M 4 Bonding I Covalent Bonding Learning objectives

M 4 Bonding I: Covalent Bonding

Learning objectives • Key Concepts: – Stable, unstable, bond, chemical bond, molecule, diatomic molecule, molecular element, – energetic stability, incomplete valence (or outer) shell, complete valence shell, – covalent bond, electron pair, shared electrons, single bond, double bond, triple bond, ion, molecular ion (polyatomic ion), dot and cross diagram, structural formula • Skills: – Describe the formation of covalent bond – Define a covalent bond – Describe and draw dot and cross diagrams for diatomic molecular elements M 4 Bonding: Covalent Bonding Slide 2 of 54

Learning objectives – Describe and draw dot and cross diagrams for some simple molecules such as CO 2, CO, H 2 O, CH 4, NH 3, HCl and other hydrogen halides. – Be able to explain and give examples of how the properties of covalent compounds can be explained by their bonding – Be able to determine, for a select covalent compounds, the name when given the formula and determine the formula when given the name – Be able to identify ionic, covalent, and metallic substance by their chemical formula or their properties – Be able to name and give the formula of a select group of molecular (polyatomic) ions – Be able to explain why molecular (polyatomic) ions are charged M 4 Bonding: Covalent Bonding Slide 3 of 54

Review • You learned that elements other than the noble gases are energetically unstable and therefore they react with other elements to be stable. That atoms of elements, apart from the noble gases, try to attain a stable state by – either losing electrons, or – gaining electrons, or – sharing electrons. • so that they have a completely filled valence (outer) shell just like the noble gases. M 4 Bonding: Covalent Bonding Slide 4 of 54

Review • A while back you observed the following reactions in the lab: 2 Na Zn (g) (s) + H 2 O (l) 2 Na 2 O + I 2 (s) Zn. I 2 (s) 2 Al (s) + 3 I 2 (s) 2 Al. I 3 2 Al (s) + 3 Br 2 2 H 2 O (l) (s) 2 Al. Br 3 (s) 2 H 2 (g) + O 2 (g) • You learned that when a metal and a nonmetal are involved in a combination reaction, as in the first four reactions above, the metal lose electrons while the non-metals gain electrons. – You learned that metals would rather lose and non-metals gain because that is the easier of the two option. M 4 Bonding: Covalent Bonding Slide 5 of 54

Review – As metals have 3 or less electrons in their valence shell, it’s easier to lose 1 or 2 or 3 electrons than gain 7 or 6 or 5 electrons. – Similarly, since most non-metals have 5 or more electrons in their valence shell, when they react with metals, it’s easier for them to gain 3 or 2 or 1 electrons than lose 5 or 6 or 7 electrons! • So, when a metal and a non-metal react the metal transfers electron(s) to the nonmetal. When two non-metals are involved however, as in the following examples, 2 H 2 (g) + O 2 C (g) + O 2 (g) 2 H 2 O (l) CO 2 (g) • they share electrons between them because neither of the non-metals have a tendency to lose electron(s). M 4 Bonding: Covalent Bonding Slide 6 of 54

Chemical Bond • Now the question is what keeps these atoms in the compound together? What about zinc and the two iodine atoms in zinc iodide? What about carbon and the two oxygen atoms in carbon dioxide? And, lastly, what about oxygen and the two hydrogen atoms in water? What keeps atoms together in in the molecule of a compound (and also in the molecule of some elements) is an attractive electrostatic (electromagnetic) force, referred to as a chemical bond or simply bond. – Remember an electrostatic force is a force between oppositely charged particles. M 4 Bonding: Covalent Bonding Slide 7 of 54

Chemical Bond • But depending on the composition of the substance, the bond between atoms in a compound is classified as either covalent, ionic or metallic. – The chemical bond in compounds made up only of non-metals is referred to as covalent bond. • We will consider covalent bonds, and look at how bonding determines the structure as well as the properties of a substance. M 4 Bonding: Covalent Bonding Slide 8 of 54

Covalent bond in MOLECULAR elements • Let’s start with the first element in the periodic table: hydrogen. Hydrogen atoms contains 1 electron in their valence shell. Electronic configuration: 1 outer shell NOT full energetically unstable What can the TWO hydrogen atoms possibly do to fill both their outer shells and become energetically stable ? M 4 Bonding: Covalent Bonding Slide 9 of 54

Covalent bond in Hydrogen • In order to be stable, in order to acquire the structure of a noble gas, hydrogen atoms must gain one electron. Since both need one electrons each, which means neither can give electron to the other!! If that were to happen, one would be stable (with two valence electrons) and the other would have no electrons at all!! What can the TWO hydrogen atoms possibly do to fill both their outer shells and become energetically stable ? M 4 Bonding: Covalent Bonding Slide 10 of 54

Covalent bond in MOLECULAR elements • They can combine to SHARE ONE PAIR OF ELECTRONS and form a hydrogen molecule, each atom contributing one electron to the shared pair. In so doing, both hydrogen atoms acquire completely filled outer shell (2 electrons) and become stable. H Electronic configuration: 1 outer shell NOT full energetically unstable M 4 Bonding: Covalent Bonding H 1 outer shell NOT full energetically unstable H 2 1+1 = 2 outer shell full energetically stable Slide 12 of 54

Why stable? • BOTH hydrogen atoms have BONDED together in order to fill their outer shells and become energetically stable. The shared electron pair remain between the two atoms because they are attracted by both the positively charged nuclei of the atoms. Structural formula 1+ M 4 Bonding: Covalent Bonding - H H 1+ The single line representing a pair of shared electrons Slide 13 of 54

Elements in Group 7 • Lets now move on to group 7 element fluorine. A Fluorine atom, like all the elements in GROUP 7, has 7 valence electrons in its outer shell. Electronic configuration: 2, 7 outer shell NOT full energetically unstable M 4 Bonding: Covalent Bonding Slide 15 of 54

Elements in Group 7: Fluorine Atoms of Fluorine, just like most other elements can become stable in one of two ways: • either by losing seven electrons (and acquiring the structure of helium gas) • Or… M 4 Bonding: Covalent Bonding Slide 16 of 54

Elements in Group 7: Fluorine – By gaining one electron (and acquiring the structure of neon gas) + M 4 Bonding: Covalent Bonding Slide 17 of 54

Single Covalent Bond • The outer shell of each atom have crossed over each other. Each atom shares its valence electron with the other atom. (co-operation) + Electronic configuration: + F F Structural Formula 2, (7 +1 = 8) outer shell full energetically stable All group 7 elements are DIATOMIC due to this BONDING. Again the two atoms are held together because both the nuclei attract the shared pair of electrons. M 4 Bonding: Covalent Bonding Slide 18 of 54

Covalent Bonding • Covalent bonds form between non-metal atoms. Covalent bonds involve sharing one or more pairs of electrons between the atoms bonded together. Noble gases, the elements in the last column of the periodic table, as have been mentioned before, have completely filled valence shells and therefore they are energetically stable. The rest of the non-metals however have less than 8 electrons (hydrogen less than 2 electrons), incomplete valence shells and therefore are energetically unstable. To become stable like the noble gases, these non-metal atoms, when they react with each other, share electrons to attain a completely filled valence shell. M 4 Bonding: Covalent Bonding Slide 19 of 54

Covalent Bond: Definition • The outer shells of each atom have crossed over each other. Each atom shares its valence electron with the other atom. (co-operation) The 2 atoms shared: CO-operated with their VALENT (valence) electrons. COVALENT BOND • The definition of covalent bond therefore is the electrostatic force of attraction between the nuclei of the bonded atoms and the shared electrons. M 4 Bonding: Covalent Bonding Slide 20 of 54

Single Covalent Bond • Other elements that share two electrons between themselves and form a diatomic molecule are: – The rest of the halogens (Chlorine, Bromine, and Iodine) – Oxygen – Nitrogen • Hydrogen and halogens (fluorine, chlorine, bromine and iodine) share only two electrons between the atoms bonded together. A covalent bond consisting of only two shared electrons it is referred to as a single bond. Oxygen and nitrogen however share 4 and 6 electrons respectively. Can you guess why? (Hint: How many electrons do oxygen and nitrogen atoms need to have a full valence shell? ) M 4 Bonding: Covalent Bonding Slide 21 of 54

Multiple covalent bonds: Double Bond • Oxygen is a non-metal. A structure for oxygen is given to the right. What is the electronic configuration of nitrogen? 2, 6 How many electrons does its outer shell have? 6 How many electrons does it need to have a completely filled outer shell and become energetically stable? 2 If two oxygen atoms are to combine and form a diatomic molecule, and become stable, how many electrons must they each contribute to the bond then? 2 How many electrons must they share between them? 4 M 4 Bonding: Covalent Bonding Slide 22 of 54

Multiple covalent bonds: Double Bond • Electronic configuration: O 2 2, (6 + 2 = 8) outer shell full energetically stable Since the oxygen atoms share 2 pairs of electrons (4 electrons), a molecule of oxygen therefore has a double bond. Determine the bonding situation in nitrogen. (Hint: It has a triple bond. ) Determine the bonding situation in diatomic molecules of chlorine, bromine and iodine as well. Additionally, draw structural formula for all the molecules considered thus far. M 4 Bonding: Covalent Bonding Slide 23 of 54

Multiple covalent bonds: Triple Bond • Nitrogen is another non-metal. A structure for nitrogen is given to the right. What is the electronic configuration of nitrogen? 2, 5 How many electrons does its outer shell have? 5 How many electrons does it need to have a completely filled outer shell and become energetically stable? 3 If two nitrogen atoms are to combine and form a diatomic molecule, and become stable, how many electrons must they each contribute to the bond then? 6 How many electrons must they share between them? 3 M 4 Bonding: Covalent Bonding Slide 24 of 54

Multiple covalent bonds • Electronic configuration: N 2 2, (5 + 3 = 8) outer shell full energetically stable Since the nitrogen atoms share 3 pairs of electrons, a molecule of nitrogen therefore has a triple bond. M 4 Bonding: Covalent Bonding Slide 25 of 54

Covalent Bonds in Compounds • We’ll start by looking at the compound formed when hydrogen and oxygen react. When a burning splint is applied to Hydrogen it will burn in oxygen to produce water. The energy in the splint will break the single bonds in hydrogen and the double bond in oxygen molecules. The atoms of hydrogen and oxygen then recombine to produce water molecules. Because both elements are non-metals, the bonds in the water molecules are covalent. Oxygen needs two electrons to have a completely filled outer shell and become stable. Hydrogen of course requires only one electron. Therefore, one oxygen atom will combine with two hydrogen atoms to form a molecule of water, H 2 O. M 4 Bonding: Covalent Bonding Slide 26 of 54

Water: Oxygen and Hydrogen Shared pairs of electrons; each pair represents a single bond H H 2 O O What would the structural formula look like? H M 4 Bonding: Covalent Bonding Slide 27 of 54

Dot and cross diagram X X • When a molecule is represented as H 2 O molecule is above, it is referred to as a dot and cross diagram. Notice a few differences between the last diagram (the one in the previous slide) and this. – The core shells have been omitted – The nucleus is omitted – The electrons from the different atoms are distinguished by representing then as either circles (electrons originating in and belonging to Oxygen) and crosses (electrons originating in hydrogen). M 4 Bonding: Covalent Bonding Slide 28 of 54

Covalent bonds in compounds: Ammonia • Nitrogen needs three electrons to have a completely filled outer shell and become stable. Hydrogen of course requires only one electron. Therefore, one nitrogen atom will combine with three hydrogen atoms to form a molecules of ammonia, NH 3. H H N H M 4 Bonding: Covalent Bonding NH 3 Slide 29 of 54

Dot and Cross diagram of Ammonia X X X Three single bonds between hydrogen and nitrogen in ammonia. • It is indeed more convenient to just show the outer electrons. What would the structural formula look like? M 4 Bonding: Covalent Bonding Slide 30 of 54

Covalent bonds in compounds: carbon dioxide • Carbon needs four electrons to have a completely filled outer shell and become stable. C: 2, 4 Oxygen of course requires two electrons. O: 2, 6 Therefore, one carbon atom will combine with two oxygen atoms to form a molecules of carbon dioxide, CO 2, each sharing 4 electrons. M 4 Bonding: Covalent Bonding Slide 31 of 54

Covalent bonds in compounds: carbon dioxide + O M 4 Bonding: Covalent Bonding + C O CO 2 Slide 32 of 54

Dot and Cross diagram of CO 2 XX XX X X XX Two double bonds in carbon dioxide. XX • Draw dot and cross diagrams for SO 2, CH 4 (methane), HCl (hydrogen chloride), and CO (carbon monoxide). If you haven’t already, draw structural formula for all the molecules considered thus far. M 4 Bonding: Covalent Bonding Slide 33 of 54

Flash Cards • For a set of covalent compounds, you need to know the formula, the electron structure diagram, the structural formula and the dot and cross diagram. Additionally, knowing them, you need to be able to apply the principle and methods involved in arriving at them to covalent compounds you haven’t encountered in your studies. And those compounds are: Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine, carbon dioxide, ammonia, sulfur dioxide, water, methane, hydrogen chloride, hydrogen bromide, hydrogen iodide, hydrogen sulfide and silicon dioxide To help you with that you need to make a few sets of flash cards. Set 1. name on one side with the question “Formula? ” at the bottom and the formula on the other side with the question “Name? ” M 4 Bonding: Covalent Bonding Slide 34 of 54

Flash Cards • Set 2. Formula on one side and the question “Dot and cross diagram? ” at the bottom and the dot and cross diagram on the other side with the question “Formula? ” Set 3. Electron structure diagram on one side and the question “Structural formula? ” at the bottom and the structural formula on the other side with the question “Electron structure? ” on the other side. Use the flash cards to help you with class work, homework assignments and to revise for tests. M 4 Bonding: Covalent Bonding Slide 35 of 54

Properties of Covalent Compounds • Few covalent compounds are solid, some are liquid, but most are gaseous. – Examples of solid covalent compounds are: phosphorus pentoxide, silicon dioxide. – Examples of liquid covalent compounds are as follows: water (H 2 O), ethanol. – Gaseous covalent compounds are: carbon monoxide (CO), carbon dioxide (CO 2), hydrogen chloride (HCl), sulfur dioxide (SO 2), nitrogen dioxide (NO 2). • The reason is that, though the bond between atoms within the molecules of covalent compounds might be strong, the attraction between the molecules are weak. For the same reason, in general they have low melting and boiling points. They are non-conductors of electricity and heat. M 4 Bonding: Covalent Bonding Slide 36 of 54

Breaking a covalent bond + + HEAT (thermal) ENERGY M 4 Bonding: Covalent Bonding Slide 37 of 54

Breaking a covalent bond + Thermal (heat) energy is absorbed (taken in) by the atoms. M 4 Bonding: Covalent Bonding + Thermal energy is transferred to kinetic (movement) energy - so the electrons and nucleuses start vibrating in all directions. Slide 38 of 54

Breaking a covalent bond + More thermal energy is transferred to kinetic energy – causing the electrons and nucleuses to vibrate further distances in all directions. M 4 Bonding: Covalent Bonding + the distance between the valence electrons and nucleuses increases – resulting in the attractive pull between them becoming weaker. Slide 39 of 54

Other covalent species: Molecular ions • SO 42 , NO 3 , NH 4+ are referred to as molecular ions or polyatomic ions. – They can be thought of as molecules that have charges because they have different number of protons and electrons. – SO 42 and NO 3 are negatively charged because they have more electrons than protons – while NH 4+ is positively charged because it has one less electron than protons. • For now you need to learn the names and formula of only the following above molecular ions. Formula Common name NH 4+ ammonium Formula SO 42 Common name sulfate CO 32 carbonate HCO 3 bicarbonate OH hydroxide NO 3 nitrate M 4 Bonding: Covalent Bonding Slide 40 of 54

Practice Questions: Multiple Choice • 1. J 04/1/8. How many electrons are shared between the atoms in the molecules of methane, CH 4, and of water, H 2 O? • 2. N 03/1/5. The table shows the electronic structures of four elements. Which element is a noble gas? M 4 Bonding: Covalent Bonding Slide 41 of 54

Practice Questions: Multiple Choice • 3. N 03/1/8. Which element is a solid non-metal? • 4. N 03/1/11. Carbon and chlorine form a chloride. What is the formula of this chloride? A. CCl 2 M 4 Bonding: Covalent Bonding B. CCl 4 C. Ca. Cl 2 D. Ca. Cl 4 Slide 42 of 54

Practice Questions: Multiple Choice • 5. N 03/1/9 The diagrams show the bonding in three covalent molecules. Which of these molecules combine to form ammonia? A. 1 and 2 • B. 1 and 3 C. 2 and 3 D. 1, 2 and 3 6. The structure of a molecule is shown below. How many different elements does the molecule contain? A 4 B 5 C 9 D 10 M 4 Bonding: Covalent Bonding Slide 43 of 54

Practice Questions: Multiple Choice • 7. Two elements represented by and can form a compound. Which diagram shows molecules of the compound? • 8. A model of a compound is shown. What is the molecular formula of the compound? A. H 2 O M 4 Bonding: Covalent Bonding B. 2 HO C. 2 H 2 O D. H 2 O 2 Slide 44 of 54

Practice Questions: Multiple Choice • 9. J 04/1/7. In the diagrams, circles of different sizes represent atoms of different elements. Which diagram can represent hydrogen chloride gas? M 4 Bonding: Covalent Bonding Slide 45 of 54

Practice Questions: Structured • 1. J 01/2/2. Natural gas is largely methane, CH 4. (a) Use information from the Periodic Table to find (i) the number of electrons in the outer (valence) shell of a carbon atom. . (ii) the number of electrons in the valence shell of a hydrogen atom. . . [2] (b) Draw a dot and cross diagram to show the electrons are arranged in a molecule of methane. Only the outer electron shells need to be shown. Use ‘o’ to represent carbon electrons. Use ‘x’ to represent hydrogen electrons. M 4 Bonding: Covalent Bonding [3] Slide 46 of 54

Practice Questions: Structured • (c) Methane burns in oxygen to produce carbon dioxide, CO 2 and water H 2 O. Draw a diagram to show the electrons are arranged in a molecule of carbon dioxide. Only the outer electron shells need to be shown. Use o to represent carbon electrons. Use x to represent oxygen electrons. M 4 Bonding: Covalent Bonding [3] Slide 47 of 54

Practice Questions: Structured • (d) With the help of the dot and cross diagrams you have drawn for methane and carbon dioxide explain why carbon bonds to different number of hydrogen and oxygen atoms. [2] M 4 Bonding: Covalent Bonding Slide 48 of 54

Practice Questions: Structured • 2. J 05/2/1. The structures of some substances are shown below. M 4 Bonding: Covalent Bonding Slide 49 of 54

Practice Questions: Structured • (a) Answer these questions using the letters A, B, C, D or E. (i) Which structure is methane? [1] • (d) Is substance D an element or a compound? Explain your answer. [1] M 4 Bonding: Covalent Bonding Slide 50 of 54

Practice Questions: Structured • 3. J 04/2/1. The diagram shows models of various structures, M 4 Bonding: Covalent Bonding Slide 51 of 54

Practice Questions: Structured • (e) Structure D represents a compound. (i) State what is meant by the term compound. (ii) Which one of the following substances is structure E most likely to represent? Put a ring around the correct answer. ammonia M 4 Bonding: Covalent Bonding hydrogen chloride methane water [2] Slide 52 of 54

Practice Questions: Structured • (f) Hydrogen chloride is a compound. (i) Draw a diagram to show the electrons are arranged in a molecule of hydrogen chloride. Show only the outer electrons. show hydrogen electrons as show chlorine electrons as x [2] (ii) State the name of the type of bonding present in hydrogen chloride. [1] M 4 Bonding: Covalent Bonding Slide 53 of 54

Practice Questions: Structured • 4. J 05/3/4 b. (iii) Draw a diagram to show the arrangement of the valency electrons in one molecule of the covalent compound hydrogen sulphide. Use ‘o’ to represent an electron from a sulfur atom. Use ‘x’ to represent an electron from a hydrogen atom. 5. N 03/3/1 e. Another compound that contains nitrogen and hydrogen is hydrazine, N 2 H 4. M 4 Bonding: Covalent Bonding Slide 54 of 54

Practice Questions: Structured (i) Draw the structural formula of hydrazine. Hydrogen can form only one bond per atom but nitrogen can form three. (ii) Draw a diagram that shows the arrangement of the valency electrons in one molecule of hydrazine. Hydrazine is a covalent compound. Use x to represent an electron from a nitrogen atom. Use o to represent an electron from a hydrogen atom. M 4 Bonding: Covalent Bonding Slide 55 of 54

Practice Questions: Structured • 6. N 01/3/5 b. The diagram shows a possible arrangement of the valency electrons in a molecule of sulphur dioxide. O represents an electron from an oxygen atom X represents an electron from a sulphur atom (i) What type of covalent bond is labelled bond 1? [1] (ii) What is unusual about the covalent bond labelled bond 2? [1] M 4 Bonding: Covalent Bonding Slide 56 of 54