Chemical Bonding PDF

CHEMISTRY Chemical Bonding What is bonding  When a chemical reaction occurs different elements will bond to gain stability.  Different elements in the periodic table bond chemically by reacting with each other to gain stability( Have a full octet of electrons according to...

CHEMISTRY Chemical Bonding What is bonding  When a chemical reaction occurs different elements will bond to gain stability.  Different elements in the periodic table bond chemically by reacting with each other to gain stability( Have a full octet of electrons according to octet rule). This is called a chemical reaction. NB: The first four elements have a octet of 2 electrons. Electronic configuration Rules Every other element gains stability First shell holds 2 electrons by gaining 8 electrons on their Second shell hold 8 electrons valance shell 3rd shell holds 8 electrons 4ths shell up to 32 electrons Why do elements bond  Elements Bond to gain stability.  Every element in groups 1-7 will bond but group 8 will not bond as they are naturally unreactive(inert) as they have a full octet of valence electrons  Elements bond because elements in the periodic table are not naturally stable but they are unstable. Therefore they need to bond with each other to gain stability we call this stability a full octet of valence electrons. What part of the atom takes part in the bond? It is not the proton or neutron that are in the nucleus that takes part in the chemical bond. Neither is it the inner electrons but it is valence electrons that are on the last shell of the atoms that participate in chemical bonding. What type of chemical bonds exist 1. Ionic Bond 2. Covalent Bond 3. Metallic bond Ionic Bond  The ionic bond is a chemical bond that takes place between a Metal and a Non metal.  The ionic bond consist of a metal atom donating( give away ) all its valence electrons to a non- metal. The metal will form a cation( positively charged ion). The non- metal atom will accept the valence electrons of the metal atom and hence the non- metal atom will form an anion( negatively charged ion). So we see that that ionic bonding is concerned with the donation of electrons by metal atoms and accepting of electrons by non- metals. The metals  Therefore when doing ionic bonding you will be using elements in groups 1( alkali metals),2( alkaline earth metals) and 3 which are your metals. Please note that Hydrogen in group 1 is not a metal but rather a non metal as it is a gas and Boron in group 3 is not a metal but rather a metalloid. Please note that you will not be using the transition metals(Elements 21-112) when bonding but you will be using only elements 1-20. The non metals  Our Non- metals in the periodic table exists in groups 4,5,6,7( Halogens) and 8( Noble gases). However when you are carrying out a chemical ionic bond you will never use group 8 because they are already stable as they have a full octet of valence electrons. Niether group 4  The substance formed when a ionic bond is done is called a compound( Substance containing two or more elements chemically bonded in a fixed ratio. Eg CaCl2 or NaCl  however due it to it being ionic its special name is a formula unit. E.C=2,1 E.C=2,7 metal Non-metal Steps in doing ionic bonding  1. Write the electronic configuration of your atom  2. Determine if your atom is a metal or a non- metal if so you will do a ionic bond  3. Draw the structure of your atom( use dots to represent the electrons on the metal atom and use crosses to represent the electrons on the non- metal atom) and write its atomic symbol in the middle with the proton number  4. Transfer all valence electrons from metal to non metal using arrows from the metal going towards non metal. Ensure that the metal atoms are stable and non metal atoms stable as well. Not every time you may be able to use just one atom, you may need more of each.  5. Redraw the new structures and new electronic configuration of the atoms and place them in brackets with the charge to the top right hand corner. Ensure that they are Stable  6. Write the chemical formula and give the name of the formula of the substance formed How to write chemical formulas and formula name for ionic substances  To write the chemical formula:  Make note of how many metal atoms you used and how many non- metal atoms. You will write the symbol of the non- metal first and the subscript for it to the bottom right hand side given how many atoms of it you used. You will do the same for the element symbol of the non metal.  To write the name of the formula simply write the name of the metal atom first and write the name of the non metal atom second but changing the last part of the name of the non metal to “ ide” e.g chlorine becomes chloride and nitrogen becomes nitride etc. Lets do some example bond the following  Sodium and chlorine  Sodium and oxygen  Ca and Cl  Li and N  Aluminum and oxygen  Aluminum and chlorine  Lithium and oxygen Next topic: Covalent Bonding  Covalent bonding is a chemical bond between two or more non- metals  The covalent bond consist of two or more non metal atoms sharing their valence electrons with each other. The substance formed after a covalent bond is done is called a molecule because it consists of only non metals. Therefore the special name for covalent compounds are called molecules. Types of covalent bond Types of covalent bond continued  Single Bond- This is a covalent bond that consists of the sharing of 1 pair of valence electrons  Double Bond- This is a covalent bond that consists of the sharing of 2 pairs of valence electrons  Triple Bond- This is a covalent bond that consists of the sharing of 3 pairs of valence electrons Steps in bonding covalently  1. Write the electronic configuration of your atoms  2. Determine if your atoms is metal or a non- metal or both non- metals, if they are both non metals you will do a covalent bond.  3. Draw the structure of your atom( use dots to represent the electrons one type of non metal atoms and crosses to represent the electrons on the other types of non- metal atoms) and write its atomic symbol in the middle with the proton number.  4. The largest atom should be placed in the center and all other atoms surround it. NB. If you are using Carbon in a covalent bond, ensure that carbon is the central atom, and if you have more than one carbon they will be joined side by side in a chain and all other atoms hanging on them..  5. Share valence electrons from one non- metal to the other non metal using the joined valance shell ( looks like a Venn diagram). Form a single bond first and check for stability( meaning for elements the element H, it stabilizes with 2 electrons, every other non- metal stabilizes with 8 electrons on their valence shell.) , if you cant stable by using a single bond then try to do so with a double bond or triple bond until you can ensure that the atoms are stable. Not every time you may be able to use just one atom, you may need more of each and more than one double, single or triple bond.  6. This is now the new structure. Ensure that the overall molecule is stable How to write chemical formulas and formula name for covalent molecules Steps in writing formula and names for covalent molecules 1. Asses the structure and determine how many atoms of each element exists in the structure. 2. Write the Formula by writing the symbol of the central atom/ atoms first and the other atoms next. Use subscripts ( numbers to bottom right)to identify the number of each. 3. Write the name of the formula by using the Prefixes that you have learnt. The prefix comes before the element name when writing the name of chemical formulas (See example 7). Please note there is no need to write mono if the first element is only one but you must use mono if the second element is only one( see example 5 and 6 ). 4. Change the last part of the name of the second element in the formula to “ide” 5. E.g CO2 is carbon dioxide 6. Eg. CO is Carbon monoxide 7. Eg. Se2Br2 is diselenium dibromide 8. More examples on slide 21 In naming a binary(binary compound is a substance composed of exactly two different elements, which are substances that cannot be simplified further by chemical means) molecular compound, the number of atoms of each element present in the molecule is noted by using prefixes. When naming, the Use of suffix prefix Prefix Number of atoms Example Mono 1 atom of that type of element in the molecule Carbon Monoxide (CO) Di 2 atom of that type of element in the molecule Carbon dioxide(CO2) Diphosphorous chloride ( P2Cl) Tri 3 atom of that type of element in the molecule Carbon trioxide(CO3) Tetr or tetra 4 atom of that type of element in the molecule Carbon tetrachloride( CCl4) Pent 5 atom of that type of element in the molecule Vanadium pentoxide( V2O5) Hex 6 atom of that type of element in the molecule Sulfur hexafluoride(SF6) Use of suffix -ide Tells us that There are only 2 types of carbon dioxide (CO2) atoms present in the molecule Special names  However many covalent substances have special names brought down by history E.g.. H2O(WATER) and not di hydrogen monoxide and NH3 Ammonia All acids do not go by the naming system you just learnt but have special names eg. HCl- (hydrochloric acid), H2SO4 ( Sulphuric Acid) and many more MANY OTHER ORGANIC COMPOUNDS EG CH4- METHANE, C2H4 ETHENE, HAVE SPECIAL NAMES OR TOTALLY DIFFERENT NAMING SYSTEMS as well. Bond the following and show the name and formulas where necessary  Cl2  O2  N2  H2O  Carbon and oxygen ( example with only names of element given and not formula- look out for these)  Sulphur and oxygen  Carbon and hydrogen  Carbon and fluorine  C2F4  Sulphur and fluorine  Carbon and chorine  C2H4  CH4 Lone pairs You also need to be able to know what are lone pairs of electrons and how to identify how many lone pairs is present in a molecule Lone pairs are pair of valence shell electrons that did not take part in the covalent bond to form the molecule. However if you take that formed molecule and react it with another substance the lone pair becomes essential as it will participate in the chemical reaction. How much does this have Lewis structure Converting dot cross structures to Lewis structures Lets try it for SF6-sulfur hexafluoride and Co2- Carbon dioxide Convert this Lewis structure to a dot- cross diagram and Identify the types of covalent bonds, lone pairs and bonded pairs of electrons present Questions  Draw Lewis structures for the following  CO2  SiCl2  BF3  H2O2  CS2  OCl2 Coordinate( Dative) covalent bonding  A coordinate or dative covalent bond is formed when both electrons in the shared pair come from the same atom.  In ordinary covalent bonding each atom contributes one electron to the shared pair of electrons. This is described as simple/ ordinary covalent bonding.  The sharing of electrons can occur in another way where all of the electrons in the bond is contributed by one of the combining atoms.  The first atom uses a pair of non- bonded electrons( lone pair) to bond with the second atom. The second atom contributes nothing to the bond.  Polarity  Molecules can be considered Polar( positive and negative charge) or non polar( no charge difference). A polar molecule arises when one of the atoms exerts a stronger attractive force on the electrons in the bond. The electrons get drawn more towards that atom, so that the molecule exhibits a slight charge imbalance. A non polar molecule consists of atoms with the same electronegativity so that there is not much charge imbalance or very little difference in charge. It is worthy to mention that all ionic substances are polar Important: When dissolving substances it is worthy to mention the all time favorite idea that: Like dissolves Like This means that you need a polar substance to dissolve a polar substance and you need a non-polar substance to dissolve a non-polar Electronegativity can be calculated from Pauling scale of electronegativity values Non-Polar covalent molecules Polar covalent molecules Other examples of polar covalent molecules include HF , HCl and NH3 (ammonia) Please know though that majority of covalent substances are non- polar Metallic bonding Metallic Bonding Metallic bonding consists of the attraction of the free-floating valence electrons from the positively charged metal ions Positive metal ions(cations) in a sea of mobile electrons. Electrons are shared amongst every metal ion. (sea of mobile electrons) Lets appreciate it better  Metallic Bond Properties e- are delocalized among metal positively charged metal cations of the Bond Formation same kind 2 Metal atoms of the same kind( eg in potassium metal there is bonding between potassium atoms and Occurs Between potassium atoms) “Electron Sea” which leads to a Type of Structure metallic lattice