UA GenChem

General ChemistryCHEM 151

Week 5

UA GenChem

Week 5 Reading Assignment

See D2L CONTENT

UA GenChem

Unit 2: Zooming In

What are they made

of?

What holds them

together?

Why are they joined

in this way?

Why are there forces between them? (intermolecular

forces)

They could be atoms, molecules,

ions

UA GenChem

Electrical Nature How do we explain this?

We assume atoms and molecules have an electrical nature

(they are made of + and - charges)

Understanding “charge distribution” in atoms and molecules is of central

importance in chemistry.

UA GenChem

PLANETARY MODEL

α

UA GenChem

PLUM PUDDING MODEL

.

α

What experimentResolved the problem?

UA GenChem

Rutherford and Marsden

UA GenChem

Current view of the atom

X1

2

4

3 1)Atomic number(Z) =# protons = # electrons= ?

11

23 1+

3 2) Mass number (A) protons plus neutrons neutrons =

Na

3) Ionic charge -- 0 if neutral atom + if electrons lost - if electrons addedElectrons? 4) Number of times previous item appears in the compound

.Na3 ?

UA GenChem

objective

• Given any element or ion find the• Number of • protons• neutrons• electrons (especially ions)• p= n= e=

32713Al

UA GenChem

Atoms are characterized by the number of protons in the nucleus (atomic number, Z).

In a neutral atom, the number of protons is equal to the number of electrons.

Mass Number, A

(protons+neutrons)

Atoms

Made of electrons (-) , protons (+), and neutrons (no charge).

old new

XAZ

O168

UA GenChem

Atomic Number, Z, and Mass Number, A

• Mass Number A= # protons + #

neutrons• A boron atom can have

A = 5 p + 5 n A

Z

10

5B

A

Z

10

5B

All atoms of the same element have the same number of protons in the nucleus, Z, but they may differ in the number of

neutrons

UA GenChem

Dalton’s model revisited

All matter is made of atoms (Greeks)All atoms of an element are the sameCompounds are whole number combinations of different atomsIn a chemical Rx. Atoms are reorganized but never made or destroyed

By all experiments Cl (chlorine) weights 35.45 amu

You CANNOT HAVE 0.45 of a mass unit

DALTON’s LAW was in trouble

UA GenChem

Isotopes Atoms of the same element (same Z) but

different mass number (A).Boron-10 (10B) has 5 p and 5 nBoron-11 (11B) has 5 p and 6 n

10B

11B

How many p and n in this

atom?C126

??20882

What is this?

UA GenChem

Atomic MassThe scale of atomic masses is defined relative to a standard:

12 amu (atomic mass units)

1 amu is exactly 1/12 the mass of an atom of or 1.66 x 10-24g

1.00728 amu 10.0129 amu

11.0093 amu 78.918 amu

Because of the existence of isotopes, the mass of an atom in the periodic table is

expressed as an average value of the mass of the different natural isotopes.

C126

C126

H11 B10

5

B115 Br79

35

UA GenChem

Average Atomic Mass

10B11B

Boron is 19.9% 10B (10.013 amu) and 80.1% 11B (11.009

amu) What is the average atomic

mass? Average Mass = fraction(1) x M1 +

fraction(2) x M2 +….= 0.199(10.013 amu) + 0.801(11.009 amu)= 10.81 amu

5B

10.811

Periodic Table

UA GenChem

By EXPERIMENT forEvery element

Symbol ClAtomic number 17Average Atomic mass 35.45 Name Chlorine

Average Atomic Mass

UA GenChem

Electrical Interactions

Protons and electrons have exactly the same charge (magnitude) but opposite

signs.

+

-

)(10602.1 19 CCoulombxq r

221

r

qqF

Coulomb’s Law

Electrons have both kinetic and potential energy.

Atoms are most stable when the total energy (kinetic + potential) is a

minimum.

attract

-

repel

UA GenChem

Interesting fact

Every element, compound, substance that has an ODD number of Electrons is magnetic!

Most but not all substances with an EVEN number of electrons are non-magnetic.

Electrons are the source of all magnetism -- how? Demo

UA GenChem

ms = +1/2 ms= -1/2

↑ ↓↓

Electrons like to form pairsBut only one pair per box

↓↑

UA GenChem

Other InteractionsProtons and

electrons have an additional property

called “spin”.

Same Spin

Additional Repulsion

Opposite Spin

SomeAttraction

Spin generates an additional force between

electrons(magnetic)

Magnetic force much less than Electric force

UA GenChem

A Look into Electronic Structure

The chemical properties of elements and compounds depend on the electronic structure of their atoms or molecules.

How are electrons distributed in the

system?(space-wise, energy-

wise)

UA GenChem

Our analytical methods are based on the use

of light or electromagnetic

radiation

Chemical AnalysisBecause we cannot see inside atoms,

we need to use indirect methods to analyze their structure.

We analyze the light emitted or absorbed by different atoms

to make models of their structure

UA GenChem

I’m glad you are learning ahead of time.

Print this page only and turn it in on Wednesday For this week’s participation points.

NAME PRINT ________________________________

Name Sign ________________________________

Dr.W

UA GenChem

Light Interactions

Because atoms have an electrical nature, they can interact with light.

Waves can be characterized

by their wavelength Wavelength (l) - distance

between the top of nearby crests. (meters-m)

UA GenChem

Electromagnetic Radiation

Frequency-Number of waves per unit time. (1/s=s-1 = Hertz=Hz)

UA GenChem

Relationships For all waves v (velocity) = νλ or frequency x wavelength

dimensional analysis =

ALL forms of light = electromagnetic radiation

ALL ELECTROMAGNETIC RADIATION TRAVELS AT THE SAME VELOCITY

V= λν = c (speed of light)

3 x 108 m/sec

3 x 1010 cm/sec186,000 miles/sec or is it /hr?

UA GenChem

γ-rays X-raysUV

VISIBLE

Infrared

microwave

Radio/TV Long waves

ROY G BIV

Energy INCREASES

VIB G YOR

UA GenChem

Page 259

ROY G BIV

UA GenChem

What does light do?

• γ-ray –destroys tissue• X-ray – penetrates tissue – knocks out e’s• UV- moves e’s; breaks bonds• Visible- moves e’s inside of atoms (jumps)• IR causes molecules to vibrate- no e jump

• Microwave- spins molecules; rotates bonds• Radio/TV – passes through tissue will flip the nucleus of

H MRI• Long waves – communications through the earth• Taos,NM HUMMMM• ELF (extreme low freq)

UA GenChem

Fig. 7.3

UA GenChem

ALL ELECTROMAGNETIC RADIATION MOVES AT THE SPEED OF LIGHT

ALL ELECTROMAGNETIC RADIATION MOVES AT THE SPEED OF LIGHT

ALL ELECTROMAGNETIC RADIATION MOVES AT THE SPEED OF LIGHT

UA GenChem

Spectra

Most heated objects (horse shoes, etc.)Give all the colors of the spectrum but in

Different amounts – we see the most intense.

UA GenChem

Light from Glowing objects

Most heated solid objects (light bulbs, electric stove)

give all the colors of the spectrum (white light) but in

different amounts – we see the most intense.“red hot” (mostly red) and “white hot” (more

blues)

UA GenChemp.295b

Heated Elements give only one color

Sodium

Strontium

Copper

UA GenChem

Metal Ions make Fireworks!!

Strontium Copper or Barium

UA GenChem

Atomic Emission SpectraWhen the light emitted by individual atoms

is passed through a prism, only a few colored lines are seen. This is called a line emission spectrum, which is characteristic

of the element.

H atoms

Why “line”Spectra?

UA GenChem

Line spectra

Each element in the gas phase has a distinctive set of

colored lines – this was the way new elements were often

found at very low concentrations

sodium

UA GenChem

Short High

Long Low

Line Spectra of Excited Atoms

How do we explain this behavior?

• Excited atoms emit light of only certain wavelengths

• The wavelengths of emitted light depend on the type of atom.

700.

nm

UA GenChem

How do we explain Atomic Line Spectra?

Niels Bohr

+Electronorbit

Niels Bohr (1905) suggested that electrons in atoms can only exist in certain discrete orbits, with discrete

energy levels, called quantized energy states.

UA GenChem

Atomic Spectra and Bohr’ Model

If electrons are in quantized energy states, then DE of states can have only certain values. This explains sharp line

spectra.

n: principal quantum number

We can use the wavelength of the light emitted to calculate the

energy difference between levels.

UA GenChem

Interaction Light-Matter

l • n= c where c = speed of light

= 2.998 x 108 m/sec

Basic Relationships

Energy per photon

E = h • h = Planck’s constant = 6.6261 x 10-

34 J•s

When light interacts with matter, energy is transmitted in packets of energy called

“photons.”

UA GenChem

Excited H atoms release light of different colors

(wavelengths)

700

nm

Consider one of the lines that is red light

UA GenChem

Electromagnetic Radiation

One H-emission has l = 700. nm (red light)

Calculate the frequency in Hertz.

s-1= Hertz (Hz)

700. nm • 1 x 10-9 m

1 nm= 7.00 x 10-7 m

Freq = 2.998 x 108 m/s

7.00 x 10-7 m= 4.28 x 1014 s-1

c=λν ν = c/λ

UA GenChem

Energy of Radiation

PROBLEM: Calculate a) the energy of a photon of red light (l= 700. nm)

UA GenChem

PerfectMatch !!

Bohr’s Model

Predictions

UA GenChem

Electromagnetic Radiation

The human eye is a complex sensing device for visible light. The optic nerve needs a minimum of 2.0 x 10-17 J of energy to trigger a series of impulses that eventually reaches the brain:

How many photons of blue light (475. nm) are needed?

. .

UA GenChem

What other properties can help us understand electronic

structure?

First Ionization Energy

Energy required to remove a first electron from a gaseous atom to

create a positively charged gaseous +1 ion

M(g) M+ (g) + e-

UA GenChem

1st Ionization Energy Data

M → M+ + 1e- E= I.E.

Trend?

UA GenChem

What trends do you see? Implications?

H

HHe

Li

1st IE increases across a period

1st IE decreases down a group

UA GenChem

Shell Model of the Atom

The trends in the first ionization energy suggest

that electrons are arranged in energy shells.

n=1n=2

Shell # of e-

n = 1 2 e-

n = 2 8 e-

n = 3 8 e-

n = 4 18 e-

Observations

UA GenChem

Summary Activity:What ion does a sodium atom form and why?

1st 2nd 3rd

0

1000

2000

3000

4000

5000

6000

7000

Ioniz

ati

on E

nerg

y

(kJ/

mol)

UA GenChem

end