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Tuesday, February 07, 2017Chemistry 2202

Unit 2: From Structures to PropertiesNotes – Part A

Classifying Compounds…A ReviewThere are three main types of compounds that we will be dealing with. They are:

1. Ionic compound – combination of a metal and nonmetals made from ions. 2. Molecular compound – combination of nonmetal and nonmetal made from

molecules. 3. Metallic compound – combination of metal and metal.

Compound Properties Samples

Molecular

Solid, liquid, or gas at STP; relatively low melting and

boiling points, do not conduct electricity in aqueous soln; may be

soluble or insoluble in water; held together by covalent

bonds

C12H22O11, H2O, C3H8

Metallic

Ductile, malleable, good conductors of heat and electricity, shiny (luster)

when freshly cut or polished; held together by metallic

bonds

CuZn (brass), all metallic alloys such as steel

Ionic

Crystalline solid at STP, high melting and boiling point, usually soluble in water,

conduct electricity in aqueous solutions; held together by ionic bonds

NaCl, CuSO4

Ductile – able to form a wire.

Malleable – indicates the substance in bendable (hammered into thin sheets).

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Luster – refers to how shiny a substance is.

NOTE: Because of the nature of ionic and covalent bonds, the materials produced by those bonds tend to have quite different macroscopic (observable to the naked eye) properties. The atoms of covalent bonds are bound tightly to each other in stable molecules, but those molecules are generally not very strongly attracted to other molecules in the material. The atoms (ions) in ionic materials show strong attractions to other ions in their vicinity. This generally leads to low melting points of molecular compounds and high melting points for ionic compounds.

Metallic Compounds

Such metals as gold, silver, aluminum, etc. exhibit a chemical bonding mechanism called metallic bonds. The atoms achieve a more stable configuration by sharing electrons in its outer shell with many other atoms. Metallic bonding occurs in elements in which the valance electrons are not tightly bound with the nucleus, namely metals. In this type of bond, each atom in a metal crystal contributes all the electrons in its valence shell to all other atoms in the crystal. Therefore, the individual atoms can slip over one another yet remain firmly held together by electrostatic forces exerted by the electrons. This is why most metals can be hammered into thin sheets (malleable) or drawn into thin wires (ductile). When an electrical potential difference is applied, the electrons move freely between atoms and a current flows.

Electron Configurations: Electron Energy Level Theory

The Bohr Model of the Atom

The number of protons and electrons in an atom is equal to the atomic number of the atom

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Electrons are located in various energy level surrounding the nucleus and are said to “orbit” the nucleus

The energy levels located furthest from the nucleus are the highest in energy and contain the highest energy electrons

The number of energy levels is equal to the period number for the atom

The outermost energy level is called the Valence Level and holds the Valence Electrons

The first three energy levels hold a maximum of 2, 8 and 8 electrons respectively, corresponding to the number of elements in each period.

Example: Bohr Model of the Nitrogen atom

Nitrogen

Atomic # = 7, 7 protons, 7 electrons

Period # = 2, 2 energy levels surrounding nucleus

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Noble Gases all have a filled valence. They all hold the maximum of 8 electrons in the valence level.

Noble Gases are inert (meaning un-reactive). They obey the Octet Rule which states that chemical stability (un-reactivity) is associated with a group of 8 valence electrons.

Exception: Helium

Helium is said to have a Duet of Electrons.

Practice: Draw Bohr diagrams for atoms of F, Na, Mg, B and S. State the number of valence electrons for each.

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Valence Electrons and the Periodic Table

The valence electrons of an atom are the only electrons of the atom involved in forming chemical bonds.

All the atoms of elements in the same group (family) have the same number of valence electrons

The number of valence electrons for an atom is equal to the second digit of the group #

Lewis Theory States that only the valence electrons of an atom are

involved in chemical bonding!

Lewis Diagrams (Electron Dot Diagrams)

A compact way to show the valence electrons of an atom An element symbol is used to represent the nucleus of the atom Dots, surrounding the element symbol, are used to represent

valence electrons in imaginary orbital spaces around the atom of 28

Thursday, February 09, 2017Bonding Capacity

The maximum # of chemical bonds that an atom can form is equal to its number of bonding electrons

Hydrogen has a bonding capacity of 1 (1 unpaired electron)

Oxygen has a bonding capacity of 2 (2 unpaired electrons)

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Arsenic has a bonding capacity of 3 (3 unpaired electrons)

Chlorine has a bonding capacity of 1 (1 unpaired electron)

Example: Draw a Lewis Diagram for Selenium, Se, and determine the # of bonding electrons, the # of non-bonding electrons and the bonding capacity.

The Periodic Table and Bond Capacity

Elements in the same group have the same bonding capacity , since atoms in the same group has the same # of bonding and non-bonding electrons.

Practice:

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(a)Hydrogen (b) Potassium

(b)Strontium (d) Antimony

(c) Tin

Monday, February 20, 2017Electronegativity and Bond Type

What is Electronegativity?

Electronegativity: is the tendency of an atom to attract valence electrons of other atoms.

Metals have low electronegativity values, typically < 2. Metals do not have a tendency to attract valence electrons of other atoms.

Non-metals have high electronegativity values, typically > 2. Non-metals have a tendency to attract valence electrons of other atoms.

Periodic Trends in Electronegativity

(1)Within a group (chemical family), electronegativity values decrease down the group as the atomic number increases.

(2)Within a period, electronegativity values increase from left to right.

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We can use electronegativity values to predict the type of bonding that will occur in a compound!!!!!

What kind of bonding will NH3 exhibit? Well, both atoms, N and H, have EN values > 2. Therefore, NH3 exhibits covalent bonding.

What kind of bonding will brass, CuPbZn, exhibit? Well, all three atoms, Cu, Pb and Zn, have EN values < 2. Therefore, brass exhibits metallic bonding.

What kind of bonding will PbI2 exhibit? Well, one atom, Pb, has an EN value < 2 and the other atom, I, has an EN value > 2. Therefore, PbI2 exhibits ionic bonding.

Practice: Consider these fictitious elements and electronegativity values.

Q = 2.5 R = 0.8 T= 3.8 X = 1.2 Z = 3.0

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Predict the type of bonding that will occur within these fictitious compounds.

(a)R2Q IONIC

(b)RX METALLIC

(c) QZ4 COVALENT

(d)T2 COVALENT (two atoms of T bonding together)

(e)XZ2 IONIC

Metallic Bonding *VERY STRONG

Metals have fewer valence electrons when compared to non-metals! Metals tend to loose electrons to gain stability. In the process they form positively charged ions called cations. More on this later!

Let’s compare the Lewis diagrams of Na and Ca (two metals) with those of Cl and O (two non-metals) to compare the number of valence electrons in metals and non-metals. .

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Properties of Metals

All solids at room temperature (Hg is an exception) Malleable (can be bent out of shape) and ductile (can be shaped and drawn

into wire) Excellent conductors of heat and electricity Shiny and opaque (opaque means you cannot see through them)

Theory of Bonding in Metals: The Electron-Sea Model

The valence electrons in metals are somewhat loosely held and free to move from one atom to the next. Since these freely moving valence electrons spend most of their time between one metal atom and the next we can visualize metals as positive ions embedded in a sea of flowing valence electrons.

The force of attraction between the positively charged metal ions and the “sea” of negative electrons is considered to be a Metallic Bond.

Evidence to support the Electron Sea Model of metallic bonding:

The major piece of evidence to support this theory is our observation of the excellent electrical conductivity of metals. Since electricity is the movement of electrons, the electron-sea model can be used to explain the movement of electrons into, through, and out of a metal.

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Metallic Structure

How are the atoms arranged in metals?

The two basic arrangements identified are:

(1) Cubic

(2) Hexagonal

Monday, February 27th, 2017

Ionic Bonding

Ions: positively charged atoms, formed by either gaining or losing valence electrons.

Cations and Cation Formation

Metal atoms loose electrons to form positively charged ions called Cations. Valence electrons are lost to acquire an octet of valence electrons and gain

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stability. In such a respect, metal atoms loose valence electrons to acquire the electron configuration of the nearest noble gas. They become isoelectronic with the nearest noble gas. Isoelectronic means to have the same electron configuration.

Let’s look at Calcium, Ca as an example:

Anions and Anion Formation

Non-metal atoms gain electrons to form negatively charged ions called Anions. Valence electrons are gained to acquire an octet of valence electrons and gain stability. In such a respect, non-metal atoms gain valence electrons to acquire the electron configuration of the nearest noble gas. They become isoelectronic with the nearest noble gas. Isoelectronic means to have the same electron configuration.

Let’s look at Oxygen, O, as an example:

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The Periodic Table and Ion Charges

All atoms in the same chemical family forms ions with the same charge, the results are summarized in the table below:

Group#

1 Alkali Metals

2AlkalineEarthMetals

13 14 15 16 17Halogens

18Noble Gases

IonicCharge

1+ 2+ 3+ 4+ 3- 2- 1- 0

Note:

Group 14 atoms (including Carbon) do not tend to form ions. They undergo covalent bonding by sharing electrons. There will be more on this later!

The Noble gases obey the Octet Rule and, hence, do not form ions at all.

Practice:

For each atom listed, predict the charge of its ion and state which noble gas each ion is isoelectronic with.

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(a) Be Be2+ Isoelectronic with Helium

(b) He No charge, A noble gas

(c) Se Se2- Isoelectronic with Krypton

(d) Te Te2- Isoelectronic with Xeon

Ionic Bond Formation (Metals loose, Non-Metals Gain)

An ionic bond is an electrostatic force of attraction between two oppositely charged atoms, a positive metal cation and a negative non-metal ion.

An ionic bond is formed when a metal transfers its valence electrons to the valence level of the non-metal.

Lewis Diagram Equations – Ionic Bonding

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Wednesday, March 1st, 2017

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Covalent Bonding – Lewis Diagrams for Covalent Compounds*More bonds mean they are stronger

(the more bonds a covalent compound has, the stronger it is) of 28

Recall that non-metals have high electronegativity’s. Thus, non-metals attract valence electrons.

Covalent Bonding: is a force of attraction between the bonding electrons of two non-metal atoms. The sharing of two bonding electrons, one contributed from each atom, results in a covalent bond.

Single Covalent Bond: the shared attraction of a single pair of bonding electrons. A single bonding electron is contributed by each non-metal in the compound to form a single bond.

Thursday, March 2nd, 2017Double Covalent Bond: the shared attraction of two pairs of bonding electrons. A total of 4 bonding electrons, 2 from each atom, are contributed to form the chemical bond.

Example: O2

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Triple Covalent Bond: the shared attraction of three pairs of bonding electrons. A total of 6 bonding electrons, 3 from each atom, are contributed to form the chemical bond.

Example: N2

Drawing Lewis Diagrams – Polyatomic Covalent Compounds

Central Atom: the atom with the greatest bonding capacity. It has the greatest number of bonding electrons. There can be more than one central atom in a compound.

Rules for Drawing Lewis Diagrams for Polyatomic Covalent Compounds:

1) Draw the Lewis diagrams for every atom in the compound.

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2) Identify the central atom in the molecule. It has the greatest number of bonding electrons (highest bond capacity).

3) Begin to draw the Lewis structure of the compound by placing the Lewis diagram of the central atom in the center.

4) Add the Lewis diagram of the atom with the next highest bonding capacity and so on until all atoms are used up. As you add atoms, pair a single bonding electron of the atom with a bonding electron of the central atom to form single covalent bonds.

5) Any unused bonding electrons on the central atom can be used to form double or triple covalent bonds between the central atoms.

6) Finally, check to ensure that each atom has an octet or duet of electrons.

Let us work through the following examples. There is only one central atom in each compound. Examples:

H2OHFNCl3HCNCO2

AsH3

BrClCH4

CHCl3CH2Br2

CH3OHCCl2O

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Converting a Lewis Diagram into a Structural Diagram

We simply use a solid line to indicate a single covalent bond. Hence, we use two solid lines to indicate a double covalent bond and three solid lines to indicate a triple covalent bond.

Draw structural diagrams corresponding to the Lewis diagrams above!

Covalent Compounds w/ 2 or more central atoms

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Frequently, you will have two draw Lewis and structural diagrams for compounds that contain more than one central atom. Let us work on the following:

H2O2

C2H6

N2H4

C2H2

N2H2

C2H4OCH3COOH

Friday, March 10th, 2017Valence Shell Electron Pair Repulsion Theory (VSEPR)

Predicting the 3D Shape of a Molecule

The basis behind VSEPR theory is a simple law of electrostatics you have encountered in junior high science. That is, like charges repel one another.

VSEPR Theory states:

Since electrons will repel each other, like similar poles of a magnet, groups of valence electrons spread out as far as possible over the surface of the central atom in order to minimize the repulsive forces between them. The degree of repulsion affects the shape of the molecule. X-ray crystallography and other techniques have been used to determine the 3D shapes of molecules, supporting VSEPR theory.

THE # OF LONE PAIRS AND BONDING PAIRS DETERMINES THE SHAPE OF THE MOLECULE.

The shape around a central atom in a molecule is determined by the number of lone pairs and bonding groups. A bonding group constitutes as either a single, double, or triple covalent bond.

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The table below can be used to predict the 3D shape of a molecule. This is an application of VSEPR theory. KNOW THIS TABLE!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Number of

Lone Pairs

Number ofBonding Groups

Shape AroundCentral Atom

Bond Angles Example

0 4 tetrahedral 109.5° CH4

1 3 pyramidal 107° NH3

2 2 Bent (V-shaped) 105° H2O

0 3 trigonal planar 120° H2CO

0 2 linear 180° CO2

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0 LONE PAIRS4 BONDING GROUPS

Tetrahedral

CH4

SHAPE DIAGRAM

4 bonding pairs groups, 0 lone pairsTetrahedral

Pyramidal


PyramidalNH3

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109.5°

Solid Wedge indicates bond is in front of the plane of the paper

Dashed Line indicates bond is behind the plane of the paper

Solid Line indicates bond is in the plane of the paper.

1 lone pair, 3 bonding pairs groupsPyramidal


Bent (V-shaped)H2O

2 lone pairs2 bonding pairs/groups


Trigonal Planar

H2CO

0 lone pairs3 bonding pairs groups

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107°

105°

120°


Linear

CO2

0 lone pairs2 bonding groups

Further Examples:

Draw Lewis Diagrams and 3D shapes for the following:

C2H4

HCNSiCl4H2SeNF3

PBr3

CS2

CH3ClCO2HC2H6

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