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Bonding and Structure


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Summary

Nuclear model of an atom



 ParticleSymbol
Relative mass
 Charge
 proton p 1 +1
 neutronn
 1 0
 electron e 1/1836 -1


  • Proton number: number of protons in an atom
  • Nucleon number: number of protons + number of neutrons 
  • Number of electrons = number of protons
  • Isotopes: atoms of the same element with same proton number but different number of neutrons
    • isotopes have slightly different physical properties but have identical chemical properties as they have the same number and arrangement of electrons
  • Valence electrons: the electrons in the outermost shell 
    • used to form chemical bonds

Electronic Configuration

  • Electrons are placed in orbits.
  • First shell contains maximum 2 electrons. Second shell and so and so for has maximum of 8 electrons.
  • To write electronic configuration we write as n.n.n.... where first n denotes the first shell, second the second shell and so and so for.
    • E.g. Sulfur has electronic configuration of 2.8.6
  • The valence electrons is the number of electrons of the outermost shell.
    • Sulphur has 6 valence electrons.

Relation with Periodic Table

  • Elements in same horizontal row: Period
  • Elements in same vertical column: Group
    • Group 1 has 1 valency
    • Group 2 has 2 valency
    • Group 3 has 3 valency and so on.
    • Group 0 has full valency which makes it having stable electronic configuration.
  • Down the group the number of shells increases.

Structure of Matter

a. Atom: Smallest particle of an element

b. Molecule: group of two or more atoms chemically joined together, e.g. chlorine molecule has 2 chlorine atoms

c. Element: Element is a substance that cannot be broken down into simpler substances by chemical nor physical methods.

Classifying elements

  • Classifying by state. E.g. some elements are solids, some liquids, some gases.
  • Classifying by metals and non-metals. E.g. most elements are metals, semi-metals are metalloids (having properties of metals & non-metals), some are non-metals
  • Classifying by periodicity.From left-right elements change from metal to non-metal

d. Compound: substance containing 2 or more elements chemically joined together
e.g. Magnesium is an element; oxygen is an element – they can only be burnt to form magnesium oxide compound.

Composition of compounds
  • Ions or molecules make up compounds
  • Ions are atoms having electrical charge
  • E.g. NaCl made up of 2 ions; positively charged Na, negatively charged Cl.

e. Mixtures: contains 2 or more substances not chemically joined together. e.g. seawater is made up of water and NaCl (salt); oxygen in air varies

Compounds vs Mixtures


Ionic bond

  • Ionic bonding is the transfer of electrons from one atom to another to become achieve an inert gas configuration, forming ions.
  • Ionic bonds are formed between METALLIC and NON- METALLIC ATOMS ONLY.
  • Metals lose electrons to form positive ions (cations)
  • Non-metals gain electrons to form negative ions (anions)
  • The formation of ions is resulted from transfer of atoms from one atom to another atom(s), which the ions produced are of opposite charges, and unlike charges attract, causing them to be held together with a strong force.
Example: Formation of NaCl
  • Sodium atom loses an electron by transferring the electron to chlorine atom, making both stable.
  • The loss of electron forms cation, Na+, and the gain of electron forms anion, Cl-.
  • The opposite charges acquired by both ions attract to each other, forming a strong ionic bond of NaCl.

Deducing formula ionic compounds

  • We can know the charge of elements by looking at groups of periodic table.
    • Group I to group III elements has charge of +1, increasing to +3, going to the right.
    • Group V to group VII elements has charge of -3, decreasing to -1, going to the right.
E.g. Aluminium sulfate

We have to balance the charges to make a stable bond.

Ions present:
                         Al3+     SO42-
                         Al3+     SO42-
                                     SO42-

Total change: 6+        6-

Therefore, the formula is Al2(SO4)3

1. The symbol of metal ion should always be first, e.g. NaCl
2. Polyatomic ion should be placed in brackets, e.g. Fe(NO3)2

Structure and Properties of ionic bond


Structure
  • Ionic substances appear as giant lattice structures which the ions are held together by electrostatic force between oppositely charged ions.
  • To find the formula of ionic bond, say sodium chloride bond, by looking at lattice structure, we count the ratio of amount of metal ions to non-metal ions.
    • E.g. in sodium chloride, the ratio Na:Cl is 1:1, therefore the ionic formula is NaCl.
Properties
  • Ionic compounds are hard crystalline solids with flat sides and regular shapes because the ions are arranged in straight rows in strong ionic bonds.
  • Ionic compounds have very high melting points and boiling points.
  • The strong forces holding ionic compounds prevents them to evaporate easily. Hence, ionic compounds have no smell.
  • Solid ionic compounds don’t conduct electricity but they do when they are aqueous or molten.
    • This is because in liquid/aqueous state the ions which conduct electricity are free to move.
    • In solids, these ions are fixed in place.
  • Ionic compounds are soluble in water but insoluble in organic compounds.
    • This is because the ions attract water molecules which distrupts the crystal structure, causing them separate & go into solution.
    • Vice versa when in organic solvent.

Covalent bond

  • Covalent bonding is the sharing a pair of electrons to gain electronic configuration of an inert gas, usually for molecules.
  • Covalent bonds occur between NON-METALLIC ATOMS ONLY.
  • In covalent bond, we try to substitute the short of electrons of two or more atoms between each other to form the 2 or 8 valence electrons.
  • The shared electrons appear in pairs
Example: H2 molecule

  • Hydrogen atom has one valency.
  • To become stable with hydrogen atom, it needs one more electron, just like helium which has 2 valency.
  • When 2 hydrogen atoms join, they share their electrons, on which, the share becomes 2 electrons, which is now a noble gas configuration, being shared between these 2 atoms.
  • We write the bond as H – H single bond, which means they share an electron pair (2 electrons).
Example: Cl2 molecule

  • Cl atom has 7 valency and needs one electron, each, to form a noble gas configuration between two Cl atoms.
  • Hence they share an electron EACH to hence share 2 electrons between the atoms.
  • Hence, each Cl atom now has 8 valency which is a noble gas configuration.
Example: O2 molecule

  • An O atom has 6 valency and needs 2 electrons, each, to form a noble gas configuration.
  • Hence, EACH SHARE THE AMOUNT OF ELECTRONS EACH SHORT OF, in this case – 2 electrons, to form stable molecule.
  • The contribution hence now become 4 electrons and what left on each oxygen atom is 4 electrons.
  • We combine each 4 electrons on oxygen atom with the 4 electrons shared and hence we get 8 valency for each oxygen atom – a noble gas configuration!
Example: H2O molecule
  • Apart from oxygen sharing between oxygen atoms, it can have electrons with other atoms.
  • Oxygen needs 2 electrons and when bonded with hydrogen, which need an atom each, they combine to provide 2 electrons on both sides of oxygen bonded with hydrogen atoms.
  • Each hydrogen with oxygen atom form a single bond: O – H.
Example: CO2 molecule

  • Carbon needs 4, oxygen needs 2.
  • We share two from oxygen part, WHICH HAS THE SMALLEST NUMBER OF SHORT ELECTRONS, TO SHARE THE AMOUNT OF ELECTRONS THAT ATOM NEEDS, to form 4 shared atoms.
  • Now oxygen is stable but carbon needs 2 more, which we now know they can get from another oxygen atom.
  • The atoms are now stable and since each bond has 2 pairs of electrons, we call this double bond: C = O.

Structure and Properties of covalent bond


Structure

a. Giant Covalent Bond

  • Diamond
    • Diamond has carbon atoms bonded with another carbon atoms in a tetrahedral arrangement which each carbon atom uses all its valence electrons to form 4 single covalent bonds with other 4 carbon atoms.
  • Silicon Dioxide
    • Silicon dioxide, SiO2, has silicon atoms bonded with another oxygen atoms in a tetrahedral arrangement which each silicon atom uses all its valence electrons to form 4 single covalent bonds with other 4 oxygen atoms.
  • Graphite
    • Graphite has flat layers of carbon atoms bonded strongly in hexagonal arrangement in which the layers are bonded to each other weakly.


Properties

  • It is a hard solid because it consists of many strong covalent bonds between atoms. This property makes it suitable as abrasives.
  • It has very high melting and boiling points.
  • It does not conduct electricity (except graphite) because there are no free electrons in covalent bonds since they are used to form bonds; hence electrons are in fixed positions. To conduct electricity, there must be free electrons.
  • All covalent structures are insoluble in water.

Metallic bond

  • Metallic bonding is bonding within atoms of metals caused by attractive force between positively charged metal ions and negatively charged free electrons.
  • The atoms are packed closely together in giant lattice structures.
BOND FORMING
  • Each atom in metal gives up valence electrons to form positive ions.
  • There are free electrons moving between the spaces and positive metal ions are attracted to the sea of electrons which hold the atoms together.

Structure and Properties of metallic bond

  • Metals can be bent (ductile) and can be stretched (malleable) because the layers of atoms in metals slide over each other when force is applied but will not break due to attractive force between electrons and metal ions.
  • Metals conduct electricity as it has free electrons which carries current.
  • Metals conduct heat as it has free electrons which gains energy when heated and moves faster to collide with metal atoms, releasing heat in collisions.
  • Metals have high melting and boiling points because the bonds between metals is very strong. Hence very high heat energy needed to break the bonds.

Simple Molecular Substances

  • Simple molecular substances are usually liquids/gases at r.t.p. because the molecules are not tightly bonded like in solids, hence free to move.
  • They have low melting and boiling points because the force of attraction is weak that they can be easily broken by heat.
  • Since they have low boiling points, they evaporate easily.
  • They don’t conduct electricity because they don’t have free electrons/ions which helps to conduct electricity.
  • Most of these are insoluble in water but soluble in organic solvent.

Noble gases

- Noble Gases like helium and argon are unreactive.

- do not form chemical bonds with other atoms as they have a stable electronic configuration.

- Atoms of other elements form chemical bonds so that they attain the electronic configuration of a noble gas

Drawing Structures

MCQ Questions

1. Which ion has the most shells that contain electrons?
a. Al3+
b. Be2+
c. N3-
d. S2-

2. Which substance contains covalent bonds but also conducts electricity?
a. brass
b. graphite
c. iodine
d. steel

3. Which electron arrangement is that of a metallic element?
a. 2, 1
b. 2, 4
c. 2, 5
d. 2, 7

4. Which molecules have the same number of protons?
a. O2 and N2
b. Cl2 and Br2
c. CO2 and SO2
d. CH4 and NH3

5. Metals have positive ions in a 'sea of electrons'. Which metal atom provides most electrons for the sea?
a. aluminium
b. magnesium
c. sodium
d. zinc

6. Which substance does not have a macromolecular structure?
a. diamond
b. graphite
c. silicon dioxide
d. sodium chloride

7. Which atom has the same electronic configuration as the strontium ion?
a. calcium
b. krypton
c. rubidium
d. selenium

8. Element X has an electronic structure 2.8.8.1.
Element Y has an electronic structure 2.8.6.
What is made when X and Y react?


      Type of compound         Formula
a.           covalent                     X2Y
b.           covalent                     XY2
c.           ionic                          X2Y
d.           ionic                          XY2

9. What is the formula of uranium(VI) oxide?
a. U6O
b. U2O3
c. UO3
d. UO6

10. Graphite is used to make
a. glass
b. cutting tools
c. electrical wiring
d. electrodes

11. Which compound has both ionic and covalent bonds?
a. ammonium chloride
b. carbon dioxide
c. ethy ethanoate
d. sodium chloride

12. Elements X and Y react to form an ionic compound of formula XY. What could be the proton (atomic) numbers of X and Y?

 X
 Y
 a38
 b6
8
 c8
16
 d12
16

13. An atom of an element contains 17 protons, 18 neutrons and 17 electrons. What is the atomic structure of another isotope of this element?


  protonsneutrons
electrons
 a 17 20 17
 b 17 18
 16
 c 18 18 17
 d 20 18 17

14. Which of the following has the highest electrical conductivity?
a. aqueous sugar solution
b. solid graphite
c. solid sodium chloride
d. gaseous carbon dioxide

15. Which of the following ions has the same number of electrons as a krypton atom, atomic number 36?
a. chlorine
b. rubidium
c. sodium
d. xenon

16. Which of the following is a compound?
a. air
b. carbon
c. oxygen
d. steam

17. In which set do the three particles each have the same total number of electrons?


 aCl-Br-
I-
 bF-
Ne
Na+
 cK+
Ca2+
Br-
 dLi+
Na+
K+

18. An ion X+ has 23 nucleons and 10 electrons. What does the nucleus of the ion X+ contain?

  protonsneutrons
 a12
11
 b11
12
 c10
13
 d9
14

19. The table shows information about particles X and Y.
  number of protons
number of neutrons
 electronic structure
 X 9 10 2, 8
 Y 17 20 2, 8, 8

Which statement is correct for both X and Y?
a. they are atoms of metals
b. they are atoms of noble gases
c. they are isotopes of the same element
d. they are negative ions

20. Which substance could be sodium chloride?
 substance melting point/oC
conduction of electricity when liquid
 conduction of electricity when in aqueous solution
 a -114 none none
 b -114 none good
 c 180 none insoluble
 d 808 good good

21. Chemical bonding involves the rearrangement of 
a. molecules
b. atoms
c. electrons
d. ions

22. Solid carbon dioxide (dry ice) is used as a refrigerating agent because it readily changes directly from the solid state into the gaseous state at a low temperature. Based on the above information, what is the main intermolecular bonding in CO2 (s) likely to be?
a. covalent bonding
b. ionic bonding
c. hydrogen bonding
d. Van der Waals forces

23. Which of the following does not contain covalent bonds?
a. graphite
b. gold
c. ice
d. diamond

24. Which atom has the same electronic configuration as the strontium ion?
a. calcium
b. krypton
c. rubidium
d. selenium

25. How does a magnesium atom form a bond with an oxygen atom?
a. by giving one pair of electrons to the oxygen atom
b. by sharing one pair of electrons, provided by the magnesium atom
c. by sharing two pairs of electrons, both pairs provided by the oxygen atom
d. by sharing two pairs of electrons, each atom donating one pair of electrons

26. In which of the following pairs do the elements form a compound by sharing electrons?
a. carbon and chlorine
b. lithium and iodine
c. neon and oxygen
d. potassium and bromine

27. Which substance does not have a macromolecular structure?
a. diamond
b. graphite
c. silicon dioxide
d. sodium chloride

28. Which substance has the highest melting point?
a. diamond
b. lead(II) bromide
c. methane
d. water

29. Substance X has a melting point higher than 500ºC. It is insoluble both in water and in organic solvents. It conducts electricity both when solid and when molten. What could X be?
a. copper
b. magnesium oxide
c. poly(ethene)
d. silica

30. Which of the following describes the structure of diamond?
a. a giant structure of atoms bonded covalently
b. a regular arrangement of oppositely charged ions
c. layers of rings of atoms
d. positively charged ions surrounded by a sea of electrons

MCQ Answers

1. d
2. b
3. a
4. d
5. a
6. d
7. b
8. c
9. c
10. d
11. a
12. d
13. a (isotopes of the same element have same number of protons but different number of neutrons)
14. b
15. b
16. d
17. b
18. b
19. d
20. d
21. c
22. d
23. b
24. b (both strontium ion and krypton have 36 electrons)
25. a (magnesium and oxygen undergo ionic bonding)
26. a
27. d (sodium chloride has a giant ionic structure made up of a lattice of positive and negative ions held tightly together by strong electrostatic forces of attraction - ionic bonds)
28. a (a lot of energy is needed to break the extremely strong C-C covalent bonds in diamond)
29. a
30. a

Structured Question Worked Solutions

1. Uranium-235 and uranium-238 are two isotopes of uranium. The table below shows the numbers of the particles in one atom of uranium-235
 particleNumber
 electron92
 neutron143
 proton92

a. suggest a reason why this is known as uranium-235

b. In the table below, give the numbers of particles found in an atom of uranium-238

 particlenumber
 electron 
 neutron 
 proton 

c. Uranium-235 is radioactive and is used in the manufacture of nuclear weapons. Give one other use of uranium-235


Solutions

a. mass number = 143 + 92 = 235

b.
 particle number
 electron 92
 neutron 146
 proton 92

c. as a nuclear fuel in the production of nuclear energy

2. Fluorine can form either covalent or ionic bonds.
a. Draw a dot-and-cross diagram to show the bonding in
i. sodium fluoride, NaF
ii. fluorine, F2
Your diagrams must show all electrons.

b. Explain why sodium fluoride has a higher melting point than fluorine


Solutions

ai.
aii.
b. In NaF, the ionic bonds are very strong. In F2, the forces of attraction between F2 molecules are very weak. A lot of energy is required to break the strong ionic bonds. Hence, NaF has a higher melting point than F2.

3. Use the information in the table below to answer the following questions
 substanceconducts electricity when solid
 melting point/oC
dissolves in water
 sodium chloride
 no 808 yes
 sulphur no 113 no
 tungsten yes 3377 no
 wax no 35-50 no
 aluminium yes 660 no

ai. Name two metals from the table
aii. How can you tell from the table that the substances you have chosen in (i) are metals?

b. How can you tell from the table that wax is a mixture?

ci. Name a compound from the table
cii. Explain the meaning of the word compound

d. Describe how the arrangement and movement of particles in aluminium change as the temperature rises from 659
oC to 661oC

e. Suggest how you could separate a mixture of sulphur and sodium chloride to produce a sample of each


Solution

ai. tungsten and aluminium
aii. They conduct electricity when solid

b. Wax melts over a range of temperatures

ci. sodium chloride
cii. a compound contains 2 or more elements chemically bonded together. The ratio of each element is fixed.

d. At 659oC, the particles are vibrating vigorously about their fixed positions. At 660oC, Al starts to melt and the particles break free from each other and begin to move about in aggregates.

e. Dissolve the mixture in water and filter. Sulphur remains as residue while NaCl is collected as filtrate. NaCl can then be obtained by crystallization.

4. The table shows the atomic structure of six particles, represented by the letters L to Q. The particles are atoms or ions. The letters are not the symbols of the elements.
 particleelectrons
protons
neutrons
 L 6 6 6
 M 2 2 2
 N 12 12 12
 O 10 12 12
 P 6 68
 Q 10 13 14

Use the letters L to Q to answer the following questions.

a. Which 2 particles are ions?

b. Which particle is a atom of a noble gas?

c. Which 2 particles are an atom and an ion of the same element?

d. Which 2 particles are isotopes of the same element?

e. Which particle has the highest atomic mass?

Solutions

a. O and Q (because O is an ion with a 2+ charge while Q is an ion with a 3+ charge)

b. M

c. N and O (N and O have the same number of protons but different number of electrons

d. L and P have the same number of protons but different number of neutrons

e. Q. (13 + 14 =27)

5. Sodium is stored under oil because it rapidly oxidises to form sodium oxide, Na2O.
Draw a dot-and-cross diagram to show the bonding in sodium oxide. (Only need to show outer shell electrons)

Solution


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