Early ThoughtsEarly Thoughts

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– Empedocles stated that matter was made of 4 elements: earth, air, fire, water.

– Democritus (about 470-370 B.C.) thought that all forms of matter were divisible into tiny indivisible particles. He called them “atoms” from the Greek “atomos” indivisible.

• The earliest models of the atom were developed by the ancient Greek philosophers.

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• Aristotle (384-322 B.C.) rejected the theory of Democritus and advanced the Empedoclean theory–Aristotle’s influence dominated

the thinking of scientists and philosophers until the beginning of the 17th century

MATTER IS MADE UP OF SOMETHING

• Nature of Matter• Theories1. Matter is Continous2. Matter is not continous; can be cut

a. Democritus argued that Silver can be cut to yield its smallest particles called atomos (not Silver anymore

b. Dalton argued farther that Silver can be cut to yield its smallest particles called atoms (they are still Silver)

Laws of Chemical Composition

• 1. Law of Conservation of Mass

• 2. Law of Constant Composition

LAW OF Conservation of Mass

• In an ordinary chemical reaction mass cannot be created nor destroyed, it can only be transformed into another form.

Law of Constant Composition

• When elements combine to form compounds they will always do so in definite amounts.

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Composition of Compounds

Composition of Compounds

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The Law of Definite Composition

A compound always contains two or more elements combined in a definite proportion by mass.

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• The percent by mass of hydrogen in water is 11.2%.

• The percent by mass of oxygen in water is 88.8%.

• Water always has these percentages. If the percentages were different the compound would not be water.

• Water always contains the same two elements: hydrogen and oxygen.

Composition of Water

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• The percent by mass of hydrogen in hydrogen peroxide is 5.9%.

• The percent by mass of oxygen in hydrogen peroxide is 94.1%.

• Hydrogen peroxide always has these percentages. If the percentages were different the compound would not be hydrogen peroxide.

• Hydrogen peroxide always contains the same two elements: hydrogen and oxygen.

Composition of Hydrogen Peroxide

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The Law of Multiple Proportions

Atoms of two or more elements may combine in different ratios to produce more than one compound.

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Mass Hydrogen(g)

Mass Oxygen(g)

Water 1.0 8.0

Hydrogen Peroxide 1.0 16.0

mass of oxygen in hydrogen peroxide 16g 2= = ¹

mass of oxygen in water 8g 1

Combining Masses of Hydrogen and Oxygen

Hydrogen peroxide has twice as much oxygen (by mass) as does water.

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• The formula for water is H2O.• The formula for hydrogen peroxide is

H2O2.

• Hydrogen peroxide has twice as many oxygens per hydrogen atom as does water.

Combining Ratios of Hydrogen and Oxygen

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Dalton’s Atomic Theory

1. Matter is made up of smallest particles called atoms2. The atoms of a given element are essentially the same with

only slight variations in their masses

Hydrogen cut

protium

deuterium

tritium

H1. ....

1

H2. ....

1

H3.....

1

H1.0079

1

atomic number

atomic mass

175.1

Dalton’s atoms were individual particles.

Atoms of each element are alike in mass and size.

Dalton’s Atomic Theory

3. The atoms of an element are different from the atoms of another element

4. In a given chemical reaction atoms cannot be destroyed

5. Compounds are formed from 2 or more elements joining together.

195.1

Dalton’s atoms were individual particles.

Atoms of different elements are not alike in mass and size.

. Compounds are formed from 2 or more elements joining together.

Matter Atoms Sub-atomicparticles

quarks

Evidenced by

Can be cut

Can be cut

Can be cut

ProtonNeutronelectron

Up quark +2/3

Down quark -1/3

DaltonExperimental evidences

Mass accelerationexperiments

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Subatomic Partsof the Atom

Subatomic Partsof the Atom

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An atom is very Small

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The diameter of an atom is 0.1 to 0.5 nm.This is 1 to 5 ten billionths of a meter.

If the diameter of this dot is 1 mm then 10 million hydrogen

atoms would form a line across the dot.

Even smaller particles than atoms exist. These are called subatomic particles.

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Subatomic ParticlesSubatomic Particles

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ElectronElectron

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• In 1875 Sir William Crookes invented the Crookes tube.

• Crookes tubes experiments led the way to an understanding of the subatomic structure of the atom.

• Crookes tube emissions are called cathode rays.

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In 1897 Sir Joseph Thompson demonstrated that cathode rays:

• travel in straight lines.• are negative in charge.• are deflected by electric and magnetic

fields.• produce sharp shadows• are capable of moving a small paddle

wheel.

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This was the discovery of the fundamental unit of charge

– the electron.

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ProtonProton

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• Eugen Goldstein, a German physicist, first observed protons in 1886:

• Thompson determined the protons’ characteristics.

• Thompson showed that atoms contained both positive and negative charges.

• This disproved the Dalton model of the atom which held that atoms were indivisible.

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NeutronNeutron

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• James Chadwick discovered the neutron in 1932.

• Its actual mass is slightly greater than the mass of a proton.

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Subatomic Particle

Symbol Charge Mass (a.m.u.)

Location Significance

proton P+ + 1.0072 nucleus Mass of the atom

neutron N0 0 1.008665 nucleus Mass of the atom

electron e- - 0.000054 space Size of the atom

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The Rutherford Experiment

The Rutherford Experiment

Rutherford in 1911 performed experiments that shot a stream of alpha particles at a gold foil.

Most of the alpha particles passed through the foil with little or no deflection.

He found that a few were deflected at large angles and some alpha particles even bounced back.

Properties of the Nucleus : Rutheford Experiment

1. Tiny2. Positively charged3. Very densed (heavy)

An atom can be identified by 2 numbers

1. Atomic number (Z)- number of positive charge of an atom;number of protons; number of electrons

an atom is neutral

2. Mass number (A)- number of protons and number of neutrons contained in the

nucleus A= Z + number of neutrons Number of neutrons = A-Z

Whole number equivalent of atomic mass

An atom undergoes change to become charged entities

( ion ; cation and or anion,

electropositive, electronegative species)

Cation – positively charge ion; more protons than electrons; electrons were lost

Anion- negatively charge ion; less protons than electrons; electrons were gained

Na+1

P+ = 11

e- = 10

n0 = 12

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22.9 ~ 23

Cl17

35.45~35

p+ = 17

e- = 18

n= 18

415.4

When one or more electrons are lost from an atom, a cation is formed.

425.4

When one or more electrons are added to a neutral atom, an anion is formed.

Fundamental Facts about the Subatomic Particles

1. Every proton and neutron are found in the center of the atom called the nucleus

2. The number of protons is called the atomic number3. The mass number (atomic mass) is the number of

protons and the number of neutrons contained in the nucleus

4. In an electrically charged neural atom the number of protons is equal to the number of electrons

5. Electrons are located in the space about the nucleus