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CHEMICAL BONDING Part 1 Introduction to Chemical Bonding

Doc Brown's Science–Chemistry Chemical Bonding GCSE/IGCSE/AS/A2 O Level  Revision Notes

The five linked pages introduce to the concept of a chemical bond and why atoms bond together, types of chemical bonds and which electron arrangements are particularly stable leading to stable chemical bonds. Through the use of dot and cross electronic diagrams is described and there are detailed notes on ionic bonding i.e. the mutual attraction of oppositely charged ions to give ionic bonds and the properties of ionic compounds, covalent bonds and the formation of small simple molecules and their properties, macromolecules like polymers and giant covalent structures like diamond, graphite and silica. Finally metallic bonding is described to explain the structure and physical properties of metals. These notes on chemical bonding are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry, KS4 science courses and a basic primer for AS/A Level chemistry courses.

(c) doc b(c) doc bPart 1 Introduction – why do atoms bond together? (this page, read first)

and sub–index for Parts 2–5 (this page)

Part 2 Ionic Bonding – compounds and properties

Part 3 Covalent Bonding – small simple molecules and their properties

Part 4 Covalent Bonding – macromolecules and giant covalent structures

Part 5 Metallic Bonding – structure and properties of metals

Part 6 More advanced concepts for advanced level chemistry (in preparation, BUT a lot on intermolecular forces in Equilibria Part 8)

(c) doc b NANOSCIENCE – NANOTECHNOLOGY – NANOCHEMISTRY (index of pages and keyword index)

(c) doc b SMART MATERIALS SCIENCE (alphabetical index at top of page)

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Keywords/phrases/names sub–index for Parts 2–5: Examples of ionic compounds described: sodium chloride NaCl (exemplar for any Li/Na/K + F/Cl/Br/I combination), magnesium chloride MgCl2 (exemplar for any Mg/Ca + F/Cl/Br combination), aluminium fluoride AlF3, potassium oxide K2O (exemplar for any Li/Na/K + O/S combination), magnesium/calcium oxide MgO/CaO and magnesium/calcium sulphide (MgS/CaS), aluminium oxide Al2O3 (exemplar for Al2S3) * Examples of covalent molecules: simple small molecule bonding e.g. water * physical properties of small molecules * giant network bonding – giant molecules e.g. carbon C–diamond/graphite, silicon Si/silica SiO2 * properties of giant covalent structures * polymers/plastics * properties of polymers * inter/intra (internal)–molecular forces * hydrogen H2, chlorine Cl2, hydrogen chloride HCl, water H2O, ammonia NH3, methane CH4, oxygen O2, carbon dioxide CO2, ethene C2H4, nitrogen N2, ethane C2H6, chloromethane CH3Cl, methanol CH3OH, carbon (diamond), carbon (graphite), carbon (buckminsterfullerene/fullerenes), silica/silicon dioxide SiO2 * examples of ionic compounds * physical properties of ionic compounds * If your ionic compound is not listed, look for a compound with a similar formula and you should be able to work it out from the example given. The use of the word exemplar implies you are dealing with the same set of outer electron arrangements (configurations), which is why you can work out lots more dot and cross diagrams of ionic compounds by understanding one example * metal bonding model element/alloys * physical properties of metals *



top(c) doc bINTRODUCTION TO CHEMICAL BONDING

When different elements (different types of atom) react and combine to form a compound (new substance) chemical bonds must be formed to keep the atoms together. Once these atoms are joined together its usually difficult to separate them.

The atoms can join together by sharing electrons in what is known as a covalent bond.

Or, they can transfer or accept electrons to form positive and negative ions and form an ionic bond.

Metals form another kind of bond in sharing electrons called a metallic bond.

The types of are briefly explained below with links to even more detailed notes with lots of examples.

Part 1 begins by explaining why atoms bond together in the first place and then the concepts broadened out to explain the different types of bonding.

Introduction to some important definitions in Chemistry eg atom, molecule, formula, element, compound etc. are all explained with examples.

Also see notes on How to write word & symbol equations, work out formula and name compounds formed by ionic or covalent bonding


Part 1. Why do atoms bond together? – 'electron glue'!

Some atoms are very reluctant to combine with other atoms and exist in the air around us as single atoms. These are the Noble Gases and have very stable electron arrangements e.g. 2, 2,8 and 2,8,8 because their outer shells are full. The first three are shown in the diagrams below and explains why Noble Gases are so reluctant to form compounds with other elements.

(c) doc b (c) doc b (c) doc b (atomic number) electron arrangement

All other atoms therefore, bond together to become electronically more stable, that is to become like Noble Gases in electron arrangement. Bonding produces new substances and usually involves only the 'outer shell' or 'valency' electrons and atoms can bond in two ways.

The phrase CHEMICAL BOND refers to the strong electrical force of attraction between the atoms or ions in the structure. The combining power of an atom is sometimes referred to as its valency and its value is linked to the number of outer electrons of the original uncombined atom (see examples later).


(a) IONIC BONDING – an ionic bond is formed by one atom transferring electrons to another atom to form oppositely charged particles called ions which attract each other – the ionic bond.

  • An ion is an atom or group of atoms carrying an overall positive or negative electric charge

    • The electric charge is shown as a superscript +, –, 2+, 2– or 3+ etc.

    • e.g. Na+, Cl, [Cu(H2O)]2+, SO42– etc.

  • If a particle, as in a neutral atom, has equal numbers of protons (+) and electrons (–) the overall particle charge is zero i.e. no overall electric charge.

  • The proton/atomic number in an atom does not change BUT the number of associated electrons can!

  • If negative electrons are lost the excess charge from the protons produces an overall positive ion.

  • If negative electrons are gained there is an excess of negative charge, so a negative ion is formed.

  • The charge on the ion is numerically related to the number of electrons transferred i.e. electrons lost or gained.

  • For any atom or group of atoms, for every electron gained you get a one unit increase in negative charge on the ion, for every electron lost you get a one unit increase in the positive charge on the ion.

  • The atom losing electrons forms a positive ion (cation) and is usually a metallic element.

  • The atom gaining electrons forms a negative ion (anion) and is usually a non–metallic element.

  • The ionic bond then consists of the attractive force between the positive and negative ions in the structure.

  • The ionic bonding forces act in all directions around a particular ion, it is not directional, as in the case of covalent bonding.

  •  (c) doc b(c) doc bThe sodium (metal) atom transfers an electron to the chlorine (non–metal) atom in forming the ionic compound sodium chloride

  • For more detailed notes on this example and lots of other examples ...

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(b) COVALENT BONDING – a covalent bond is formed by two atoms sharing electrons so that the atoms combine to form molecules.

The bond is usually formed between two non–metallic elements in a molecule. The two positive nuclei (due to the positive protons in them) of both atoms are mutually attracted to the shared negative electrons between them forming the covalent bond. They share the electrons in a way that gives a stable Noble Gas electron arrangement like helium (2) or neon (2.8) etc..


(c) METALLIC BONDING isn't quite like ionic or covalent bonding, although the metal atoms form positive ions, no negative ion is formed from the same metal atoms, but the immobile positive metal ions/atoms in the lattice are attracted together by the free moving negative electrons between them. So, like ionic bonding, you do get attraction between positive and negative particles and this is the metallic bond.


(d) INTERMOLECULAR FORCES – INTERMOLECULAR BONDING

Between all particles, but with particular reference to covalently bonded molecules, there always exists some very weak electrical attractive forces known as intermolecular forces or intermolecular bonding.

These constantly acting attractive forces or intermolecular bonds are very much weaker than full covalent or ionic chemical bonds (approximately 1/30 to 1/20th in comparative attractive force).

For example, although the oxygen and hydrogen atoms are very strongly bonded in water to make a VERY stable molecule, BUT this does NOT account for the existence of liquid water and ice!

It is the weak intermolecular forces that induces condensation below 100oC and freezing–solidification to form ice crystals below 0oC.

In the reverse process, when ice is warmed, the intermolecular forces are weakened and at 0oC the intermolecular bonds are weakened enough to allow melting to take place.

Above 0oC (evaporation), and particularly at 100oC (boiling), the intermolecular forces are weak enough for 'intact water molecules' to escape from the surface of the liquid water.

It is VERY important to realise that the chemical hydrogen–oxygen covalent bonds (O–H) in water are NOT broken and the state changes ...

solid <== freezing/melting ==> liquid <== condensing/boiling ==> gas ...

are due to the weakening of the intermolecular forces/bonds with increase in temperature OR the strengthening of the intermolecular bonds/forces decrease in temperature.

For more details see Covalent Bonding – small simple molecules and properties


(e) WHY DO SOME ATOM DO NOT READILY FORM CHEMICAL BONDS?

As explained at the start of Part 1, NOBLE GASES are very reluctant to share, gain or lose electrons to form a chemical bond ie they do NOT readily form a covalent or ionic bond with other atoms. They are already electronically very stable. For most other elements the types of bonding and the resulting properties of the elements or compounds are described in detail in Parts 2 to 5. In all the electronic diagrams ONLY the outer electrons are shown.

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New bonds formed! Poetry in motion!

Lots of energy released when metals like magnesium bond with oxygen!

Ionic Bonding Poem – a snippet of chemical poetry

(anon Y11 student, Whitby Community College, Oct 31st 2002)

How do I long for a full outer shell!

being chlorine having seven, is a horrid hell

but my name is sodium and I have one spare!

I want to lose it, can we not share?

No? for are we not a perfect match

chuck it to me, I promise to catch

then we can live our separate ways

and live with full shells to the end of our days!

and so our tale comes to an end

as positive and negative we shall remain friends


Its a good idea to have some idea of where the elements are in the periodic table, and their electronic structure, before looking at the theoretical electronic models for ionic, covalent or metallic bonding

The black zig–zag line 'roughly' divides the metals on the left from the non–metals on the right of the elements of the Periodic Table.

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The electronic structures of the first 20 elements of the Periodic Table

You need to know about these to understand the details of chemical bonding whether it is ionic or covalent etc.

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

metals => non–metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1

1H  Note that H does not readily fit into any group

2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
Gp 1 Alkali Metals  Gp 2 Alkaline Earth Metals  Gp 7 Halogens  Gp 0 Noble Gases

Each page has bonding comments about selected elements highlighted in white

e.g. the type of chemical bond an element forms with another element (or with itself)

 


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Granddaughter Baby Niamh at nearly 6 months – first experiment in molecular modelling?

No teething dribbling on the structure please! The greatest chemistry of all – the chemistry of life!


Revision notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10) basic aid notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA OCR Edexcel Salters CIE, CCEA/CEA & WJEC advanced level courses for pre–university students (equal to US grade 11 and grade 12 and Honours/honors level courses)


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