students scorebooster series videos waec, ssce, gce, · pdf filefor radicals, the valency is...

Students ScoreBooster Series Videos

WAEC, SSCE, GCE, JAMB (UTME),

NECO and NABTEB

Chemistry

Presented by

Chemical Combination I

A.A.S Lateef

Chemical combination

• Learning objectives, at the end of this lecture,

Students should be able to explain the concept of bonding.

Students should be able to apply the concept of valency in

forming simple compounds.

Naming of simple compounds is also expected

Properties of the compounds formed should be highlighted and

adequately explained.

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The Periodic Table

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Why do atoms form bond

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Sodium and chlorine are both elements.

When sodium is heated and placed in a jar

of chlorine, it burns with a bright flame

The result is a white solid that has to be scraped

from the sides of the jar. It looks completely

different from the sodium and chlorine.

So a chemical reaction has taken place. The white solid is sodium chloride.

Atoms of sodium and chlorine have bonded (joined together) to form a compound. The

equation for the reaction is:

sodium + chlorine ---------sodium chloride

Why do atoms form bonds

Looking at the elements of the group 0

Atoms bond with each other in order to gain a stable arrangement of outer -

shell electrons, like the atoms of Group 0 (or 8).

In other words, they bond in order to gain 8 electrons in their outer shell (or

2, if they have only one shell).

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6

How sodium and chlorine form ions

Sodium has only one electron in the outermost shell, so it loses one

electron to obtain a stable configuration of 8 outer electrons.

For Chlorine, it attains its stable 8 electrons by accepting 1 electron

because it has 7 electrons in its outermost shell.

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7

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Ions

An atom becomes an ion when it loses or gains electrons.

An ion is a charged particle. It is charged because it has an unequal

number of protons and electrons.

Valency: This is defined as the combining power of elements. It is, in other

words, the ions of the elements. For radicals, the valency is the power on

the group that makes up the radical.

Sometimes the group to which any element belongs can be used to

determine the valency of such elements.

Examples: Na+, Cl-, K+, Mg2+, NO3- etc

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IA family: Each element has one electron in its outermost shell, so it forms

ions with the loss of its one electron.

IIA family: Each element has two electrons in its outermost shell, so it forms

ions with the loss of the two electrons.

IIIA family: Each element has three electrons in its outermost shell, so it

forms ions with the loss of the three electrons.

VA family: Each element has five valence electrons, so it gains three

electrons to form an anion with a 3- charge.

VIA family: Each element has six valence electrons, so it gains two electrons

to form an anion with a 2- charge.

VIIA family: Each element has seven valence electrons, so it gains one

electron to form an anion with a 1- charge.

Determining the number of electrons that members of the transition metals

(the B families) lose is more difficult. In fact, many of these elements lose a

varying number of electrons so that they form two or more cations with

different charges.

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Ions and their charges

Forming Chemical Bond

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A chemical bond is the physical process that causes atoms and molecules to

be attracted to each other, and held together in more stable chemical

compounds.

There are two ways of forming chemical bond; by balancing the charge and

crisscross rule.

Balancing the charges

The charges on the cations of the participating elements must be balanced

with that on the anion and vice versa.

For example, consider the reaction between calcium and chlorine to form

calcium chloride; (i) Ca = 2+, Cl = -1. Forming the compound, each Ca

atom requires 2 atoms of chlorine to form a neutral compound, hence we

have CaCl2. (ii) Sodium with sulfur to form sodium sulfide reacts as

follows: Na = +1, S = 2-. Two sodium atoms is required to react with one

atom of Sulphur forming Na2S.

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THE CRISSCROSS RULE A quick way to determine the formula of an ionic compound is to use the

crisscross rule: Take the numerical value of the metal ion’s superscript

(forget about the charge symbol) and move it to the bottom right-hand side

of the non-metal’s symbol — as a subscript and vice versa. (Note that if

the numerical value is 1, it’s just understood and not shown.)

Forming Compounds (Crisscross rule)

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Consider aluminium reacting with oxygen as illustrated below;

Compounds involving polyatomic ions work in exactly the same way. For

example, here’s the compound made from the ammonium cation (NH4+) and

the sulfide anion (S2–): (NH4)2S

NB: Because two ammonium ions (two positive charges) are needed to

neutralize the two negative charges of the sulfide ion, the former is enclosed in

the parentheses and a subscript 2 is added.

Also, after the crisscross rule is used, the subscripts are reduced by a common

factor, if possible, to get the right formula. e.g. Mg2O2 = MgO.

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Class Exercise

Using the crisscross rule together with figures on pgs 10 & 11,

write at least 12 ionic compounds.

1. Electrovalent (Ionic) Bonding

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Electrovalent bond is a type of bond that involves transfer of electron

between a highly electropositive metal and electronegative non-metal. e.g

sodium and chlorine. It can also involve a highly electropositive metal and

a moderately electronegative non-metal. e.g magnesium and oxygen,

potassium and hydrogen etc. The electronegativity value is the key here.

The electronegativity value difference must be wide enough to permit

transfer of electrons between the participating elements.

Electrovalent (Ionic) Compounds

The compounds which contain ionic or electrovalent bonds are known as

Electrovalent or Ionic Compounds. Mainly electrovalent compounds are

formed due to reaction between highly electropositive and highly

electronegative atoms. e.g. sodium/ potassium/calcium/magnesium with

chlorine/fluorine.

Creating Ionic Compounds Ionic bonding, the bonding that holds the cations and anions together, is

one of the two major types of bonding in chemistry.

NB: An ionic bond occurs between a metal and a non-metal. The metal

loses electrons (to becomes a positively charged cation), and a non-metal

gains those electrons (to become a negatively charged anion). The ions

have opposite charges so they’re attracted to each other. This attraction

draws them together into a compound.

The transfer of an electron creates ions — cations (positive charge) and

anions (negative charge). Opposite charges attract each other, so the cations

and anions may come together through an ionic bond. An ionic bond is a

chemical bond (a strong attractive force that keeps two chemical elements

together) that comes from the electrostatic attraction (attraction of opposite

charges) between cations and anions. Together, the ions form a compound.

For instance, consider the reaction of sodium with chlorine done earlier!

17

Creating Ionic Compounds (cont’d)

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For instance, sodium, a metal, can fill its octet and achieve stability by

losing an electron. Chlorine, a non-metal, can fill its octet by gaining an

electron. If the two are in the same container, then the electron that sodium

loses can be the same electron that chlorine gains. The Na+ cation attracts

the Cl– anion and forms the compound NaCl, sodium chloride.

Compounds that have ionic bonds are commonly called salts. In sodium

chloride — table salt — a crystal is formed in which each sodium cation is

surrounded by six different chloride anions and each chloride anion is

surrounded by six different sodium cations.

Different types of salts have different crystal structures.

Characteristics of electrovalent compounds

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Crystal Structure: In solid state of electrovalent compounds, anions and

cations are arranged in regular manner called crystals.

Electrical Conductivity: In molten and solution forms electrovalent

compounds conduct electricity because ions flows in molten and

solution forms.

Appearance: Ionic or electrovalent compounds are generally hard and

their hardness increases with increasing ionic charge and decreasing

distance between ions.

Solubility: Ionic or electrovalent compounds are soluble in polar

solvents like water and insoluble in non polar solvents like benzene,

ether, alcohol.

Melting point and boiling point: Electrovalent or ionic compounds have

high Melting and boiling points because large amount of energy is

needed to break strong ionic bonds.

Naming ionic compounds

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In naming inorganic compounds, the name of the metal is put first which

is then followed by that of the non-metal with -ide at the end (for

compounds involving monatomic ions).e.g. MgO == Magnesium Oxide.

For polyatomic radical-containing compound, the principle is the same

except that –ide is not necessary. e.g. (NH4)2CO3 ----ammonium

carbonate.

For metals with varying oxidation state, e.g. transition metals, the name of

the metal is written first and the oxidation state is put in parenthesis

followed by the name of the attachment. e.g. Fe(CN)3: iron(III) cyanide

Elements react together to attain a complete and stable octet (8 electrons) or

duplet (2 electrons) configuration.

Formation of a compound results from reaction between elements.

Compound formation either involves transfer or sharing of electrons.

Valency is the combining power of element.

Metals lose electrons while non-metals gain electrons.

The number of electron lose by metal is the valency while the one gained

by non-metal is the valency of that non-metal.

Ionic bonding involves lose of electron(s) by the metal and transfer of such

electron(s) to the non-metal.

Compounds are formed by interchanged of charges and the charges need to

be balanced for such compounds to be neutral.

Ionic compounds are electrolyte in molten and solution forms. They are

soluble in polar solvents etc

The name of the metal is mentioned first and the oxidation number is put in

parenthesis (if required) before the name of the negative ion or radical.

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Points to note:

Examples

•

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JAMB 1988, Q2

JAMB 1998, Q9

The diagram above represents element

that can combine with chlorine to form

a. Covalent bond

b. electrovalent bond

c. Hydrogen bond

d. Coordinate bond

Examples

•

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JAMB 1988, Q2

JAMB 1998, Q9

The diagram above represents element

that can combine with chlorine to form

a. Covalent bond

b. electrovalent bond

c. Hydrogen bond

d. Coordinate bond

Solution

M =3, X= 4, by crisscross

rule, the compound formed

is M4X3. Option C.

Hint: The element has 2

valence electrons, hence a

group metal. Reacting with

Cl will form electrovalent

bond. Option B

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