UNIT 6 : THE CHEMICAL BOND
MÂŞ Eugenia Becerril Quintero Science Departament I.E.S. Clara Campoamor
1. OCTET RULE
The octet rule says that atoms tend to gain, lose or share electrons so as to have eight electrons in their outer electron shell. This is because a full outershell with eight electrons is relatively stable. Atoms need to obtain an electron configuration like that of the nearest noble gas. Except for helium, noble gases have eight electrons in their valence sheels. The octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as its nearest noble gas.
1- Read the octet rule and answer the following questions: a- How many electrons make atoms be stable? b- Which atoms always have eight electrons in their outer shell? c- Do atoms want to become noble gas? d- What is a " valence shell"?
2. LEWIS STRUCTURE A Lewis symbol is a symbol in which the electrons in the valence shell of an atom or ion are represented by dots placed around the letter symbol of the element. Each dot represents one electron.
For ions, the chemical symbol is surrounded by the number of valence electrons present in an ion. The whole structure is then placed within square brackets, with a superscript to indicate the charge on the ion. 2- Write the Lewis structure of the following atoms: a- Li b- Be c- B d- C e- N f- O g- F h- Ne i- Li+ j- F_ k- Be++
3 . IONIC BONDING Ionic bonding takes place between metals and non-metals. This is because all atoms wish to get a full outer shell of electrons, like the noble gas elements. Ionic bonding involves the transfer of electrons from one atom to another. Metal atoms lose electrons to get a full outer shell of electrons. Overall, they become positively charged. Non-metal atoms gain electrons to obtain a full outer shell, and so they become negatively charged. The attraction between these two oppositely charged ions is called an ionic bond and holds the compound together. For example, sodium reacts with chlorine to make sodium chloride: sodium
+
chlorine
sodium chloride
•
The sodium atom transfers one electron from its outer shell to the chlorine atom.
•
Both the sodium and the chlorine atoms have full outer shells.
•
The sodium atom has lost an electron so it has a 1+ charge. It is now a sodium ion.
•
The chlorine atom has gained an electron so it has a 1- charge. It is now a chloride ion.
•
The attraction between these two oppositely charged ions is called an ionic bond and holds the compound together.
Ionic compunds have a regular structure . Their properties are: •
The strong forces of attration make ionic compunds have high melting and boiling points.
•
When dissolved in water, ionic compounds form solutions in which ions can move. This means that these solutions can conduct electricity.
•
Molten ionic compounds can also conduct electricity.
3. How many outer electrons do elements in Group 1 have?
4. How many outer electrons do elements in Group 6 have? 5. If an element in Group 1 loses an electron , what charge does it have? 6. If an element in Group 6 gains one electron, what charge does it have?
7. Draw a Lewis diagram to show sodium and chlorine
8. Draw a Lewis diagram ( dot and cross) to show sodium chloride.
9. What kind of bonding occurs between calcium and sulphur?
10. Draw a Lewis diagram to show calcium and sulphur.
11. Draw a dot and cross diagram (Lewis diagram) to show calcium reacting with sulphur to make calcium sulphide.
12. Complete this passage. Atoms wish to get a ............................ outer shell of ..................... .Ionic bonding involves the transfer of ..................... . Metal atoms ..................... electrons to gain a ....................... charge. Non-metal atoms ................... electrons to gain a ................... charge . An ionic bond is the ........................... between these oppositely charged ions. Sodium chloride is a ........................ formed from the metal ..................... and the non-metal ..................... . When sodium reacts with chlorine, sodium atoms ................. an electron to form sodium ........................ which have a ................... charge. Chlorine atoms .................. an electron to form chloride ions that have a ...................... charge . These ions are then held together by ................... bonds.
13.True or false? 1. Ionic bonding involves the sharing of electrons 2. Oxide ions have a 2+ charge 3. Potassium ions have 1+ charge
14. Which of these statements about ionic compounds is true or false? a- They usually dissolve in water b- They do not conduct electricity when molten c- They have low melting points d- They contain bonds that involve shared pairs of electrons
4. COVALENT BONDING Covalent bonding occurs between atoms of non-metal elements and forms molecules. The atoms share pairs of electrons so that all the atoms gain a full outer shell of electrons. Each shared electron belongs to each atom which takes part in the bond.
• •
Two hydrogen atoms form the molecule of the element hydrogen H2. Two hydrogen atoms share a pair of electrons to have the same elctronic configuration as helium.
Covelent bonding is very strong. Covalent compounds have the following properties: • • • •
They have low melting and boiling points. Most are gases or liquids at room temperature. They don't conduct electricity. They tend to be insoluble in water.
15. What type of bonding occurs between non-metal atoms?
16. Draw a Lewis diagram to show hydrogen reacting with chlorine.
17. Draw a Lewis diagram to show carbon reacting with hydrogen.
18. Draw a Lewis diagram to show nitrogen reacting with hydrogen.
19. What's the reason why non-metals share electrons in covalent bonding and gain electrons in ionic bonding?
20. Use the words below to complete the table. Word (s)
Description
Ionic bond
a)
A shared pair of electrons
Covelent bond
b)
The charge an ion has when it has gained an electron during a chemical reaction
Electrons
c)
The attration between oppositely charged ions
Negative
d)
Particles shared between atoms in a covelent bond
5. METALLIC BONDING In metals, the metal atoms lose their outer electrons to form metal cations. The electrons from all the metal atoms form a "sea" of electrons that can flow around these metal cations. These electrons are often described as delocalised electrons.
As the metal cations and electrons are oppositely charged , they will be attracted to each other. These electrostatic forces are called metallic bonds, and these are what hold particles together in metals.
Metallic bonding means that metals have several very useful properties: •
The free electrons make metals good electrical conductors
•
The free electrons also make metals good thermal conductors.
•
They have high melting points.
•
They have a high density.
•
They can be mixed with other metals to form alloys.
21. Which of these properties do you not expect of a metal? a- a good electrical conductor b- a good thermal conductor c- low melting points d- brittle 22- What is a mixture of metals called? a- emulsion
b- compound
c- oxide
d- alloy
23- Why are metals able to conduct heat and electricity?
24. Consider the following statements and decide if each one is true or false: a- Metals generally have a low density b- All metals are solid at room temperature c- Metals have high melting and boiling points.
25. As you can see, the picture belongs to metallic bonding of aluminium, can you describe it?
GLOSSARY Alloy
/'ælɔɪ/
Bond Brittle
/'brɪtḷ/
Delocalised To dissolve
/dɪ'zɑ:lv /
To flow To hold
/həʊld/
To involve
/ɪn'vɑ:lv /
To occur
/ə'kɜ:r /
Oppositely Outer Overall
/'əʊvərɔ:l/
To share
/ʃeə(r)/
Stable Whole
/'steɪbəl/
GLOSSARY Alloy
/'ælɔɪ/
Bond Brittle
/'brɪtḷ/
Delocalised To dissolve
quebradizo deslocalizado
/dɪ'zɑ:lv /
To flow
fluir
To hold
/həʊld/
To involve
/ɪn'vɑ:lv /
To occur
mantener involucrar
/ə'kɜ:r /
Oppositely Outer Overall
/'əʊvərɔ:l/
To share
/ʃeə(r)/
Stable Whole
en conjunto
/'steɪbəl/ todo, entero