of the atom. He performed many experiments to study how elements join together to form new substances.

William crookes Accidental discovery of the cathode ray Discovery led to the development of the television

History of the atom ELECTRON Joseph John Thompson found that atoms could sometimes eject a far smaller negative particle which he called an What particle did Thompson discover? ELECTRON

History of the atom PLUM PUDDING MODEL Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere This sphere contained a soup of positive charge to balance the electron's charge electrons similar to plums in a pudding or chocolate chips in ice cream. PLUM PUDDING MODEL Negative electron Positive charge spread over sphere

Robert Millikan Developed the charge of an atom… e-

Ernest Rutherford Rutherford shot alpha () particles at gold foil Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Most particles passed through So, atoms are mostly empty space Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass Rutherford disagreed with the plum pudding model. Devised an experiment to investigate the structure of positive and negative charges in the atom.

Bohr’s model Electrons orbit the nucleus in “shells” Electrons can be bumped up to a higher shell if hit by an electron or a photon of light

What is in an Atom? There are 3 types of subatomic particles. electrons (e–) protons (p+) neutrons (n0) They have: no charge Elements are often symbolized with their mass number and atomic number The periodic table give you all this information For now, round the mass # to a whole number # of protons = # of electrons = atomic number Mass number – atomic number = # of protons E.g. Oxygen: O 16 8 Nuclear Symbol

Atomic structure Particle Charge Mass Proton + charge 1 Neutron No charge Electron nil

He 4 2 ATOMIC STRUCTURE EXAMPLE Atomic mass the number of protons and neutrons in an atom Atomic number 2 the number of protons in an atom All of these pieces of information (numbers) are obtained from your periodic table Atomic number = number of electrons = number of protons 14

Practice Atomic Mass p+ n0 e– Ca Ar Br

Assignment- Copy the table right below your notes and fill in the information that is required of you. Use the periodic table to fill in this information. Symbol Atomic number Mass p+ n0 e- Calcium Neon Magnesium Sodium Lithium Oxygen Carbon Potassium Silicon Chlorine

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