Review for unit 2 testReview for unit 2 test

How do elements differ from each How do elements differ from each other?other?

Most obviously in their properties Most obviously in their properties (reactivity, m.p., b.p., solubility, (reactivity, m.p., b.p., solubility, density, relative weight.)density, relative weight.)

They also differ in their They also differ in their identity/makeup, but that is easiest identity/makeup, but that is easiest to consider at the atomic level.to consider at the atomic level.

What unit would be used to discuss What unit would be used to discuss the mass of an atom?the mass of an atom?

a.m.u. (atomic mass units).a.m.u. (atomic mass units). Example: A copper atom has a mass Example: A copper atom has a mass

of 63.5 amuof 63.5 amu A lead atom has a mass of 207.2 A lead atom has a mass of 207.2

amu.amu.

We did a worksheet comparing We did a worksheet comparing their weights:their weights:

Originally, every element was Originally, every element was weighed in terms of the lightest weighed in terms of the lightest element (hydrogen)element (hydrogen)

Helium has a mass 4 times that of Helium has a mass 4 times that of hydrogen, thus helium has a mass of hydrogen, thus helium has a mass of 4 amu.4 amu.

Lithium has a mass 7 times that of Lithium has a mass 7 times that of hydrogen; lithium’s mass is 7 amu.hydrogen; lithium’s mass is 7 amu.

Today the relative mass is not Today the relative mass is not based on hydrogen.based on hydrogen.

The reason is simply in order to be The reason is simply in order to be more exact.more exact.

The weights are based off of The weights are based off of (compared to) the carbon-12 isotope.(compared to) the carbon-12 isotope.

Because atoms are so small, and Because atoms are so small, and have such little mass, we have such little mass, we

introduced the mole concept. introduced the mole concept. What is a mole?What is a mole?

Simply stated, it’s just a quantity Simply stated, it’s just a quantity (a number)(a number)

What is special about the mole What is special about the mole quantity?quantity?

It’s the number of particles it takes It’s the number of particles it takes to get the same mass in grams as to get the same mass in grams as what one particle’s mass is in amu’s.what one particle’s mass is in amu’s.

Examples:Examples: 1 sodium (Na) atom is 22.99 1 sodium (Na) atom is 22.99 amusamus.. To get 22.99 To get 22.99 gramsgrams of sodium, you of sodium, you

need 1 moles worth of sodium need 1 moles worth of sodium atoms.atoms.

So, a mole is the So, a mole is the quantityquantity of sodium of sodium atoms needed to get 22.99 gramsatoms needed to get 22.99 grams

Examples:Examples: 1 mercury (Hg) atom is 200.59 1 mercury (Hg) atom is 200.59 amusamus..

To get 200.59 To get 200.59 gramsgrams of mercury, of mercury, you need 1 moles worth of mercury you need 1 moles worth of mercury atoms.atoms.

So, a mole is the So, a mole is the quantityquantity of of mercury atoms needed to get 200.59 mercury atoms needed to get 200.59 grams of mercurygrams of mercury

Why is the mole concept useful to Why is the mole concept useful to us?us?

It allows us to “count by weighing”.It allows us to “count by weighing”.

ExampleExample

If you have 5 lbs of nails, how many If you have 5 lbs of nails, how many nails do you have?nails do you have?

You first need to know the mass of You first need to know the mass of one nail.one nail.

If the mass of one nail is 0.02857 lbsIf the mass of one nail is 0.02857 lbs 5 lbs/0.02857 = 175 nails.5 lbs/0.02857 = 175 nails.

We do the same counting method We do the same counting method using moles.using moles.

If you have 200. grams of iron, how If you have 200. grams of iron, how many iron atoms do you have?many iron atoms do you have?

A mole of iron is 55.845 gramsA mole of iron is 55.845 grams 200/55.845 = 3.58 moles of iron200/55.845 = 3.58 moles of iron This would be (3.58)*(6.022x10This would be (3.58)*(6.022x102323) = ) =

2.16 x 102.16 x 102424 iron atoms iron atoms

What are three basic parts of the What are three basic parts of the periodic table?periodic table?

Metals (most elements are metals.Metals (most elements are metals. Nonmetals.Nonmetals. A handful of “semi-metals” or A handful of “semi-metals” or

“metaliods.“metaliods.

The Periodic tableThe Periodic table

Why is it called the “periodic table”?Why is it called the “periodic table”?

Because it has a “periodic” Because it has a “periodic” (repeating) quality to it.(repeating) quality to it.

The repeating patterns are seen The repeating patterns are seen within the group/family (vertical within the group/family (vertical columns)columns)

So: Li, Na, K, Rb, Cs all have similar So: Li, Na, K, Rb, Cs all have similar properties-hence the term the “alkali properties-hence the term the “alkali metal” family.metal” family.

Who arranged the “periodic table”?Who arranged the “periodic table”?

Dmetri Mendelev (1869)Dmetri Mendelev (1869)

How did Mendelev arrange his How did Mendelev arrange his table of elements?table of elements?

In order of generally increasing In order of generally increasing atomic weight (with a few atomic weight (with a few exceptions)exceptions)

With repeating properties—most With repeating properties—most notably the property of “valence”.notably the property of “valence”.

Also considered were properties such Also considered were properties such as density, melting point, boiling as density, melting point, boiling point, atomic weightpoint, atomic weight

What is valency?What is valency?

It’s the It’s the ratioratio in which atoms in which atoms combine-particularly with other combine-particularly with other elements…especially hydrogen, elements…especially hydrogen, oxygen, and chlorineoxygen, and chlorine

Example: Nitrogen combines with Example: Nitrogen combines with hydrogen in a 1:3 ratio, so we say hydrogen in a 1:3 ratio, so we say nitrogen has a “valency” of 3.nitrogen has a “valency” of 3.

Given the following relationship, Given the following relationship, what is the “valency” of the what is the “valency” of the

elements below?elements below?

4.0 grams of element X combines 4.0 grams of element X combines with 39.2 grams of chlorine.with 39.2 grams of chlorine.

Chlorine has a relative mass that is Chlorine has a relative mass that is 3.27 times that of element X.3.27 times that of element X.

What is the valence of X?What is the valence of X?

How to solve:How to solve:

First, compare the mass ratios. 39.2/4.0 = First, compare the mass ratios. 39.2/4.0 = 9.89.8

Next, compare the relative masses. If 9.8 Next, compare the relative masses. If 9.8 times as much chlorine is needed for every times as much chlorine is needed for every gram of X, divide the 9.8 by the fact that it gram of X, divide the 9.8 by the fact that it is 3.27 times heavier,and you get 2.99 is 3.27 times heavier,and you get 2.99 which is very close to 3. which is very close to 3.

This means 3 Chlorines were needed for This means 3 Chlorines were needed for every X, so X has a valency of 3.every X, so X has a valency of 3.

Developing “atomic” theoryDeveloping “atomic” theory

What is an “atom”?What is an “atom”?

The smallest particle of an element The smallest particle of an element that retains the properties of that that retains the properties of that elementelement

John Dalton (early 1800s) John Dalton (early 1800s) thought atoms were the thought atoms were the

smallest particles that existed.smallest particles that existed.

List Dalton’s postulatesList Dalton’s postulates

3.When a chemical reaction occurs, 3.When a chemical reaction occurs, atoms c_____, s______ or are atoms c_____, s______ or are re__________.re__________.

4. Atoms cannot be subdivided, 4. Atoms cannot be subdivided, created or destroyed.created or destroyed.

5. When atoms combine they do so 5. When atoms combine they do so in small f____, w____ n____ r____.in small f____, w____ n____ r____.

Are these true?Are these true?

1. Everything is made up of small 1. Everything is made up of small particles called atomsparticles called atoms

2a.Atoms of different elements _____ 2a.Atoms of different elements _____ in their size, mass, properties.in their size, mass, properties.

2a.Atoms of identical elements have 2a.Atoms of identical elements have _____ size, mass, properties _____ size, mass, properties

List Dalton’s postulatesList Dalton’s postulates

3.When a chemical reaction occurs, 3.When a chemical reaction occurs, atoms c_____, s______ or are atoms c_____, s______ or are re__________.re__________.

4. Atoms cannot be subdivided, 4. Atoms cannot be subdivided, created or destroyed.created or destroyed.

5. When atoms combine they do so 5. When atoms combine they do so in small f____, w____ n____ r____.in small f____, w____ n____ r____.

Are these true?Are these true?

3.When a chemical reaction occurs, 3.When a chemical reaction occurs, atoms c_____, s______ or are atoms c_____, s______ or are re__________.re__________.

4. Atoms cannot be subdivided, 4. Atoms cannot be subdivided, created or destroyed.created or destroyed.

5. When atoms combine they do so 5. When atoms combine they do so in small f____, w____ n____ r____.in small f____, w____ n____ r____.

Now we know that particles smaller Now we know that particles smaller than an atom exist.than an atom exist.

The first of these particles to be The first of these particles to be discovered was the “electron”.discovered was the “electron”.

J.J. Thomson gets credit for the J.J. Thomson gets credit for the discovery.discovery.

What is an “ion”?What is an “ion”?

A charged particleA charged particle

How was the electron discovered?How was the electron discovered?

Experiments using a CRT Cathode Experiments using a CRT Cathode Ray TubeRay Tube

Experiments with the CRTExperiments with the CRT

When an object was placed in the When an object was placed in the middle, it cast a ______. Meaning:middle, it cast a ______. Meaning:

Something was being blocked.Something was being blocked. When a paddle wheel was placed in When a paddle wheel was placed in

the middle, it moved to toward the the middle, it moved to toward the anode. Meaning:anode. Meaning:

Something was pushing it toward the Something was pushing it toward the anode.anode.

Experiments with the CRTExperiments with the CRT

When a magnet was near the beam, the When a magnet was near the beam, the beam moved. Meaning:beam moved. Meaning:

The stuff in the beam has magnetic The stuff in the beam has magnetic properties.properties.

When a negatively charged plate was When a negatively charged plate was placed near the tube, the beam moved placed near the tube, the beam moved _____. Meaning:_____. Meaning:

Whatever is in the beam has a negative Whatever is in the beam has a negative charge (because like charges repel.)charge (because like charges repel.)

What is an “ion”?What is an “ion”?

A charged particle (not neutral)A charged particle (not neutral)

Are all atoms of the same element Are all atoms of the same element identical?identical?

No. Different isotopes of each No. Different isotopes of each element exist.element exist.

Usually one isotope is far more Usually one isotope is far more prevalent than others.prevalent than others.

What is a “isotope”?What is a “isotope”?

A “variety” of an atom that has a A “variety” of an atom that has a specific number of neutrons.specific number of neutrons.

When the same type of atom has a When the same type of atom has a different number of neutrons, that is different number of neutrons, that is a different isotope of the same a different isotope of the same element.element.

Example C-12, C-13, C-14Example C-12, C-13, C-14

Are all isotopes stable?Are all isotopes stable?

No. Ususally one isotope is stable, No. Ususally one isotope is stable, and the others are not stable.and the others are not stable.

Unstable isotopes are “radioactive”.Unstable isotopes are “radioactive”. Example C-12 is a stable isotope,Example C-12 is a stable isotope, C-14 is an “unstable” isotope.C-14 is an “unstable” isotope.