Introduction to bonding

Group 1 Li Na K Rb Cs Fr Group 2 Be Mg Ca Sr Ba Ra Group 7 F Cl Br I At All elements in the same group have the same number of electrons in their outer shell. Group 1 = 1 Group 2 = 2 Group 3 = 3 Group 4 = 4 Group 5 = 5 Group 6 = 6 Group 7= 7 Group 8 = 8 Group 3 B Al Ga In Tl Group 4 C Si Ge Sn Pb Group 5 N P As Sb Bi Group 6 O S Se Te Po Group 8 He Ne Ar Kr Xe Rn

. Group 1 Li 2s1 Na 3s1 K 4s1 Rb 5s1 Cs 6s1 Group 7 F 2s2 2p5 = 7 Cl 3s2 3p5 = 7 Br 4s2 4p5 = 7 (ignoring d orbitals) I 5s2 5p5 = 7 (ignoring d orbitals)

Group 8 all have a complete outer shell, this makes them very stable and unreactive, hence they are called “inert” or “noble” gases. He 2 Helium has two electrons, 1s2, so the first shell is full. Ne =10 Neon has 8 electrons, 2s2 2p6, so the second shell is also full. Ar = 18 Argon has 18 electrons, only the first part of the third shell is full, 3s2 3p6, but this is still very stable. Kr = 36 Krypton has 36 electrons, the third shell is full, as is the first part of the fourth shell, 4s2 4p6.

Inert gases and bonding Most elements are not as stable as the inert gases. They react, forming chemical bonds, so that they will have a complete outer shell and hence be as stable as an inert gas. Two main types of bonds are formed in the process; 1) Ionic 2) Covalent

Ionic bonding In ionic bonding one atom loses an electron(s), so that it has a complete outer shell (ie has the same number of electrons as an inert gas), forming a positive ion. M → M + + e The other element accepts an electron(s), to complete its outer shell, forming a negative ion. X + e → X - Metals generally form positive ions. Non metals generally form negative ions.

Group 1 Li Na K Rb Cs Fr Group 2 Be Mg Ca Sr Ba Ra Group 3 Al To obtain a complete outer shell metals loose electrons to form positive ions. Group 1 lose 1 electron Group 2 lose 2 Aluminium lose three.

Group 8 He 2 Ne 10 Ar 18 Kr 36 M+ ion Li+ 2 Na+ 10 K+ 18 Rb+ 36 M atom Li 3 Na 11 K 19 Rb 37 They will then have the same number of electrons (isoelectronic) as an atom of a stable inert gas in group 8. Eg Group 1.

Group 8 He 2 Ne 10 Ar 18 Kr 36 M2+ ion Mg2+ 10 Ca2+ 18 Sr2+ 36 M atom Mg 12 Ca 20 Sr 38 Likewise for Group 2…

Group 8 He 2 Ne 10 Ar 18 Kr 36 Xe 54 X Atom F9F9 Cl 17 Br 35 I 53 X- ion F- 10 Cl- 18 Br- 36 I- 54 To get a complete outer shell non metals gain electrons to form negative ions. Eg Group 7

Group 8 He 2 Ne 10 Ar 18 X atom O8O8 S 16 X2- ion O2- 10 S2- 18 N7N7 P 15 X3- ion N3- 10 P3- 18 Ne 10 Ar 18

Generally opposite charges attract. This means that the positive ion and the negative ion will attract each other. This attraction is an ionic bond. The ions attract each other strongly, so the ionic bond is a very strong bond.

Dot and cross diagrams; The outermost electrons of one atom are represented by a dot, those of the other by a cross. Eg; Lithium Hydride Li H Li = 1s 2 2s 1 = Li. H = 1s 1 = H X Li H = (Li) + (.x H) -

Covalent bonding In covalent bonding atoms share electrons to get a complete outer shell of electrons (ie the electronic configuration of an inert gas.) Eg; H 2 H = 1s 1 = H. H 2 = H:H

Group 8 He 2 Ne 10 Ar 18 Kr 36 Xe 54 X Atom F9F9 Cl 17 Br 35 I 53 X 2 molecule F : F = 10 each Cl : Cl = 18 each Br : Br = 36 each I : I = 54 each To get a complete outer shell non metals share electrons to form covalent molecules. Eg Group 7

Group 6 elements need a share of two additional electrons to achieve a complete outer shell. O atom O6O6 Group 8 He 2 Ne 8 O 2 molecule ::O :: O:: Group 5 elements need a share of three additional electrons to achieve a complete outer shell. N5N5 :N.::. N: Ne 8

Group 4 elements need a share of four additional electrons to achieve a complete outer shell. C4C4 1 1 C 1 1 H1H1 H1H1 H1H1 H1H1 Ne 8

Valency and covalent bonding The number of extra electrons that an atom needs to share is also known as its valency or combining power. Hydrogen needs 1 electron and has a valency of 1. Chlorine also has a valency of 1. Oxygen needs 2 and has a valency of 2. Nitrogen has a valency of 3. Carbon has a valency of 4

Multiple bonds Each covalent bond involves sharing a pair of electrons. So in a double bond two pairs of electrons are shared. Eg O 2 O = 1s 2 2s 2 2p 4 To complete its shell each oxygen must gain two electrons. O::O

Electron deficient compounds Beryllium, aluminium and boron, all form compounds in which they do not have a full outer shell. Eg Beryllium chloride Be = 1s2 2s2 Cl = 1s2 2s2 2p6 3s2 3p5 Be uses its two outer electrons to make two covalent bonds with Cl. Giving it = 4, not 8, electrons in its outer shell.

Electron rich compounds Some elements, such as sulphur, can form covalent compounds in which they have a share of more than 8 electrons in their outer shell. Eg; Sulphur hexaflouride (SF 6 ) S = 1s2 2s2 2p6 3s2 3p4 F = 1s2 2s2 2p5 Sulphur uses each of its six outer electrons to form a covalent bond with chlorine, giving it a share of 12, not 8, electrons.

Dative bonds A dative bond is a covalent bond in which one atom provides both electrons to be shared. :NH 3 H+H+ Ammonia has a lone pair on N that is not involved in bonding as N has a full shell. Protons have no electrons. :NH 3 H An ammonium ion, NH 4 + forms by N sharing both the lone pair electrons with the proton. +