02a Atoms, Molecules, Ions 6.pdf

(1)

Chapter 2 : Part 1

(2.1 - 2.7)

Atoms, Molecules, and Ions

Alpharetta High School

Dr. Sonha Payne

Why do scientists

use models?



Models

may be used to represent things that are difficult

to visualize.



Scaled-down models

allow you to see something too large

to see all at once, (

solar system

) or something that hasn’t

been built yet.



Scaled-up mode

ls

are used to visualize things that are too

small to see. (

atoms

)

1

st

Model of the

Atom

Democritus (400 BC)

atomos

,

meaning uncuttable,

or cannot be divided

Just as a brick is basic to the structure of a wall,

an atom is basic to the structure of matter.

Foundations of Atomic Theory

Law of

Conservation

of Mass

(Lavoisier)

Mass

is neither

created

nor

destroyed

during ordinary chemical reactions.

Mass products = mass reactants

Foundations of Atomic Theory

Law of

Definite

Proportions

(Proust)

A chemical compound contains the

same

elements

in the same

percent

by

mass

regardless of the size of the sample or

the source of the sample.

(2)

Foundations of Atomic Theory

Law of

Multiple

Proportions

(Dalton)

When elements combine, they do so in

the ratio of small

whole

numbers

.

CO

2

and CO

H

2

O and H

2

O

2

Principles of Dalton

’

s Atomic Theory of

Matter (1808)

1.

All

matter

is composed of extremely small, indivisible

particles called

atoms

.

2.

All atoms of a given element have identical properties

that differ from those of other elements.

3.

Atoms cannot be created, destroyed, or transformed into

atoms of another element.

4.

Compounds are formed when atoms of different

elements combine with one another in small

whole-number ratios.

5.

The relative numbers and kinds of elements are constant

in a given compound.

Dalton

’

s Billiard Ball Model of the Atom

Billiard Ball Model

because the atom

is likened to a

billiard ball - it is a

single, complete

unit of matter.

A solid, indivisible,

indestructible sphere

Voltage source

No Charge Flows When the Glass Tube is Empty

(Vacuum, No Gas)



Cathode Ray Tube: 2 metal plates sealed inside a

glass tube connected to a source of electricity



Since glass is an insulator

,

no charge was

observed to flow when the tube was empty

.



Some means of conduction was needed if charge

was to flow.

2.2

Charge Flows (a Ray is observed)

in the Presence of ANY GAS

When a small amount of

any gas

was placed

in the glass tube and the power source was

turned on

, a ray was observed

striking the

phosphor-coated end of the tube and

emitting a flash of light.

Voltage source

+

-

The

current carriers in the gas are

“

invisible

”

and are visualized by the phosphor-coated

screen which fluoresces as the current

carriers strike it.

For current to flow, mobile,

charged particles are required.

Voltage source

+

(3)



Observation

: A fluorescent screen glows (emits a flash of

light) when struck by the

“

ray

”

.



Conclusion

: Invisible particles traveling from the cathode to

the anode are carrying electric current and are visualized

by the fluorescent screen.

 Observation: When a tiny object is placed in the middle of the tube, a shadow is cast on the screen at the anode

 Conclusion: the particles travel in a straight line.

Cathode Rays

Originate at the

Cathode

(

Negative

Electrode)



The rays were called

cathode rays

because

they originated at the negative electrode

(aka the cathode) and moved to the

positive electrode (aka the anode).



The entire apparatus is now known as a

cathode ray tube

(CRT

).



Observation

: In the presence of a magnetic field, the

cathode rays bend.



Conclusion

: The cathode ray consists of charged particles.



Observation

: In the presence of an external electric field,

the cathode rays bent towards the positive plate.



Conclusion

: The particles are negatively charged.

ALL gases

used in the tube were

found to produce identical rays, so

the negatively charged particles

of the cathode rays were

determined to be part of all

(4)



Observation

: A paddle wheel in the

middle of the cathode ray tube turns.



Conclusion

: The

particles have mass

.

JJ Thomson (1897)



Credited with

discovery of the electron



Found the

ratio

of

the

mass

of the particles

to

the

charge

of the particles in cathode rays

m/e

= -5.6856 x 10

-9

g/C



Conclusion

: all cathode rays are composed of identical,

negatively-charged particles

.



Conclusion

: these negative particles are fundamental

particles of matter. (electrons)

In 1897, JJ Thomson discovered that

negatively

charged electrons

were part of all matter.

Dalton Postulate 1: All matter is composed of extremely small,

indivisible particles called atoms.

This is no longer valid.

Atoms are NOT indivisible particles, but CONTAIN ELECTRONS.

Millikan Determined the Charge on an Electron

by Examining the Motion of Tiny Oil Drops (1909)

 Small drops of oil with a negative charge are examined.  The diameter of the drop is measured. From this, the volume is

determined (4/3pr3_).

 The mass is calculated using the known density of the oil.  By adjusting the electric field to balance the force of gravity, the

charge on the drop is calculated.

Observation

The charge of each drop is a whole number multiple of some

common number.

Conclusion

Each number is divisible by 1.92 x 10

-19

. This is the

charge of

an electron

. (modern value =

1.602 x 10

-19

C

)

Charge in Coulombs

13.458 x 10-19 ₌₇_{x (}_{1.92 x 10}-19₎

15.373 x 10-19 ₌₈_{x (}_{1.92 x 10}-19₎

17.303 x 10-19 ₌₉_{x (}_{1.92 x 10}-19₎

15.378 x 10-19 ₌₈_{x (}_{1.92 x 10}-19₎

17.308 x 10-19 ₌₉_{x (}_{1.92 x 10}-19₎

28.844 x 10-19 ₌15x (1.92 x 10-19₎

11.545 x 10-19 ₌₆_{x (}_{1.92 x 10}-19₎

19.214 x 10-19 ₌₁₀_{x (}_{1.92 x 10}-19₎

Millikan Determined the Mass of an Electron

Knowing the charge, Millikan was able to use Thomson’s charge-to-mass ratio to determine the mass of an electron.



  _

  19

28 8

charge 1.6022 10 C

(5)

Inferences from the

Properties of Electrons



Atoms are neutral, so

there must be positively

charged particles

to balance the negatively charged

electrons.



Electrons have a tiny mass,

so atoms must contain

other particles that account for most of their mass

.

Even the lightest atom, H, has a mass of 1.7 x 10-24 _g,

compared to a mass of only 9.1 x 10-28 _{g for an electron.}

That is, the lightest atom is almost 10,000 x heavier than an electron.

Thus, most of the mass of an atom had to come from somewhere else.

Thomson

’

s Plum Pudding Model of the Atom

•Since electrons are negatively

charged, and atoms are neutral,

atoms must also contain a positively

charged substance.

•Thomson

’

s model of the atom: the

positively charged substance fills the

atom and the electrons are

embedded throughout the substance

like

“

raisons in plum pudding

”

.

Rutherford's gold foil experiment (1911)

Discovery of the Nucleus



(

helium nuclei

)

a

-particles

were known to be

heavy positive

particles.



When the particles hit the screen, brief flashes of light

were seen.

Rutherford

’

s Expected and

Actual Results

If plum-pudding

model was correct.

Actual results.

Rutherford

’

s Expected and Actual Results

•

Observation: Most particles went straight through the foil

undeflected.

•

Conclusion: The

atom is mostly empty space.

•

Observation: Some particles bounced back!

•

Conclusion

•

To deflect the energetic

a

-particles,

most of the mass and

all of the positive charge must be in a dense central region

which he called the nucleus.

•

For the fraction deflected to be small, the

nucleus must be

small

, relative to the overall size of the atom.

•

Since atoms are neutral particles, the

charge of the nucleus

must be equal to the sum of the negative charges of the

electrons.

1911

Rutherford’s

Gold Foil

Experiment



Conclusion: An atom is

mostly empty space occupied by

electrons

, and

centrally located

within that space lies a

tiny

region, which he called the

nucleus

, that

contains all the

positive charge and essentially all the mass

of the atom.



Rutherford proposed that

positive particles

lay

within the

(6)

Summary: The Rutherford Experiment (1911)



Alpha particles (helium nuclei) fired at a thin sheet of

gold

 Assumed that the positively charged alpha-particles were bounced back if they approached a positively charged atomic nucleus head on

(Like charges repel one another)



Very few particles were greatly deflected back from the

gold sheet

 Atoms contain very small, very dense, positively-charged nuclei.  The electrons are in clouds surrounding the nucleus at relatively

large distances.

 Most of the atom is empty space, and if the nucleus were the size of a ladybug, it would be in an atom the size of the Georgia Dome.

Moseley,1913

X-rays, Atomic Number and the Proton



A beam of electrons shot at a sample of an element gives off x-rays.



Observation

 The frequency of x-rays given off is unique to that element.

 Higher energy rays are given off when the nuclear charge is higher.



Conclusion

 The atomic number can be determined from the x-ray data

 Each element on the periodic chart differs from the next by having one more positive charge in the nucleus

 These fundamental positive charge units arethe protons

Modern Atomic Theory

1.

All matter is made up of very tiny particles called

atoms.

2.

Atoms of the same element are chemically alike.

3.

Individual atoms of an element may not all have the

same mass. However, the atoms of an element have

a definite average mass that is characteristic of the

element.

4.

Atoms of different elements have different average

masses.

5.

Atoms are not subdivided, created, or destroyed in

chemical reactions.

Modern

Atomic

Theory

 An atom is an electrically neutral, spherical entity composed of a positively charged central nucleus surrounded by one or more negatively charged electrons. The “cloud” of rapidly moving, negatively charged electron occupies virtually all of the atom’s volume and surrounds the tiny nucleus.

 The nucleus is very dense as it contributes 99.97% of the atom’s mass but occupies only about one ten-trillionth of its volume

 An atom’s diameter (~10-10_{m) is about 10,000 times the diameter of}

it’s nucleus (~10-14_m).

What are the particles that make up an

atom and where are they located?

Particle Symbol Charge Relative Mass (amu) Actual Mass (g)

Proton p+ ₊₁ ₁ _{1.7 x 10}-24

Neutron n 0 1 1.7 x 10-24

Electron e- _-1 ₀ _{9.1 x 10}-28

Atomic Mass Unit (amu)

: the Unit Used For

Masses of Atoms and Subatomic Particles

Atoms have an very small mass

(e.g. hydrogen 1.67 x 10

–24

g)

It is hard to work with these small numbers

so a relative mass scale was introduced:

atomic mass unit (amu or u)

1 u = 1/12 the mass of a carbon-12 atom

Particle Symbol Charge Relative Mass (amu) Actual Mass (g)

Proton p+ ₊₁ ₁ _{1.7 x 10}-24

Neutron n 0 1 1.7 x 10-24

(7)

# Protons

Defines an

Atom

The number of protons distinguishes

atoms of one element from

atoms of all of the other elements.

Atomic number = # protons in the nucleus

2.3

What Distinguishes One Element from

Another Element? The # of Protons!

For a Neutral Atom,

# protons = # electrons

For a neutral atom,

the

positive charges

(

+1 per proton

)

and

negative charges

(

-1 per electron

)

must add up to zero.

Complete the table.

Element Atomic

number

Protons Electrons

Pb

82

8

30

Mass Number

=

protons

+

neutrons

The mass number is NOT on the periodic table!

Always a whole number

Nuclear Symbol and

Hyphen Notation

are Interchangeable

Carbon-12

(8)

Atoms of the Same Element That Have

Different # of Neutrons are ISOTOPES

Isotopes

of

elements have the

same # of protons

but

different # of

neutrons

.

Magnesium-24

Magnesium-25

Magnesium-26

An

isotope

of an element is

identified by the mass number

45

Which of the following statements regarding

Dalton

’

s atomic theory are still believed to

be

true

?

I. Elements are made of tiny particles called atoms.

II. All atoms of a given element are identical.

III. A given compound always has the same relative numbers and types of atoms.

IV. Atoms are indestructible.

How many protons, electrons, and neutrons do

each of these isotopes have?

Determine # p

+

, # e

-

, # n

0

for the following.

Name each isotope and write its symbol.

Element

Atomic

number

Mass number

Neon

10

22

Calcium

20

46

Oxygen

8

17

Iron

26

57

Zinc

30

64

(9)

Complete each of the following

isotope symbols:

206

84

?

a.

224

?

Ra

b.

197

?

Au

c.

84

36

?

d.

The Modern Periodic Table

Groups contain elements with similar properties

and are arranged in vertical columns.

Periods are the horizontal rows of elements.

Two Numbering Systems for Groups



The old method uses the letter A for the representative

elements (1A to 8A) and the letter B for the transition

elements.



The new method numbers groups 1–18 from left to right.

Special Names of

Groups

Alkali Metals

Halogens

(10)

Metals, Nonmetals, and Metalloids

The

heavy zigzag line

separates

metals

and

nonmetals

.

 Metalsare located to the left.

 Nonmetalsare located to the

right.

 Metalloidsare located along

the heavy zigzag line between

the metals and nonmetals (except for Al)

56

Metals



solids at room temperature, except Hg



reflective surface (shiny)



conduct heat and electricity



Malleable (can be shaped)



Ductile (can be pulled into wires)



lose electrons and form cations in

reactions



About 75% of the elements are metals.

57

Nonmetals

Exist as s, l, g

poor conductors of heat and electricity

Solids are brittle.

gain electrons in reactions to become anions

upper right on the table

 except H Sulfur, S(sBromine, Br) ₂(Chlorine, Cll) ₂(g)

Metalloids



show some properties of

metals and some of

nonmetals



also known as

semiconductors

Properties of Silicon shiny conducts electricity does not conduct heat

well brittle

Properties of Metals, Nonmetals,

and Metalloids

Metals Metalloids Nonmetals

Shiny (s) Dull (s, l, g)

Ductile,

malleable Brittle

Good

conductors Better conductors than nonmetals, but not as good as metals