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CHAPTER- 2 BONDING

Standard Level

A chemical bond is formed when outer-shell electrons of different atoms comeclose enough to each other to interact and re-arrange themselves into a more

stable arrangementone with a lower overall chemical potential energy. All

chemical bonds are based on the electrostatic attractionbetween positive and

negative particles. When two or more atoms approach one another to form a

bond, it is their outer-shell electrons that generally interact. Outer-shell

electrons are so important in the generation of chemical bonds that they are

given a special namevalence electrons.

There are three kinds of bonds-a. Metal atoms and Non-Metal atoms combine to form ionic bonds

b. Metal atoms and Metal atoms combine to form metallic bondsc. Non-Metal atoms and Non-Metal atoms combine to form covalent bonds

Bond dissociation enthalpy can be defined as the enthalpy required to breakthe bonds between 1 mole of bonded atoms. The larger the value, the stronger

the bond.

1. IONICBONDINGMETALS NON-METALS

Metals tend to be shiny, have high

melting and boiling points, and aregenerally good conductors of heat and

electricity. Metal atoms tend to have low

num

bers of electrons (usually one ortwo) in their valence shells and, for this

reason, have a tendency to lose these

electrons so as to gain an outer-shell

octet of electrons.

Non-metals are generally poor

conductors of electricity and heat. Theyhave low melting points, and several are

gases at room temperature. They have

high num

bers of electrons in theirvalence shells and will readily accept

further electrons in order to reach the

desired outcome of eight outer-shell

electrons.

Ionic bonding occurs as the result of a metal atom donating its valence electron(s)

to a non-metal atom. As the metal atom loses electrons, it will gain an overall

positive charge, becoming a positively charged ion (a cation). Similarly, the non-

metal atom accepts electrons, becoming a negatively charged ion (an anion). The

positive and negative ions are attracted to each other by electrostatic attraction,

producing a neutral compound.The ionic compound formed is hard and brittle and is

often described as a salt.

Eg. Sodium reacts explosively with chlorine gas to produce the white crystalline

solid sodium chloride. Sodium is a highly reactive metallic element in group 1 of theperiodic table. It has an electron arrangement of 2,8,1. The one valence electron is

lost when sodium forms a bond and the sodium ion, Na+, is formed. Chlorine has 17electrons and an electron arrangement of 2,8,7. Chlorine has a strong affinity (or

attracting power) for electrons and will readily accept one additional electron to fillits valence shell, resulting in the chloride ion, Cl.


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The size of the positive sodium ion is smaller than the parent ion because one electronis lost and thus it looses one energy level. In the case of chlorine, the addition of one

electron has completed the octet and made it a negative ion. The presence of extraelectron adds to the greater repulsion and thus the anion is bigger than the neutral

atom.

4.1.2: Determine which ions will be formed when metals in groups 1, 2 and 3 lose

electrons.

The formation of ions is a process that makes the atoms electronic structure more

stable. Metal atoms form groups 1, 2 and 3 lose all of their outer shell electrons when

forming ions. As each electron corresponds to a negative charge then the loss of one

electron produces a positive ion with one positive charge. The loss of two electrons

produces a positive ion with two positive charges. Etc

y Group 1 metals form single positive ionsy Group 2 metals form double positive ionsy Group 3 metals form triple positive ions

4.1.3: Determine which ions will be formed when elements in groups 6 and 7 gain

electrons.

Non-metals gain just enough electrons to fill their outer shells. It may be seen from

the electronic configuration of group 6 elements that their outer shells with 6

electrons need another two electrons to be filled. Each electron gained corresponds to

a negative charge and so group 6 non-metals form double negative ions.

Sulphur (2,8,6) --> [sulphide ion (2,8,8)]2-

Similarly group 7 elements need only one electron for a full outer shell and so form

single negative ions.Chlorine (2,8,7) --> [chloride ion (2,8,8)]-

4.1.4: State that transition metals can form more than one ion. Restrict examples to

simple ions eg Fe2+ and Fe3+.

- Variable ion formation

As stated above the normal 'rules' do not apply to the transition metals as they

can form more than one type of ion. The reason for this lies in the electronic

configuration, which involves electrons from the 3rd level called 'd' electrons.

Examples

Fe2+ and Fe3+

In these two ions the Iron atoms have lost either two electrons (in the case of

Fe2+) or three electrons (in the case of Fe3+). This type of behaviour is possible

for all of the transition metals. Iron II compounds (containing Fe2+ ions) are

usually pale green in colour whereas Iron III compounds (containing Fe3+ ions)

are usually yellow, orange or red.


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negatively charged ions, and the ions with the same charge are as far apart as possible.

This arrangement serves to maximize the electrostatic attraction between the positive

and negative ions and minimize the repulsion between like charged ions, thuslowering the overall chemical potential energy of the lattice.

Sodium chloride provides a good example of an ionic lattice. Each positive sodium

ion is surrounded by six chloride ions, and each chloride ion is surrounded by sixsodium ions. A sodium chloride crystal is cubic in shape.

METALLICBONDING

Metal ions, formed when atoms lose their valence electrons, are arranged in a three-

dimensional lattice. This array of ions is surrounded by freely moving electrons that

form a sea of mobile electrons. These electrons are said to be delocalized, as they

are not confined to a particular location but can move throughout the structure.

Electrons are attracted to positively charged ions. This electrostatic attraction holds

the lattice together, and prevents the ions pushing each other apart due to the

electrostatic repulsion of like charges. This type of bonding is called metallicbonding.

Why do the metal atoms release their valence electrons to form the sea of electrons?

Metal atoms achieve greater stability by releasing their valence electrons. Without

their valence electrons, the metal atoms achieve a noble gas configurationan outer-

shell octet of electrons. When non-metals are present, these valence electrons are

transferred to the non-metal atoms, giving rise to the ionic bonding.

PROPERTIES OFMETALS

Conductivity- The electrical conductivity of metals can be explained by the presence

of the sea of delocalized electrons that surrounds the lattice of positive metal ions. Inthe solid state these electrons can move freely and will respond to the application of a


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Describe the covalent bond as the electrostatic attraction between a pair of electrons and positively

charge nuclei?

Describe how the covalent bond is formed as a result of electron sharing?

chapter 2 bonding sl

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