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”

Democritus

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?

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.

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.

PIECES OF CARBON

CARBON ATOM

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.

Accidental discovery of the cathode ray Discovery led to the development of

the television

Joseph John Thompson

found that atoms could sometimes eject a far smaller negative particle which he called an

ELECTRONELECTRON

What particle did Thompson discover?

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

PLUM PUDDING

MODEL

electrons similar to plums in a pudding or chocolate chips in ice cream.

Negative electron

Positive charge spread oversphere

Developed the charge of an atom… e-

• 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

Radioactive substance path of invisible -

particles

Rutherford shot alpha () particles at gold foil

Lead block

Zinc sulfide screen Thin gold foil

• 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

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: O16 8 Nuclear Symbol

Particle Charge Mass

Proton + charge 1

Neutron No charge 1

Electron - Charge nil

ATOMIC STRUCTURE EXAMPLEATOMIC STRUCTURE EXAMPLE

the number of protons in an atom

the number of protons and neutrons in an atom HeHe

22

44Atomic mass

Atomic number

All of these pieces of information (numbers) are obtained from your periodic table

Atomic number = number of electrons = number of protons

Br

Ar

Ca

e–n0p+MassAtomic

Practice

Symbol Atomic number

Mass p+ n0 e-

Calcium

Neon

Magnesium

Sodium

Lithium

Oxygen

Carbon

Potassium

Silicon

Chlorine

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.