october 15 th 2009

October 15th 2009

Objectives SWBAT Identify oxidation numbers for given ions SWBAT State the definition of electronegativity SWBAT explain electronegativity trends on the

periodic table

Catalyst Write down the stable ion that will form for the

following elements: Be able to explain why… Na, Ar, O, Mg

Label the ions above as cations or anions

Catalyst Review

Na+1, because sodium needs to lose

1e- to be stable Ar, because Argon is already stable,

it does not lose or gain any e-. O2-, because Oxygen needs to gain

2e- to be happy Mg2+, because magnesium needs to

lose two electrons to be happy

Agenda

Catalyst Review (15min) Welcome to Unit 4 (5min) Compounds and Chemical Bonds

(10min) Oxidation numbers (5min) Electronegativity (20min) Practice (25min) Exit Ticket (10min)

New word

Oxidation Number – the positive or negative charge on an ion What is the oxidation number of Sodium? Argon?

Na+1

Ar O2-

Mg2+

What to expect in Unit 4 You will learn about…

Compounds (ionic and covalent) Chemical formulas Electronegativity, ionization energy, polarity Naming compounds Valence electrons continued Lewis structures

You will get to… Make Ionic Personal Ads! Speed Date!

Atom - the smallest particle of an element

What is a compound?

A compound is a pure substance

A compound is a combination of two or more different elements that are chemically combined

Water moleculesAdd to Vocabulary Log!

Familiar Compounds and Their Formulas

VinegarAcetic acidCH2COOH

Grain AlcoholEthanolC2H5OH

Cane sugarSucroseC12H22O11

Stomach acidHydrochloric acidHCl

What is a compound?

A compound is a pure substance

A compound is a combination of two or more different elements that are chemically combined

• Remember! Electrons do all the work in chemical bonds!

• Atoms do 3 things with electrons:

1. Give them away

2. Take them3. Share them

What is a chemical bond?

A chemical bond is the force that holds two atoms together in a compound (attractive force)

There are two main types of bonds: Ionic Covalent

Chemical Bond Spotlight: NaCl (table salt)

2 different elements: Sodium (Na) Chlorine (Cl)

1 compound: Sodium

chloride (NaCl)

Na Cl

NaCl

Chemical Bond Spotlight: NaCl (table salt)

Chemical Formula A chemical formula is a

combination of chemical symbols used to represent a compound.

A chemical formula contains the number of each atom in the compound.

Ratio Examples KBr has 1 atom of K and 1 atom of Br

H2O has 2 atoms of H and 1 atom of O

CO2 has 1 atom of C and two atoms of O

Subscript Numbers Subscript numbers are small numbers placed to the lower right of element symbols and are used to show how many atoms of each element are present

H2O CO2

Chemical Formulas What is a chemical formula?

What is a subscript number? What does the subscript number

tell us?If there is no number, it is an implied

ONE Example: Na2CO3

2 atoms Na (sodium) 1 atom C (carbon) 3 atoms O (oxygen) 6 total atoms (2 + 1 + 3 = 6)

RATIO2 Na : 1 C : 3 O

Chemical Formula Practice Determine the number of each

atom in the following compounds.1. N2

2. CO3. H2CO3

4. KOH5. CaCO3

6. KNO3

What is electronegativity? Electronegativity is…

The ability of an atom to attract electrons to itself in a chemical bond

How much an atom wants electrons Electronegativity is expressed in numerical

values of 4.0 or less These numerical values are called Paulings Higher number means the atom will attract electrons

in a chemical bond

Rb has an electronegativity value of .82 Paulings

Br has an electronegativity value of 2.96 Paulings Which one wants an electron more?

Graphing Electronegativity On two white boards in your

groups: Draw a line graph

X-axis: Element Y-axis: Electronegativity

Part 1 – points = dots Part 2 – points = squares

Elements and their Electronegativity (1)(Logistics manager copy down the chart for your group)Element Electronegativity

(Paulings)Hydrogen 2.20Lithium 0.98Sodium 0.93

Potassium 0.82Rubidium 0.82Cesium 0.79

Francium 0.70

Elements and their Electronegativity (2)

Element Electronegativity (Paulings)

Lithium 0.98Beryllium 1.57

Boron 2.04Carbon 2.55

Nitrogen 3.04Oxygen 3.44Fluorine 3.98

What trend(s) doyou notice?

Electronegativity

Electronegativity decreases as you go down the periodic table and increases as you go left to right across the periodic table.

Practice Problems

Rank the following elements in order of increasing electronegativity based on location on the periodic table (smallest to biggest)Mg, Sr, Be, Ra

Cl, Si, Al, S, PRadon, Strontium, Magnesium, BerylliumAluminum, Silicon, Phosphorus, Sulfur, Chlorine

Let’s Think about the Trends…

Discuss the following question with your group…10 class points if you come up with the right answer. Why do Alkali metals have such low

Electronegativities and Fluorine such a high electronegativity?

Why aren’t electronegativities listed for the Noble gases?

The Answer…

Electronegativity tells us how much an atom wants to TAKE an electron. (- charge)

The alkali metals have low electronegativities because in order to be stable (happy) they need to GET RID of an electron. (+ charge)

Halogens have a high electronegativity because in order to be stable (happy) they need to GAIN an electron (- charge)

Noble gases are already happy, so they do not want to gain or lose an electron (EN of zero)

How does atomic size related to electronegativity?

Atomic size is indirectly related to electronegativity

ELECTRONEGATIVITY

ATOMIC SIZE

As electronegativity increases, atomic size decreases!

Why is this relationship true? Atoms with HIGH

ELECTRONEGATIVITIES hold their electrons very close!

Sooooo, the atomic size decreasesHigh or low

electronegativity?

Large or small atomic size?

Practice Time! (15min)1. T or F? Electronegativity decreases as you move left across the

periodic table.2. T or F? As you move down the Periodic Table, atoms get more

electronegative.3. Rank the following sets of elements in order of increasing

electronegativity (small big). Set A: Bh, Mn, Re, Tc Set B: Sb, I, Ag, Ru Set C: Y, Ti, Sg, Ta

4. Rank the following sets of elements in order of decreasing electronegativity (big small).Set A: Cl, At, I, F, Br Set B: Te, Xe, Sn, InSet C: Rb, K, Sr, Ca

5. Why do Alkali metals have a lower EN then Halogens?6. Why don’t the Noble Gases have an EN?7. What is the relationship of atomic size to electronegativity?8. Why does this relationship exist?9. What are the oxidation numbers for the following elements?

A. Fluorine, Barium, Francium, Helium, Arsenic