Chemical BondingUnderstand why atoms form compounds
Understand chemical stability and the octet rule
Understand how Ionic bonds and Ionic compounds are formed
How Elements Form Compounds
Compounds: a chemical combination of two or more different elements joined together in a fixed proportion
Collisions between particles of the atom cause reactions
Reactions between atoms Involve Only The Electron Cloud!
Remember, chemical properties of elements on the periodic table repeat because the pattern of valence electrons repeat in each period
Valence electrons of colliding atoms react to form compounds
Compounds form when electrons in atoms rearrange to achieve stable electron configurations.
Chemical Stability
Nobel Gasses: they are almost completely un-reactive
None of these have ever been found naturally in the environment as a compound.
They are extremely un-reactive or stable
Group 18 (except He) all have 8 valence electrons
Electron arrangement determines chemical properties
The electron arrangement of the noble gasses is the cause of their stability (extremely un-reactive)
Check Your Understanding
Turn and discuss the following with the person on your right, then answer each of the questions in your notebook.
Identify each of the following as a compound or not a compound
Water
Nitrogen
Why do elements form compounds?
Which group of elements rarely forms compounds?
Explain why.
Answers
Identify each of the following as a compound or not a compound
Water--- compound
What subatomic particles are involved in forming compounds?--- valence electrons
Why do elements form compounds? --To rearrange electrons in order to achieve a stable electron configuration
Which group of elements rarely forms compounds?
Explain why. ---Noble gasses, because they already have a stable electron configuration
The Octet Rule
more stable
Atoms become stable by having eight electrons in their outer energy levels (He is an exception w/ 2 because it is so small)
Atoms become stable by achieving a noble gas configuration
Achieving Chemical Stability
Collisions between atoms, which involve enough energy, can cause valence electron rearrangements
Forming a stable octet
Establishing Stable Octets
Sharing electrons between atoms
Example
Na- sodium is a shiny gray metal which quickly oxidizes upon exposure to the atmosphere
Cl - Chlorine gas is a dense, pale yellowish-green, poisonous, gas
Na and Cl react to form–salt
Electron Transfer On a Subatomic Level
Na- a group 1 element, with 1 valence electron
Cl - a group 17 element, with 7 valence electrons
How can the valence electrons be rearranged to provide a stable octet for each?
Chlorine Gets a Stable Octet
If the Cl atom gains an electron from the Na atom, it will achieve a noble gas configuration
It will be stable
It will have a complete octet
It will also have a negative charge, because it has an extra electron
It will be an ion
Na Gets a Stable Octet
If Na gives away its 1 valence electron, what will it have for a valence number?
11 electrons minus 1 electron = 10 electrons
1s2 2s2 2p6 - 8 valence electrons
It will be stable
It will have a complete octet
It will also have a positive charge, because it has one less electron
It will be an ion
Transfer of the Electron Forms an Ionic Bond
An Ionic Bond
1 positively and 1 negatively charged
Ionic bond: A strong attractive force between atoms with opposite charges which is formed by an electron transfer between atoms.
Ionic bonds form ionic compounds
A compound that is made up of ions
Na+ Ions are attracted to all nearby Cl- Ions and visa versa thereby forming crystal structures
Cl-
Na+
The Results of Ionic Attraction
Affects properties of the compound
Example; sodium chloride (salt) is a crystal because of intermolecular forces of attraction between ions, it is a solid at room temperature
Melting Ionic Compounds: Breaking the strong crystal structure requires a lot of energy, therefore the melting point of NaCl is more than 800°C
Hardness and Brittleness: It takes a great deal of force to break the structure of an ionic crystal
Naming Binary Ionic Compounds
Binary compounds are compounds with only two different elements.
First write the name of the positively charged ion (usually a metal)
Then add the name of the negatively charged ion (nonmetal)
Modify the negatively charged ion name to end in -ide
Example NaCl is called sodium chloride
NOT sodium chlorine
Check Your Understanding
Mg+ Cl-
Cl- Ca+
Cl- K+
Mg+ Cl- Magnesium chloride
Cl- Ca+ calcium chloride
Cl- K+ potassium chloride
Charged atoms or compounds are called Ions
The total charge on the ion is known as the Oxidation Number of the atom
Examples:
Mg+2 is magnesium ion, the charge is positive 2, the oxidation number is 2+
F- has an oxidation number of 1-
Some metals have the same oxidation number in all compounds (memorize this)
Group 1 elements , oxidation number = 1+
Group 2 elements , oxidation number = 2+
Aluminum, oxidation number = 3+
The Charge of Ionic Compounds
In Ionic Compounds, the total positive charge is equal to the total negative charge
One Mg2+ ion will combine with 2 Cl- ions
Forming MgCl2, The total positive charge is 2+, the total negative charge is 2-
+2+-2 = 0
In a correctly written formula, the sum of the total positive charges and the total negative charges = 0
Predicting Oxidation Numbers
Oxidation numbers for most elements can be predicted from their position on the periodic table.
Groups 3-12 Transition metals are difficult as many of these elements Have more than one oxidation number depending on the reaction
Group 13 elements have 3 valence electrons, so will lose 3 and have an oxidation number of 3+
Group 14 may have 2+ or 4+ oxidation number
Groups 15, 16 and 17 tend to gain electrons to complete the octet since they are already ½ full
Their oxidation numbers are 3-, 2-, and 1- respectively
They can also lose electrons and have positive oxidation numbers
The tendency to lose electrons increases as you move down the column
Check for Understanding
Al
N
Cl
Mg
S
Na
K
O
Ga
P
Se
Br
Answers
Al 3+
N 3-
Cl 1-
Mg 2+
S 2-
Na 1+
K 1+
O 2-
Ga 3+
P 3-
Se 2-
Br 1-
Writing Chemical Formulas For Ionic Compounds
The key to writing formulas is to make the oxidation numbers add to zero, making a neutral compound.
Example:
The formula for a compound of these elements is
CaF2
The formula of a compound tells:
what elements make up the compound
and how many of each element are present in one unit of the compound
Example : H2O two H for each O
Progress Check
Sodium Fluoride
Potassium Chloride
Rubidium bromide
Sodium selenide
Potassium oxide
Lithium sulfide
Strontium fluoride
Calcium Chloride
Potassium Chloride (K+1 Cl -1) KCl
Rubidium bromide (Rb+1 Br-1) RbBr
Sodium selenide (Na +1 Se -2) Na2Se
Potassium oxide (K+1 O-2) K2O
Lithium sulfide (Li+1 S -2) Li2S
Strontium fluoride (Sr 2+ F 1-) SrF2
Calcium Chloride (Ca 2+ Cl 1-) CaCl2
The Formation Of Ionic Compounds
Lab Learning Objective: To model the transfer of electrons thereby achieving noble gas configurations and the formation of stable ionic compounds
Apply what you have learned!
The Formation Of Ionic Compounds (MiniLab 4.2)
Question: What other atoms give up and gain electrons (creating ions, forming ionic bonds) to form ionic compounds?
Pre-Lab Procedure
You and your lab partner must locate the following atoms on the periodic table: Li, S, Mg, O, Ca, N, Al and I.
Using the information on the Periodic Table, and what you have learned, complete the table with the Valence electrons, class of element, Lewis dot diagram, and electron configuration for the given atoms
Based on what you have discovered about the atoms above, construct a hypothesis predicting which of the atoms will give up electron(s) and which will receive electron(s) when forming compounds.
REMEMBER a hypothesis is an if… then… statement which answers the lab question
Have your hypothesis checked before you begin the lab.
The Formation Of Ionic Compounds
Learning Objectives:
To model the transfer of electrons thereby achieving noble gas configurations and the formation of stable ionic compounds
To name ionic compounds
To identify formulas of ionic compounds
Identify a pattern which can be used to predict which atoms will form Ionic compounds
Follow the Lab instructions to complete your investigation.
Prepare for a poster presentation
Polyatomic Ions
Some Ions are formed by more than one or two types of atoms
They are called polyatomic ions
Some of these are very common, so recognizing them will be very helpful to you.
Let’s look at some
Some do not follow simple naming rules, often because they were named before rules for naming were written
These we need to use a reference sheet for, or memorize!
Use your reference sheets to help name complex polyatomic ionic compounds
Conventions for Naming Polyatomic Ionic Compounds When Given the Formula
Steps
Name positive ion (cation) first, then negative ion (anion) second.
If one of these ions has more than 1 atom in it, look up the name on the chart
You should plan on memorizing the ones I have starred!
Determine if you need a Roman numeral in the name
If cation is NOT a transition metal, then NO Roman numeral
If the cation is a transition metal then see if it can have more than 1 oxidation number, if not then go to step 4
If yes then go to step 3
Determining the Roman numeral, usually the number of anions = the charge on the cation and the number of cations =anions.
Example: Fe2[SO4}3
There are three SO4 sulfate ions -- charge of 2-= 6 -
There are two Fe atoms – charge of ? = must total 6
Fe has to be 3+ so use Roman Numeral III Fe(III)
Name is Iron (III) sulfate!
Check to see that the sum of oxidation numbers= 0
If yes then correct!
6- + 6+ = 0 !!!!!
NH4Cl
Look at your list of common polyatomic ions, do you see any here?
Yes NH4 is ammonium it is a cation – 1+
Follow step 1---
Ammonium is the first part of the name
Chlorine is the second atom, the anion- 1-, becomes chloride (just like before)
So far we have ammonium chloride
Follow step 2---
Follow step 4
Now you try it alone!
Name the following :
NO3 nitrate = 1-
Three anions, so the charge needs to be 3+ so Roman numeral is III
Check
1 Fe(III) has charge of 3+
3 NO3 ions have a charge of 3 x 1- = 3-
3+ + 3- = 0 correct! Iron(III)nitrate
Let’s Try Another
Sharing electrons between atoms
Example:
Has one valence Electron
A group 16 element
Has 6 valence electrons
Could they achieve a stable octet by transferring electrons?
Could H give up its only electron and have a noble gas configuration?
No electrons!
When atoms collide with enough energy to cause a reaction
And neither atom attracts electrons strongly enough to take electrons from the other atom (small difference in electronegativity)
The atoms combine by sharing valence electrons
Reaction of Hydrogen and Oxygen
Hydrogen needs one more electron to have the same electron configuration as He
Oxygen needs two more electrons to have the same electron configuration as Ne
Hydrogen and Oxygen can share one electron from each atom.
This makes Hydrogen stable
Sharing to Make Oxygen Stable
Oxygen gets a complete octet by sharing an electron with another hydrogen
This explains the formula of H2O
Electrons always rearrange in a chemical reaction
Covalent Bond
The attraction of two atoms for a shared pair of electrons is called a covalent bond
Notice, neither atom has an ionic charge
A compound whose atoms are held together by covalent bonds is called a covalent compound
A molecule is an uncharged group of two or more atoms held together by covalent bonds
Sharing More Than Two Electrons
More than two electrons can be shared.
The reaction Between Carbon and Oxygen for example
You can arrange these 16 valence electrons to produce a molecule in which all three atoms have a noble gas configuration (a complete octet)
Sharing two pairs of Electrons: Double Covalent bonds are formed
Atoms can also form Triple bonds
Molecular Elements
Molecular Elements: a molecule that forms when atoms of the same element bond together
They are not compounds (not two elements)
7 nonmetal elements naturally found as diatomic molecules:
Hydrogen
Nitrogen
Allotropes: molecules of an element that form different crystalline structures.
The properties of allotropes are usually different, because structure is important
Formulas and Names of Covalent Compounds
Binary inorganic compounds (2 elements, not carbon)
The Suffix –ide
Write first nonmetal followed by the name of the second nonmetal with its ending changed to – ide
The element closest to the left of the periodic table is written first ( some exceptions with H)
If both are in the same group, name the one lower on the column 1st.
Indicating the number of atoms
Add a prefix to the name of each element indicating the number of each element
Add prefix to indicate number of atoms
If only one atom of the first element is listed, the mono is usually left out
If adding the prefix creates double vowels, the first is usually omitted
Example of Naming Protocol
General Atomic Bonding Trends
Two nonmetallic elements usually achieve stability by sharing electrons to form a covalent compound
Reacting atoms, when one is a metal and one a nonmetal, are much more likely to transfer electrons and form an ionic compound.
Comparing Ionic and Covalent bonds
When elements combine they form either ions or molecules
This changes them dramatically, this is why compounds have different properties from the elements that make them up
Properties of Ionic Compounds
They are crystalline solids at room temperature
They are generally hard, rough and brittle
They have high melting points
They are generally soluble in water and form a solution which conducts electricity (called an electrolyte)
In liquid state they also conduct electricity
Properties of Covalent Compounds
Relatively weak Interparticle forces: forces between particles that make up a substance
The molecules have no ionic charge so the attractive force between them is weak
Many are liquid or gas at room temperature
If they are solid at room temp, they have low melting point
Ex. Sugar
They do not conduct electricity
Many do not dissolve in water (ex vegetable oil), but some do (sugar)
Generally Less soluble in water
Interparticle Forces Make the Difference
Interparticle Forces are the key to determining a substances state of matter at room temperature as well as many other properties
Compare and Contrast Properties
Ionic Compounds
Covalent Compounds
Ions held tightly in solid state by strong interparticular forces- solid at room temp
Good conductors in liquid form and when dissolved in water
Water soluble- ions are attracted by water
Much weaker interparticular forces hold them together more loosely- liquid or gas at room temp (few solids)
Not electrical conductors
Check your Understanding
Describe two processes by which elements can form stable compounds? What is the type of bonding that occurs from each process?
An unknown compound dissolves in water but does not conduct electricity. Is the compound more likely ionic or covalent? Explain
Why does NaCl have to be heated to 800 degreesC before melting, and candle wax starts to melt at 50 degrees C.
Explain why ionic compounds conduct an electric current in solution but covalent compounds do not?
Answers
Two processes by which elements can form stable compounds are transferring valence electrons (Ionic Bonds) or sharing valence electrons (Covalent Bonds)
It is more likely to be covalent because ionic compounds in solution would be made of mobile ions that would conduct an electrical current
NaCl is an ionic compound and is held together by strong interparticle forces which have to be broken in order to melt, candle wax is a covalent compound with very weak interparticle forces so melting is easy.
To conduct electricity there must be ions that are free to move so that they can pass the electrons along, this does not happen with covalent compounds but does with ionic compounds in solution.
Predicting the Type of Bond
The type of bond formed depends on two properties
Electron configuration
These bonds are called polar covalent bonds
They have some ionic character
The unequal sharing creates 2 poles (- end and + end)
The less electronegative atom will be positive, less attraction for the electron
Differences in electronegativity can determine bond type
Differences greater than 2.0 makes ionic bond
Differences between 0.8 and 2.0 makes polar covalent
Differences between 0.0 and 0.5 covalent
Lets See how This Works!
The bond between calcium and oxygen, is it ionic, covalent or polar covalent?
Ca electonegtaivity is 1.0
O electonegtaivity is 3.5
3.5 - 1.0 = 2.5
2.5 is greater than 2.0 so it is an ionic bond!
Applying Our Knowledge
Using figure 9.2 0n page 302, determine the electronegativity difference to classify the bonds between the following atoms as ionic, covalent or polar covalent.
NiO
BN
CaCl2
FeSi
NaF
Zn3P2
Answers
NiO Ni 1.8, O 3.5, 1.8 – 3.5 = 1.7 polar covalent
BN B 2.0, N 3.0, 2.0 – 3.0 = 1.0 polar covalent
CaCl2 Ca 1.0, Cl 3.0, 1.0 – 3.0 = 2.0 ionic
FeSi Fe 1.8, Si 1.8, 1.8 – 1.8 = 0.0 covalent
NaF Na 0.9, F 4.0, 4.0 – 0.9 = 3.1 ionic
Zn3P2 Zn 1.6, P 2.1, 1.6 – 2.1 = o.5, covalent
Bonding in Metals
Bonding in metals DOES NOT RESULT IN COMPOUNDS
It results in an interaction that holds metal atoms together and accounts for some of their common properties.
Remember, metals are Malleable (thin sheets) and ductile (drawn into wires) and conduct electricity
Conductivity is a measure of how easily electrons can flow through a material- electrical current.
THESE PROPERTIES ARE THE RESULT OF THEIR BONDS
How Do They Bond?
Metal to Metal bonding does not involve valence electron transfer, as it does in metal-nonmetal bonding
Metal atoms release their valence electrons in a sea of electrons, shared by all of the metal atoms
This is called a metallic bond
Positively charged nucleus surrounded by loosely associated electrons shared with all surrounding nucleus
Properties Associated with Metallic Bond
Electrons are bonded to a large network, not to a single atom.
More freedom of movement
Easy, free movement of electrons makes conductivity easy to understand
Comparing and Contrasting Ionic, Covalent and Metallic Bonds
Ionic Bonds
Covalent Bonds
Metallic Bonds
N/A
Molecules, or molecular elements
no
yes