Chapter 9

Chemical Bonds

Elements and Compounds

• Most of the matter that you find on earth is not in the form of elements, but in the form of compounds.

• Compounds are combinations of elements that are “bonded” together. e.g. water is H2O is made up of two H (hydrogen atoms) and one O (oxygen atom).

• Compounds, like elements, can be solids, liquids, or gases at room temperature, 25oC.

Elements

• Elements can be mono atomic (single atom) or diatomic (two atoms bonded together) when found in nature or produced in the lab.

• Most elements are mono atomic with the exception of:

H2, N2, O2, and the halogens, F2, Cl2, Br2, I2.

• Some even occur in a tri atomic form: O3, an allotrope of oxygen, called ozone.

• Phophorous occurs as P4 and sulfur occurs as S8.

Compounds• Compounds, which occur when you have two

different types of elements together, consist of two types of bonding:

Ionic Bonding, where ions of the elements are held together by electrostatic interactions, since a positive charge attracts a negative charge. e.g. NaCl is made up of Na+ and Cl-. For ionic bonding to occur there must be a metallic element present which will from a positive ion. The other ion will be a negative ion from either a single nonmetallic element, occasionally a metalloid, or a group of nonmetals which form ions called polyatomic ions. e.g. LiNO3, composed of Li+ and NO3

-. These compound consist of units called formula units.

Compounds

Covalent Bonding, where there are no ions present. Only atoms of nonmetallic elements and metalloids will engage in covalent bonding. Electrons are shared between two atoms so that each atom can satisfy its desired octet of electrons and be stable like a noble gas. e.g. H2O, CO2. These compounds consist of units called molecules.

Ionic Compounds• Using the group number as a guide, you must determine

the number of valence electrons in atoms of representative elements only. The number of valence electrons is equal to the group number.

• All elements desire to have 8 electrons in their valence shell, the last energy level where electrons are present.

• For metals in groups IA, IIA, and IIIA, the way for them to achieve this is to lose 1, 2, or 3 electrons respectively, so that they can go to the previous energy level where there will be 8 electrons and they will therefore have an octet of electrons.

• In group IVA the metals lead (Pb) and Tin (Sn) can lose 4 electrons but sometimes they only lose 2 electrons (an exception).

Ionic Compounds

• For nonmetals in groups VA, VIA, and VIIA, the way for them to achieve this is to gain 3, 2, or 1 electrons, respectively, so that they can end up with 8 electrons, and octet.

• In group IVA the nonmetal carbon (C) can gain 4 electrons to achieve an octet.

• The elements in group VIIIA are the noble gases and they neither gain or lose electrons, since they already have an octet of electrons and are therefore satisfied.

Crystal Structure formed by sodium ions , Na+

And chloride ions, Cl- .

Crystals of NaCl magnified shows you the shape of the cubic structure.

Ions of Representative Elements

Group #

IA

IIAIIIA

VIAIA

VIIA

Li and Br form LiBr, since the Li+ and Br- get together in a 1:1 ratio and the net charge of the compound is “0”.

One electron from Li is transferred to Br.

Examples of Ionic Compounds• Mg and O get together to form MgO, since Mg2+

and O2- can be in a 1:1 ratio so that the sum of the charges, +2-2=0. The compound is neutral.

• Ca and I get together to form CaI2, since the Ca2+ and the I- need to get together in a 1:2 ratio so that +2-1-1=0.

• Al and S get together to form Al2S3, since the Al3+ and the S2- get together in a 2:3 ratio so that +3+3-2-2-2=0.

• The electrons that are lost by the metal travel to the nonmetal.

• The charge for the metal becomes the subscript for the nonmetal and vice versa.

Binary Ionic Compounds• What compounds are formed between:

• Ca and S • CaS

• Al and Br • AlBr3

• K and O• K2O

• Mg and N• Mg3N2

Formation of ionic compounds

Transition Metal Ions and Group IVA Metal ions

• Most of the transition metal ions can have more than one charge, with the exception of four of them.

• In addition there are two metallic elements in group IVA, tin (Sn) and lead (Pb), which can also have more than one charge. They can have a +4 charge, as expected from the group number, or they can have a +2 charge.

• These ions which can have more than one charge also get together with negative ions in ratios needed to end up with a charge of 0.

Polyatomic Ions

• There are ions which also form ionic compounds which consist of more than one element. These are clusters of atoms together which have a charge, so they can take part in ionic bonding.

• Most of the polyatomic ions are negative, only one, the ammonium ion, NH4

+, is positive. It is the only polyatomic ion that can take the place of a metal ion in an ionic compound.

Ionic Compounds with Polyatomic Ions:

• If Ca and sulfate get together to form an ionic compound:

Ca is in group IIA, so ion formed is Ca2+. Sulfate is SO4

2-. The compound formed is CaSO4, since +2-2=0.

• If aluminum and nitrate get together to form an ionic compound:

Al is in group IIIA, so ion formed is Al3+.

Nitrate is NO3-. The compound formed is

Al (NO3)3, since +3-1-1-1=0.

Naming Ionic Compounds

• Name the first element, then the second element followed with ending –ide.

• Never use prefixes.• If polyatomic ion is present name the polyatomic ion in

the order it appears.• If the metal ion can have more than one charge (most

transition metal ions and also tin and lead), indicate the charge with a roman numeral.

NaCl is sodium chloride MgSO4 is magnesium sulfate. (NH4)3PO4 is ammonium phosphate

CaI2 is calcium iodide CuO is copper (II) oxideAl2S3 is aluminum sulfide KOH is potassium hydroxide

Ionic Compounds

• Ionic compounds are all solids at room temperature.

• When they dissolve in water they are dissociated into the ions that make it up (ionized).

• Because you have ions in water these solutions of ionic compounds conduct electricity and are electrolytes.

NaCl(s) water Na+

(aq) + Cl-(aq)

Ionic compound dissolved in water

Covalent Compounds• Contain only nonmetals or metalloids. No metals

allowed.• The bonds between the atoms do not consist of

electrostatic attractions of positive and negative ions like in ionic compounds. Instead, they consist of a sharing of electrons between the atoms. The net effect is that each atom will be exposed to 8 electrons, an octet, which is the most desired configuration of electrons.

• You cannot always predict what the formulas will be based on the number of valence electrons of the atoms. In fact, you can have more than one formula involving the same elements. e.g. CO and CO2; PCl3 and PCl5.

• Covalent compounds are made up of molecules, which are distinct groupings of atoms. This is different from ionic compounds, which contain individual ions. Ionic compounds do not consist of molecules, they consist of formula units, which are the simplest ratios of ions that will give a compound with a charge of 0.

Bonding in Covalent Compounds

• Chlorine, Cl2 bonds by sharing 2 electrons between the 2 chlorine atoms:

. . . . :Cl : Cl : Cl-Cl Single bond between Cl . . . .

• Water, H2O has two bonds between H and O, and in each the H and O share 2 electrons:

. . H:O:H H-O-H single bonds between H and O . .

Double and Triple Bonds

• Sometimes atoms share 2 or 3 pairs of electrons and that gives rise to double bonds and triple bonds.

.. ..O::C::O or O=C=O.. ..

:N:::N: or N N

Naming Covalent Compounds

• Name the first element, then the root of the second element with the ending –ide.

• Use a prefix in front of each of the two elements, but do not use one if the first element is only one.

• If two vowels follow each other between the prefix and the name of the element, eliminate the vowel in the prefix.

S2O8 is disulfur octoxidePCl5 is phosphorous pentachlorideN2S5 is dinitrogen pentasulfide

Electronegativity• Every element has an electronegativity (EN)

value. • The higher the value the more the attraction that

that element has for electrons.• Fluorine has the highest electronegativity and is

assigned a value of 4.• The alkali metals have the lowest EN.• The halogens, oxygen and nitrogen have the

highest EN.• If two different elements are covalently bonded

together, the more electronegative one will pull electrons closer to it, away from the less electronegative one.

Electronegativity

• Electronegativities increase from left to right in the periodic table (nonmetals are more electronegative than metals), and from bottom to top in the periodic table. Remember that fluorine (F) is the most electronegative atom and it is in the upper right hand corner of the periodic table.

• Hydrogen, as a nonmetal, has a much higher electronegativity than the other elements in group IA. However, its electronegativity is lower than F, O, Cl, N, and C.

Bond Polarities

• When two nonmetals and/or metalloids are bonded together in a covalent bond, the more electronegative atom will pull electrons closer to it.

H : O : H The oxygen becomes slightly negative and each hydrogens becomes slightly positive.

This is what is called a polar bond.

Types of bonds

• Depending on the difference in the EN values of the atoms which are bonded to one another, there are 3 possible bonds that form:

Types of bonds

Examples:

Water has bonds between H and O, so the EN is 3.5-2.1=1.4.Therefore this is a polar covalent bond.

Sodium chloride has a bond between Na and Cl, so the EN is 3.0-.9=2.1. This is an ionic bond.

Methane, CH4, has bonds between carbon and hydrogen, so the EN is 2.5-2.1=.4. This is a covalent bond.

Review Problems

• p. 249-250 Applying the Concepts:# 1, 2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 17, 19, 21.p. 251-252 Parallel Exercises Group A:#1, 5, 6, 7.New Book: p. 267-270 # 1, 2, 3, 4, 5, 7, 8, 9, 10,

12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53.

Parallel Exercises, Group A, p. 271: #1, 2, 3a and d, 4a-d, 5, 6, 7.

Review• Mono atomic and diatomic

elements.• Ionic bonding, covalent bonding,

formula units, molecules.• If metal present, ionic compound.

Otherwise, covalent compound.• Relationship between group

number, number of valence electrons and charges of ions.

• Formation of ionic compounds (formulas) when given two elements, one metal and one nonmetal.

• Ions with more than one charge: Transition metal ions (except Ag+1, Zn2+, Cd2+) and lead and tin.

• Polyatomic ions• Formulas of ionic compounds

having polyatomic ions.• Ionic compounds are

crystalline, solids, conduct electricity when dissolved in water only, (electrolytes)

• Naming ionic compounds.• Bonding in covalent

compounds: Sharing electrons.• Single, double and triple

bonds.• Naming covalent compounds.• Electronegativity• Polar covalent bonds• Distinguishing between ionic,

polar covalent and covalent bonds based on EN.