chemistry chapter 3 · democritus and leucippus’s theory of matter: a) point #1 - all matter is...

Chemistry Chapter 3

Atoms: The

Building Blocks

of Matter

I. From Philosophical

Idea…

…to Scientific Theory

History of the Atom

• The Ancient Greeks were the first to come up with the idea of the atom.

• Democritus and Leucippus believed that all matter was made up of tiny particles, which they called atomos.

• They reasoned that if you kept breaking sand into smaller and smaller pieces, eventually you would get a piece that couldn’t be broken anymore.

• Sand On A Seashore

History of the Atom:

Democritus and Leucippus’s Theory of Matter:

a) Point #1 - All matter is made up of

undividable particles called atoms.

b) Point #2 - There is a void, which is empty

space between atoms.

c) Point #3 – Atoms are completely solid

d) Point #4 – Atoms are homogeneous, with

no internal structure

e) Point #5 – Atoms vary in size, shape,

weight

History of the Atom: Aristotle: Greek Philosopher that believed

that all matter was composed of four

elements:

Earth,

Wind,

Fire,

Water.

And that those four elements in different

proportions would give you the varying types

of matter

FIRE wind

water Earth

Cool

and

Heavy

Light Hot

Wet

Even though Democritus and Leucippus

were closer to being right, Aristotle won

the argument.

Why?

Who was right?

1) Greeks did not experiment, they

argued--Aristotle was more famous

so…He won!

2) His ideas carried throughout the

Middle Ages.

…but alchemists, chemists, and some science

hobbyists made discoveries that changed

everything and paved the way for a new theory.

1) By the late 1700’s, scientists had determined:

1)An element could not be broken down by

ordinary chemical means

2)Compounds were composed of elements

3)Elements have different physical and

chemical properties.

2) The transformation of a substance or

substances into one or more new substances

is known as a chemical reaction.

Foundations of Atomic Theory

3. Antoine Lavoisier -

credited with the discovery of

the Law of Conservation of Mass.

He focused on the measuring of the weights of reactants

and products during combustion reactions.

He concluded that during the combustion of phosphorus

and sulfur in air, the products weighed more than the

original because the air combined with the reactants in the

reaction.

Visual Concept

Law of Conservation of Mass

Mass is neither created nor destroyed during

ordinary chemical reactions or physical

changes

75038.html

5) Law of definite proportions: a chemical compound is always composed of the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

(The mass of the compound is the sum of the masses of the elements that make it.)

Pure water has the same mass ratio no matter how much you have or how it was made.

Salt (NaCl) has the same mass ratio no matter if it is in a mine or on your table.

6) Law of multiple proportions: when different compounds are formed by a combination of the same elements, different masses of one element combine with the same fixed mass of the other element in a ratio of small whole numbers.

https://youtu.be/D6HbmG8nIrU

https://youtu.be/D6HbmG8nIrU

Dalton’s Atomic Theory

• Unlike the Greeks, Dalton

performed experiments to

test and correct his atomic

theory.

• He studied the ratios in which

elements combine in

chemical reactions and

formulated hypotheses and

theories which could be

tested

John Dalton Was an English School Teacher

1) All matter is composed extremely small

particles called atoms.

2) Atoms of a given element are identical in

size, mass and other properties; atoms from

different elements differ in size, mass, and

other properties.

3) Atoms cannot be subdivided, created or

destroyed.

4) Atoms of different elements combine in

simple whole-number ratios to form chemical

compounds.

5) In chemical reactions, atoms are combined,

separated, or rearranged.

Dalton’s Postulates (Parts of his theory):

Chemical Reaction

Most of Dalton’s Atomic Theory is

accepted today…

Dalton said:

Atoms cannot be subdivided, created, or destroyed

However!

…one major change is the fact that an

atom can be divided

And

A given element can now have atoms with different masses (Isotopes)

II. The Structure

of the Atom

History of the Atom: • In the 1800’s, scientists proved that the

atom could be divided, and that the

number and arrangement of these

particles determine that atom’s chemical

and physical properties.

• An atom is the smallest particle of an

element that retains the chemical properties

of that element.

1. The nucleus is a very small, positively

charged, region located at the center of an

atom.

a. Made up of at least one positively charged

particle called a proton and one or more neutral

particles called neutrons.

Atomic Structure

2) Surrounding the nucleus are shells occupied

by negatively charged particles called

electrons.

3) Protons, neutrons, and electrons are often

referred to as subatomic particles.

Atomic Structure

1) Cathode Rays and Electrons

a) Experiments in the late 1800s showed

that cathode rays were composed of

negatively charged particles.

b) These particles were named electrons.

Discovery of the Electron

Discovery of the Electron

• J.J. Thomson an English physicist discovered the existence of electrons in 1897.

By bringing positively charged metal plates near the cathode ray, the path was altered. Since unlike charges attract each other, Thomson determined that the cathode ray was made up of negatively charged particles.

• He performed experiments using a cathode ray

tube.

• A sealed glass tube containing different gases

that when connected to high voltage electricity

a glowing beam called a cathode ray is created.

• Since no difference was found by using

different gases in the tube, and different metals

for the plates. Thomson concluded that all

atoms contain electrons.

Thomson’s Experiment

Voltage source

+ -

Vacuum tube

Metal Disks


Voltage source

+ -

Passing an electric current makes a beam

appear to move from the negative to the

positive end

Voltage source

+ -


Voltage source

By adding an electric field:

+

-


Voltage source

By adding an electric field

+

-


Voltage source

By adding an electric field he found that the

moving pieces were negative

+

-


Thomson’s Atomic Model

Thomson could not find a positively charged particle, so he believed that the electrons were like plums embedded in a positively charged “pudding,” thus it became known as the “plum pudding” model.

Click below to watch the Visual Concept.

Visual Concept

Thomson’s Cathode-Ray Tube Experiment

75044.html

1909 – Robert Millikan determines the mass of the electron.

•Robert Millikan sprayed

very fine drops of oil into

the drum, where they

dropped through a very

small hole.

•The drum had two electric

plates on the inside.

1909 – Robert Millikan determines the mass of the electron.

Millikan watched

through a scope

and measured

the speed at

which the drops

fell.

When adding an electrical charge the

drops fell slower and he could actually

add enough charge to cause the drops to

stop completely in mid-air.

Through his

experiments, he

determined both

the mass and the

amount of

charge for the

electron

The oil drop apparatus


Visual Concept

Millikan’s Oil Drop Experiment

75498.html

Conclusions from the Study of the Electron

1. Cathode rays have identical properties

regardless of the element used to

produce them. All elements must contain

identically charged electrons.

2. Atoms are neutral, so there must be

positive particles in the atom to balance

the negative charge of the electron

3. Electrons have so little mass, that atoms

must contain other particles that account

for most of the mass.

Discovery of the Atomic Nucleus

1) Ernest Rutherford’s gold

foil experiment led to the

discovery of a very

densely packed bundle

of matter with a positive

electric charge.

a) Rutherford called this

positive bundle of

matter the nucleus.

Ernest Rutherford and the Nucleus

Ernest Rutherford and the Nucleus

• (1910)—Rutherford believed in the plum

pudding model of the atom.

• He wanted to see how big atoms were, so

he used radioactivity and shot alpha

particles, (positively charged pieces given

off by uranium) through gold foil which can

be made a few atoms thick.

Lead

block Uranium

Gold Foil

Florescent

Screen

What he expected-- The alpha particles

would pass through without changing

direction very much

Because, he thought

the mass was evenly

distributed in the

atom (Plum Pudding)

What he got:

•However what he found was

completely unexpected.

•Many of the particles did go

straight through the gold foil, but

several were deflected, some were

turned at 90 degrees or more!

A new model must be devised!

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Resources Chapter menu

Rutherford Concluded :

a) The atom is mostly

empty space

b) It has a small dense,

positively charge

center

c) Alpha particles are

deflected when they

get near this center or

nucleus

+



+

Rutherford concluded


Visual Concept

Rutherford’s Gold Foil Experiment

75046.html

2) In 1886 twelve years before Thomson, a man named

Eugen Goldstein observed in a cathode ray tube rays

flowing in the opposite direction. He called these

rays canal rays and concluded that they were

composed of positive charges. This lead eventually

to the discovery of the proton

3) In 1932 English physicist James Chadwick confirmed

the existence of another subatomic particle, the

neutron

Canal Ray Tube

Composition of the Atomic Nucleus

• Except for the nucleus of the simplest

type of hydrogen atom, all atomic nuclei

are made of protons and neutrons.

a) A proton has a positive charge equal

in magnitude to the negative charge of

an electron.

b) Atoms are electrically neutral because

they contain equal numbers of protons

and electrons.

• A neutron is electrically neutral—they have no charge, or are neutral (they are NOT neutrally charged!)

• Atoms of different elements have a different number of protons, thus the number of protons determines that atom’s identity.

Composition of the Atomic Nucleus

1) Electrons are

negatively (-)

charged subatomic

particles. They are

found orbiting around

the nucleus in shells.

Electron Cloud

2) In a neutral atom of any element, the

number of protons in the nucleus and the

number of electrons orbiting around the

nucleus is always equal—this number is

known as the Atomic Number.

NEUTRAL ATOM : PROTONS = ELECTRONS


Visual Concept

Parts of the Atom

75047.html

III. Counting

Atoms

A. Atomic Number

1) Atoms of different elements have different numbers

of protons.

• Atoms of the same element all have the same

number of protons.

• The atomic number of an element is the number of

protons of each atom of that element.

Atomic Number

B. Isotopes

1) Isotopes are atoms of the same element that have

different masses.

• The isotopes of a particular element all have the

same number of protons and electrons but different

numbers of neutrons.

• Most of the elements consist of mixtures of isotopes.

C. Mass Number

1) The mass number is the total number of protons

and neutrons that make up the nucleus of an

isotope.

2) MASS NUMBER = PROTONS + NEUTRONS

Mass Number

D. Designating Isotopes

1) Hyphen notation: The mass number is written with a

hyphen after the name of the element.

Uranium-235

2) Nuclear symbol: The superscript indicates the mass

number and the subscript indicates the atomic

number.

235

92 U

Designating Isotopes, continued

3) The number of neutrons is found by subtracting the

atomic number from the mass number.

mass number atomic number = number of neutrons

235 (protons + neutrons) 92 protons = 143 neutrons

4) Nuclide is a general term for a specific isotope of an

element.

Sample Problem

How many protons, electrons, and neutrons are there in

an atom of chlorine-37?

Given: name and mass number of chlorine-37

Solution:

atomic number = number of protons = number of electrons

mass number = number of neutrons + number of protons

Unknown: numbers of protons, electrons, and neutrons

Sample Problem Solution

mass number of chlorine-37 atomic number of

chlorine = number of neutrons in chlorine-37

An atom of chlorine-37 is made up of 17 electrons, 17

protons, and 20 neutrons.

mass number atomic number = 37 (protons plus

neutrons) 17 protons = 20 neutrons

Solution continued:

E. Relative Atomic Masses

1) Because the masses of atoms are extremely

small numbers, chemists instead compare

the masses of atoms using simple whole

numbers called Atomic Mass Units

F. Average Atomic Masses of Elements

1) Average atomic mass is the weighted average of

the atomic masses of the naturally occurring isotopes

of an element.

2) Calculating Average Atomic Mass

a) The average atomic mass of an element

depends on both the mass and the

relative abundance of each of the

element’s isotopes.

1) Copper consists of 69.15% copper-63, which

has an atomic mass of 62.929 601 amu, and

30.85% copper-65, which has an atomic

mass of 64.927 794 amu.

• Average Atomic Mass = (Mass Number x

Relative Abundance) + (Mass Number x

Relative Abundance) …

(0.6915 62.929 601 amu) + (0.3085 64.927 794 amu)

= 63.55 amu

b) The calculated average atomic mass of

naturally occurring copper is 63.55 amu.


Visual Concept

Average Atomic Mass

75051.html

G. Relating Mass to Numbers of Atoms

1) The Mole

a) The mole

(abbreviated mol) is

the SI unit for amount of

substance.

Section 3 Counting Atoms Chapter 3

a) Avogadro’s

number—6.022 1415

1023—is the number

of particles in exactly

one mole of a pure

substance.

2) Avogadro’s Number

Amadeo Avogadro

3) Molar Mass

c) The molar mass of an element is numerically equal

to the atomic mass of the element in atomic mass

units.

b) Molar mass is usually written in units of g/mol.

a) The mass of one mole of a pure substance is

called the molar mass of that substance.

a) Chemists use molar mass as a conversion factor

in chemical calculations.

4.00 g He2.00 mol He = 8.00 g He

1 mol He

1) To find how many grams of helium there

are in two moles of helium, multiply by

the molar mass.

b) For example, the molar mass of helium is 4.00 g

He/mol He.

4) Gram/Mole Conversions

a) Avogadro’s number can be used to find the

number of atoms of an element from the amount in

moles or to find the amount of an element in moles

from the number of atoms.

b) In these calculations, Avogadro’s number is

expressed in units of atoms per mole.

5) Conversions with Avogadro’s Number

Avogadro's Number and

The Mole

https://youtu.be/H3VKLRNAQuY

Sample Problem

What is the mass in grams of 3.50 mol of the element

copper, Cu?


Given: 3.50 mol Cu

Unknown: mass of Cu in grams

Solution: the mass of an element in grams can be

calculated by multiplying the amount of the element

in moles by the element’s molar mass.

moles Cu

grams Cu

moles Cu= grams Cu

Sample Problem Solution, continued

The molar mass of copper from the periodic table is

rounded to 63.55 g/mol.

3.50 mol Cu

63.55 g Cu

1 mol Cu= 222 g Cu



Section 3 Counting Atoms

Sample Problem

A chemist produced 11.9 g of aluminum, Al. How

many moles of aluminum were produced?

Chapter 3





Given: 11.9 g Al

Unknown: amount of Al in moles

Solution: moles Al

grams Al = moles Algrams Al

1 mol Al11.9 g Al =

26.0.441

98 g Al mol Al

Chapter 3

The molar mass of aluminum from the periodic

table is rounded to 26.98 g/mol.




Sample Problem

How many moles of silver, Ag, are in 3.01 1023

atoms of silver?

Chapter 3





Given: 3.01 1023 atoms of Ag

Unknown: amount of Ag in moles

Solution:

moles Ag

Ag atoms = moles AgAvogadro's number of Ag atoms

23

23

1 mol Ag3.01 10 Ag atoms

6.022 10 Ag at

0.500

=

m

oms

ol Ag

Chapter 3




Sample Problem

What is the mass in grams of 1.20 108 atoms of

copper, Cu?

Chapter 3





Given: 1.20 108 atoms of Cu

Unknown: mass of Cu in grams

Solution:

The molar mass of copper from the periodic table is

rounded to 63.55 g/mol.

moles Cu grams Cu

Cu atoms = grams CuAvogadro's number of Cu atoms moles Cu

14

8

23

1 mol Cu 63.55 g Cu1.20 10 Cu atoms =

6.022 10 Cu atoms 1 mol Cu

1. 27 10 Cu g

–

Chapter 3



End of Chapter 3 Show

chemistry chapter 3 · democritus and leucippus’s theory of matter: a) point #1 - all matter is...

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