1-5 Bonding In Compounds (part 6)

V. Bonding in compounds

Part 6

Bonding in compounds

Page 1

Topic

V.

Part 6

Reference Reading

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 78–87

Objectives

1.5.2.6 – 1. 5.2.12 – describe the formation of ions by the gain or loss of electrons to attain the nearest noble gas structure (for elements in groups I, II, VI and VII in the Periodic Table) – recognise that ions of opposite charges in ionic compounds are held together by electrostatic forces and such linkage is known as ionic bonding – recognise that an ionic solid exists as a giant lattice rather than as an ion pair – draw electronic diagrams and name monatomic cations and anions – deduce the formulae for binary ionic compounds with known cations and anions – recognise that ionic compounds may be made of ions containing more than one element – name and write the formulae of ions containing more than one element: hydroxide, nitrate, nitrite, hydrogencarbonate, carbonate, sulphate, sulphite, phosphate, permanganate, dichromate, silicate and ammonium – recognise that an ionic solid exists as a giant lattice rather than as an ion pair – predict the formulae of compounds when the group numbers of constituent elements are given

Notes

2.

Giant ionic structure

Ionic bond in solid sodium chloride In sodium chloride, the particles are held together by ionic bond, bonding between ions. A solid sodium chloride crystal consists of numerous sodium ions (Na+) and chloride (Cl-) ions sticking together.

A sodium ion particle (Na+) contains 11 protons and 10 electrons, therefore it is positively charged. Charges on sodium ion = charges carried by protons + charges carried by electrons = (+11) + (-10) = +1 A chloride ion particle (Cl-) contains 17 protons and 18 electrons, therefore it is negatively charged. Charges on chloride ion = charges carried by protons + charges carried by electrons =(+17) + (-18) = -1

a) Electrostatic attraction between ions Because sodium ion (Na+) and chloride ion (Cl-) are oppositely charged, they attract each other by electrostatic attraction. If there are numerous sodium ions and chloride ions, they attract each other and form a very large structure called giant ionic structure. A single crystal of sodium chloride contains over 1,000,000,000,000,000,000 ions. The sodium ions (Na+) and chloride ions (Cl-) are arranged regularly in the crystal forming a giant ionic lattice. (Lattice means boxes with regular arrangement). In sodium chloride, the lattice formed is cubic in shape.

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Electron transfer in ion formation

Reaction between sodium atom and chlorine atom Sodium metal reacts with chlorine gas readily to form solid sodium chloride. i.e. sodium atom in metal reacts with chlorine atom in chlorine by electron transfer. There is 1 electron in the outermost shell of sodium atom and 7 electrons in the outermost shell of chlorine atom. By losing 1 electron to chlorine atom, the outermost shell of the sodium ion will become completely filled. Similarly, by gaining one electron from sodium atom, the outermost shell of the chloride ion will also become completely filled.

The bracket and the charge is added for the ion in the electron diagram. N.B.

Since sodium ion is much more stable than sodium atom, actually no free sodium atom can be found in the natural. All sodium metal used in the laboratory is obtained from electrolysis of molten sodium chloride. The above example only shows how an ion is obtained if we start from an atom.

The electron transfer process may be represented as follow:

In a reaction, only the outermost shell electrons are involved, For simplicity, only the outermost shell electrons are needed to be shown in the electron diagram.

or even more simple

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Reaction between magnesium atom and fluorine atom Similarly, if a magnesium want to have a completely filled outermost shell, it will lose the 2 electrons in the outermost shell. Because each fluorine atom accepts only 1 electron., the ratio of number of Mg atoms to F atoms must be 1 to 2.

OR

N.B.

Since the electronic configuration of Mg2+ ion (2,8) and F- ion (2, 8), they are known to be isoelectronic to each other. (i.e. with the same electronic configuration).

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4. Naming of ionic compound Naming of an ionic compound is based on the name of the constituent ions in the compound. e.g. common salt is composed of sodium ion Na+ and chloride ion Cl- (not chlorine atom Cl), it is called sodium chloride. Therefore, it would be beneficial to understand the naming system of the ion first.

a) Naming of ion Depending on the structure, ions can be divided into 2 categories - simple ion and polyatomic ion Simple ion - charged atom is called simple ion (monatomic ion) Polyatomic ion - charged molecule is called polyatomic ion. In the polyatomic ions, different atoms are joined together by covalent bond.

[ ]

[

2-

Simple ion e.g. oxide ion, O2no. of proton in O = 8 no. of electron = 10 Total charge = (+8) + (-10) = -2

Single covalent bond

]

-

Polyatomic ion e.g. hydroxide ion, OHno. of proton in O + no. of proton in H = 8 + 1 = 9 no. of electron = 10 Total charge = (+9) + (-10) = -1

Ions can also be classified according to the charges carried. A positively charged ion is called a cation and a negatively charged ion is called an anion.

(1) Naming of simple ion Naming of a simple ion is rather simple. A simple cation has the same name as the element. e.g. Na+ is called sodium ion and Ca2+ is called calcium ion. If a single element is capable to form more than one kind of ion, a Roman index is added to indicate the charge. e.g. Hg+ is called mercury(I) ion and Hg2+ is called mercury(II). The name of a simple anion is also closely related to the name of the element. The name of ion is formed by dropping the end of name of the element and replace it by suffix -ide. e.g. O2- is called oxide ion, H- is called hydride ion and N3- is called nitride ion. Simple cations

Simple anions

H+ Ag+ Pb2+ Mn2+ Zn2+ Cr3+ Cu+ Cu2+ Hg+ Hg2+ Fe2+ Fe3+

HClO2-

N.B.

hydrogen ion silver ion lead(II) ion manganese(II) ion zinc ion chromium(III) ion copper(I) ion copper(II) ion mercury(I) ion mercury(II) ion iron(II) ion iron(III) ion

hydride ion chloride ion oxide ion

chlorine atom Cl is very different from chloride ion Cl-, they have totally different properties.

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(2) Naming of polyatomic ion Although the naming of polyatomic ion is more difficult, they follows certain pattern. There is only one common polyatomic cation, ammonium ion (NH4+). Ammonium ion is formed when ammonia molecule (NH3) react with hydrogen ion (H+). i.e. NH3 + H+ → NH4+ There is a lot of different polyatomic anions. The followings are some basic polyatomic ions, their name have to be learnt by heart. The name of other polyatomic ions are derived from them. Polyatomic cations

Polyatomic anions

NH4+

OHCNNO3MnO4ClO3SO42CO32Cr2O72PO43-

ammonium ion

hydroxide ion cyanide ion nitrate ion permanganate ion chlorate ion sulphate ion carbonate ion dichromate ion phosphate ion

Very Important ! ! ! An oxygen containing polyatomic anion is usually ended with -ate or -ite.

The names of other polyatomic ions are derived by adding appropriate prefix or suffix to the basic name.

Meaning of some prefixes and suffixes

Example

dithio hydrogen-

dichromate ion, Cr2O72thiosulphate ion, S2O32hydrogencarbonate ion, HCO3-

two replace O atom by S atom H+ ion

Oxygen containing compounds

Example

per-ate -ate -ite hypo-ite

perchlorate ion, ClO4chlorate ion, ClO3chlorite ion, ClO2hypochlorite ion, ClO-

O containing - even more O O containing - more O O containing - less O O containing - even less O

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Name of common cations and anions

Cation - Positive ion Charge

Formula Na+ K+ Cu+ Ag+ Hg+ H+ NH4+

Name sodium ion potassium ion copper(I) ion silver ion mercury(I) ion hydrogen ion ammonium ion

2+

Mg2+ Ca2+ Ba2+ Pb2+ Fe2+ Co2+ Ni2+ Mn2+ Cu2+ Zn2+ Hg2+

magnesium ion calcium ion barium ion lead(II) ion iron(II) ion cobalt(II) ion nickel(II) ion manganese(II) ion copper(II) ion zinc ion mercury(II) ion

3+

Al3+ Fe3+ Cr3+

aluminium ion iron(III) ion chromium(III) ion

1+

Anion - Negative ion Charge

1-

2-

3-

Formula HClBrIOHNO3NO2HCO3HSO4CNMnO4ClO4ClO3ClO2ClO-

Name hydride ion chloride ion bromide ion iodide ion hydroxide ion nitrate ion nitrite ion hydrogencarbonate ion hydrogensulphate ion cyanide ion permanganate perchlorate ion chlorate ion chlorite ion hypochlorite ion

O2oxide ion S2sulphide ion SO42sulphate ion SO32sulphite ion S2O32thiosulphate ion 2SiO3 silicate ion CO32carbonate ion CrO42chromate ion Cr2O72dichromate ion

N3P3PO43-

nitride ion phosphide ion phosphate ion

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b) Formula Formulas are frequently used in chemistry to represent a particle. Like equation, there are many different kind of formulas depending on the usage. Empirical formula - showing the simplest whole number ratio of atoms present (applicable to all kind of compound). e.g.

sodium chloride (ionic compound) NaCl

water (molecular compound) H2O

ethane (molecular compound) CH3

Ionic formula - showing the simplest whole number ratio of the ions present (applicable to ionic compound only) e.g.

sodium chloride (ionic compound) Na+Cl-

Molecular formula - showing the acutal number of different atoms in a molecule (applicable to molecular compound only) e.g.

water (molecular compound) H2O

ethane (molecular compound) C2H6

Structural formula - showing the connection of the atoms in a molecule (applicable to molecular compound only) e.g.

water (molecular compound)

ethane (molecular compound)

H H H

O

H

H C C H H H

Besides the compound, a formula may also be used to represent the smallest unit of the compound. e.g. Na+Cl- (or NaCl) may be used to represent the compound sodium chloride or a pair of Na+ ion and Cl- ion. The actual no. of particles represented by a formula is also known as the formula unit of the compound.

Formula

Macroscopic meaning - substance

Microscopic meaning - formula unit

NaCl H2O C2H6 Ag

sodium chloride water ethane gas silver metal

1 sodium ion and 1 chloride ion 1 molecule of water consists of 2 H and 1 oxygen atoms 1 molecule of ethane consists of 2 C and 6 H atoms 1 atom of silver

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Deduction of ionic formula of an ionic compound

All stable substances are electrically neutral. If a substance is not electrically neutral, it will attract another substance towards it until the whole substance is electrically neutral. Like ordinary substance, a stable ionic compound is also electrically neutral. The charges possessed by all the cation must be the same as the charges possessed by all the anion. For a stable compound, total positive charge ≡ total negative charge. If there is 3 trillion cations with one positive charge M+, there must be another 3 trillion anions with one single charge X-, or 1.5 trillion anions with two negative charge, or 1 trillion anions with three negative charge. Instead of looking at the actual number of ions present, we may put the focus on the ratio instead. If the ionic compound is formed between M+ and X3- the formula of the compound should be M+3X3- (ionic formula) or M3X (empirical formula). In sodium chloride, the ratio of the number of Na+ ion to Cl- ion must be 1 : 1. Therefore, the formula of sodium chloride is Na+Cl- (ionic formula) or NaCl (empirical formula) In magnesium fluoride, the ratio of the number of Mg2+ ion to F- ion must be 1 : 2. Therefore, the formula of magnesium fluoride is Mg2+F-2 (ionic formula) or MgF2 (empirical formula)

MgF2

Examples of formulae of some ionic compounds Compound aluminium oxide potassium hydrogencarbonate ammonium chloride

Ionic formula Al3+2O2-3 K+HCO3NH4+Cl-

Empirical formula Al2O3 KHCO3 NH4Cl

ammonium sulphate aluminium sulphate magnesium hydroxide

(NH4+)2SO42Al3+2(SO42-)3 Mg2+(OH-)2

(NH4)2SO4 Al2(SO4)3 Mg(OH)2

In the last three examples, bracket is used to prevent the confusion between the number of ions present and the number of atoms in an ion. (OH-)2 = OH2= N.B.

2 hydroxide ion (correct) a polyatomic ion consists of 1 oxygen, 2 hydrogen and 1 negative charge (wrong)

Bracket is used only if there is more than one polyatomic ion of the same kind. e.g. in ammonium chloride NH4+Cl-, bracket is not used because there is only 1 ammonium ion and 1 chloride ion.

crystal electrostatic attraction giant ionic structure lattice isoelectronic simple ion (monatomic ion) polyatomic ion ionic compound molecular compound empirical formula structural formula formula unit

Glossary

Past Paper Questions 90

2 D 2

A cation of a certain element has 22 electrons and a mass number of 55. If the charge on the cation is +3, the number of neutrons in the cation is A. 19. B. 23. C. 25. D. 30.

giant ionic lattice bracket cation anion formula ionic formula molecular formula

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V. Bonding in compounds Part 6 91 4 A 4 Which of the following groups of ions/atoms has the same number of electrons ? A. K+, Ca2+ B. Cl-, S C. H+, He D. O2-, Ar 92 3 C 3 Which of the following electron diagrams is correct ? A. B. C. D. 93

94

1 D 1

Which of the following pairs of atoms/ions has the same number of electrons ? A. Mg2+ and F B. Cl- and Ne C. K+ and O2D. Cl- and S2-

36 A 36 X, Y and Z are three different elements. The electronic diagram (showing electrons in the outermost shells only) of the compound formed by X, Y and Z is shown below :

Which of the following statements are correct ? (1) There is one electron in the outermost shell of an atom of X. (2) There are five electrons in the outermost shell of an atom of Y. (3) There are eight electrons in the outermost shell of an atom of Z. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

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V. Bonding in compounds Part 6 95 2 D 2 The electronic structure of a compound formed between an element X and chlorine is shown below.

(Only electrons in the outermost shells are shown.) What would be the formula of the compound formed between X and magnesium ? A. MgX B. MgX2 C. Mg2X3 D. Mg3X2 95

34 A 34 Which of the following particles is/are present in a hydrogen ion ? (1) proton (2) neutron (3) electron A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only

96

2 B

2

Which of the following can represent the electronic structure of potassium sulphide ? A. B. C. D.

96

3 A 3

The mass number of atom X is 27. X forms a cation with a charge of +3. If the number of neutrons in the cation is 14, what is the number of electrons in the cation ? A. 10 B. 13 C. 14 D. 17

97 2 B

2

Elements X and Y form a compound having the following electronic structure: (Only outermost shell electrons are shown.) Which of the following combinations is correct ? X Y A. Na S B. Mg Br C. Al Cl D. Si O

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V. Bonding in compounds Part 6 97 5 B 5 Which of the following diagrams best represents a part of the giant lattice of sodium chloride crystal ? (In these diagrams, • represents Na+ ion and o represents Cl- ion.)

98 18 B

98 45 A

18 Which of the following ions has the same number of protons as the hydroxide ion, OH-? A. O2B. FC. Na+ D. Mg2+

45 Element X (atomic number 11) reacts with element Y (atomic number 16) to form an ionic compound

99 4 B

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4

Element X and element Y belong to the first three periods of the Periodic Table. The compound formed between these two elements has the following electronic structure:

(Only outermost shell electrons are shown.) How many electrons are there in the outermost shell of an atom of X and that of Y ? X Y A. 1 3 B. 1 5 C. 2 8 D. 3 5

Each atom of X loses one electron and each atom of Y accepts two electrons to form a compound with formula X2Y.

V. Bonding in compounds Part 6 99 5 D 5 Consider the information concerning particle X and particle Y listed below: Particle Number of Number of Number of protons electrons neutrons X 16 16 18 Y 16 18 18 Which of the following statements is correct ? A. X and Y are atoms of the same element. B. X and Y are atoms of different elements. C. X is a cation of Y. D. Y is an anion of X. 99 19 B

19 Ionic compound X has the formula AB2, where A and B represent the cation and anion respectively. If both A and B have the same electronic arrangement, X may be A. potassium sulphide. B. magnesium fluoride. C. silicon dioxide. D. calcium bromide.

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