What is matter ?

Democritus ((& Leucippus)) ~ 400 BCDemocritus ((& Leucippus)) ~ 400 BC

• Early philosophers thought that the material world must be made up of tiny indivisible particles called atomos, (meaning indivisible or uncuttable) that move through space

– Later Democritus could not answer questions from Aristotle regarding his notion of empty space and the “atomic” view of matter faded away for centuries

Pg 87-91

To describe the Democritus model of the

atom

To describe the Democritus model of the

atom

Linked 1 Linked 2

Scientists studying gases…you can feel the wind, thus air must

be composed of invisible particles of air

John Dalton - Devised an atomic theory based on the Greek idea of “atomos”

Atom-smallest particles of anelement that retain the chemical identity of the element

Pg38-39To describe the Dalton model of

the atom

To describe the Dalton model of

the atomLinked

Dalton’s Atomic Theory1) Each element is composed of extremely

small indivisible particles called atoms2) All atoms of a given element are identical

to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

3) Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

4) Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms.

Dalton’s Theory Explained several simple laws of chemical combination known at the time

• Law of constant composition (definite proportions)– in a compound the relative numbers and kinds of atoms are constant (pure H2O is 11% H and 89% O by mass)

• Law of conservation of matter – matter is neither created nor destroyed

total massbefore = total massafter

Law of Multiple Proportions

if 2 elements A&B combine to form more than one compound, different masses of B combine with the same mass of A in the ratio of small whole numbers

Pg38-39There is twice as much

oxygen by mass in H2O2 as there is in

H2O

A problem with Dalton’s Theory

Are atoms “really” the smallest particles we know?

Are atoms “really” indivisible?

What are atoms made up of?

Are atoms of the same elements “really” identical?

Cathode Rays an ElectronsJ J ThompsonTo describe the Thomson model of

the atom

To state relative charge & mass of

electron and proton

To describe the Thomson model of

the atom

To state relative charge & mass of

electron and proton

Cathode Rays

• Partially evacuated tubes produced radiation under high voltage

• Called cathode rays = initiated from cathode side

• Rays could not be seen but made materials fluoresce

Pg 92-94

Cathode Rays• Experiments showed cathode rays are deflected by

electric or magnetic fields• Thomson found that cathode rays are the same

regardless of what material the cathode is made of

• How do these findings help us answer the question…are cathode rays radiation (energy) or particles?

Link

Link 1

Electrons

• Cathode rays deflected by “field” = must be electrical particle not pure energy

• Deflected TOWARD positive field = must be negative particle

• Rays were the same no matter what substance cathode was made of = must be a fundamental particle (all the same)

“So how do all these particles that make up matter go together?

JJ Thomson’s “Plum Pudding” – Atomic Model

• Small negatively charged electrons embedded in a positively charged atom

• Like seeds (negative electrons) embedded in a watermelon (positive atom)

Pg42

Finding the Center“Ernest Rutherford and Gold Foil”

To describe the Rutherford model of

the atom

To state relative charge & mass of

electron, proton and neutron

To describe the Rutherford model of

the atom

To state relative charge & mass of

electron, proton and neutron

Radioactivity & Rutherford

Revealed three types of radiation:alpha (α) - beta (β) -gamma (γ) –

How does each type respond to electric/magnetic field? And what does

that imply about it?

Pg41-42

Rutherford’s Gold Foil Experiment• almost all α particles passed directly through

the foil without deflection• a small % slightly deflected, on the order of

1 degree, consistent with Thomson’s plum-pudding model

Pg42

                                                                                           

           

Rutherford’s Gold Foil Experiment

•BUT eventually … a small amount of scattering was observed at large angles & some particles were even scattered back in the direction from which they had come

What implicationsdoes this findinghave about the atom?

Pg42

Linked

Rutherford’s Atom

• most of the mass of each atom and all of its positive charge reside in a very small, extremely dense region

• Rutherford called this region the nucleus

Pg42

LiNkeD

ProtonsSubsequent experimental studies led to the discovery of positive particles (called protons) in the nucleus.

• Protons were discovered in 1919 by Rutherford.

• HOWEVER, when the nuclear mass was computed based on the number of protons, the mass value was much less than the actual mass…

• What does this then imply about the atom???

Pg42-43LiNkeD

The Neutron

…and subsequent experimental studies led to neutral particles (called neutrons) in the nucleus.

• Neutrons were discovered in 1932

by the British scientist James Chadwick (1891–1972).

Pg42-43

“Planetary” Model

• Every atom has an equal number of electrons and protons, and so atoms have no net electrical charge.

• The vast majority of an atom’s volume is the space in which the electrons reside.

• The electrons are attracted to the protons in the nucleus by the force that exists between particles of opposite electrical charge.

Pg43

LinKed

Rutherford’s Paradox• You cannot simply explain electron

as “orbiting” the nucleus…because (as per classical physics)

Pg 228-229

the e- will loose energy and be more attracted toward nucleus

Properties of Particles

• charge of electron is −1.602 × 10−19 C and that of a proton is +1.602 × 10 −19 C (charges of particles are expressed as multiples of this charge)

• mass of atoms very small so instead of grams we use amu (“atomic mass units”) (1 amu =1.66054 × 10−24 g )

• Atoms are extremely small (SI unit angstrom; Å is used) (1 Å = 0.0000000001m)

Pg43-44Link

Atomic Notation Pg45

To draw a diagram of an atom given its

atomic notation

To explain & illustrate the

concept of isotopes

To draw a diagram of an atom given its

atomic notation

To explain & illustrate the

concept of isotopes

PracticeHow many protons , neutrons and electrons are there in each?

Pg45

p+: p+: p+:n0: n0: n0:e-: e-: e-:

Another Problem with Dalton’s Theory

• Most elements are uniform mixtures of 2 or more unique substances called isotopes.

• Isotopes of an element have very similar chemical properties but their atoms have slightly different massesWhat about the different atoms, of the same element, could make their masses different???

Pg45-46

Link 1

Link 2

Link 3

Atomic Mass Scale

• Masses of atoms are measured in atomic mass units (amu)

• 1 amu = 1.66054x10-24g• 1g = 6.02214x1023amu

• Defined by assigning a mass of exactly 12amu to an atom of the 12C isotope of carbon.

Pg46-48

Average Atomic Weight

• Every sample of an element has the same isotopic composition (the average mass per atom is the same from sample to sample)

• So…we can use the masses of each isotope and its relative abundance to determine the average atomic weight for the element!

Pg47-48

Average Atomic Weight

Naturally occurring chlorine is 75.78% 35Cl which has an atomic mass of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine.

Solution The average atomic mass is found by multiplying the abundance of each isotope by its atomic mass and summing these products. Because 75.78% = 0.7578 and 24.22% = 0.2422 we have

Average atomic mass = (0.7578)(34.969 amu) + (0.2422)(36.966 amu) = 26.50 amu + 8.953 amu

= 35.45 amu

This answer makes sense: The average atomic mass of Cl is between the masses of the two isotopes and is closer to the value of 35Cl which is the more abundant isotope

Pg47-48

To calculate the atomic weight for an element given the

mass and abundance the

naturally occurring isotopes.

To calculate the atomic weight for an element given the

mass and abundance the

naturally occurring isotopes.

Practice

Example: Copper occurs naturally as two isotopes Cu-63 and Cu-65. Given the atomic mass and the % natural abundance of each isotope below, calculate atomic weight of Cu (as on the periodic table).

Isotope mass natural abundanceCu-63 62.930 amu 69.09%Cu-65 64.928 amu 30.91%

Radioactivity – spontaneous emission of radiation (high energy & particles)

Pg41-42

To state the properties of alpha, beta and gamma

radiation

To state the properties of alpha, beta and gamma

radiation

• In a nuclear reaction, In a nuclear reaction, protons and neutrons are protons and neutrons are rearranged rearranged

• In a chemical reaction, In a chemical reaction, electrons rearrangedelectrons rearranged

Often, at least one isotope is unstableunstable – the – the strong force of the nucleus is overcome by strong force of the nucleus is overcome by positive-positive repulsion of the+ protonspositive-positive repulsion of the+ protons

Radioactive Decay - breakdown of atom

– Nucleus becomes unstable– Radiation is emitted until nucleus

becomes stable– Stable, non-radioactive atom is formed

– Can occuroccur naturallynaturally, or be causedcaused by bombarding the atom with energy

LinKeD

Particles

Charge

Mass Stopped By

AlphaAlpha 2 p+

2 no

no e-

+2 4 Paper or light clothing

BetaBeta 1 e-

no p+ no no

-1 0 Heavy Clothing, Sunscreen or Lead (1cm thick)

GamGammama

None None 0 Concrete or Lead (10cm thick)

3 Main Types of Radiation3 Main Types of Radiation

101

e

242

00

Penetrating AbilityPenetrating Ability

• HALF-LIFE is the is the time that it takes HALF-LIFE is the is the time that it takes for 1/2 a sample to decomposefor 1/2 a sample to decompose

• The rate of a nuclear transformation depends The rate of a nuclear transformation depends only on the “reactant” (original compound) only on the “reactant” (original compound) concentration. concentration.

• For each duration (half-life), For each For each duration (half-life), For each duration (half-life), one half of the substance duration (half-life), one half of the substance decomposesdecomposes

To relate the amount of radioactive sample, or its

radioactivity, to a given half-life.

To relate the amount of radioactive sample, or its

radioactivity, to a given half-life.

For exampleFor example

Ra-234 has a half-life of 3.6 days; if Ra-234 has a half-life of 3.6 days; if you start with 50 grams of Ra-234, you start with 50 grams of Ra-234, how much do you have …how much do you have …

After 3.6 days : After 3.6 days :

After 7.2 days :After 7.2 days :

After 10.8 days :After 10.8 days :