Chapt-3B - Formation of binary ionic compounds ighly...

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Formation of binary ionic compounds Na . Na + + e - F . . . . . . . + e - F . . . . . . . . - =BeF 2 Be . . F . . . . . . . F . . . . . . . + + Be .. F . . . . . . - . . F . . . . . . - 2+ Mg . . Mg 2+ + 2e - N . . . . . + 3e - N . . . . . . . . 3- Mg . . 3Mg 2+ + 6e - 3 N . . . . . + 6 e - N . . . . . . . . 3- 2 2 Na . Na + F . . . . . . . F . . . . . . . . - + 3 Mg + 2N = Mg 3 N 2 (a). (b). (c). Highly electronegative and electropositive elements readily form ionic compounds = NaF
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Covalent bonding and Lewis structures Elements of intermediate electronegativity prefer to form covalent compounds. In covalent compounds, electrons from different atoms are shared, instead of being given out or taken away. (a). Hydrogen exists as H 2 , because each hydrogen atom “feels” a pair of electrons by sharing one electron each H . + . H H : H H-H A single covalent bond is formed by sharing two electrons between two atoms (b). Oxygen exists as O 2 , because, each atom “feels” an octet of electrons by sharing two unpaired electrons each. O=O double bond A double bond is formed by sharing four electrons between two atoms single bond O . . . . . . + O . . . . . . O . .. . O . . . . ::
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Covalent compounds ..contd (c). Nitrogen exists as N 2 because, each nitrogen atom “feels” octet of electrons by sharing three electrons each N . . . . . N . . . . . + N .. . N . . . . triple bond A triple bond is formed by sharing six electrons between two atoms (d). Halogens exist as diatomic molecules because each halogen atom “feels” octet of electrons by sharing one electron each. F-F (e). Strength of bonds : Triple bond>double bond >single bond (f). Length of bonds : Triple bond<double bond <single bond F . . . . . . . F . . . . . . . + F . . . . . . F . . . . . . : N N
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How many covalent bonds can an atom form? Group 4 elements have four unpaired valence electrons, so they can form 4 single bonds, 2 double bonds or 1 triple bond and a single bond. Example: CH 4 ,CO 2 ,HCN etc Group 5 elements have three unpaired (total five) valence electrons , so they can form 3 single bonds, 1 double bond and a single bond or 1 triple bond. Example: NH 3 , Cl-N=O, N 2 etc Group 7 elements have one unpaired (total seven) valence electrons, so they normally form only one 2 5 Group 6 elements have two unpaired (total six) valence electrons, so they can form 2 single bonds, or 1 double bond. Example: H 2 O, O 2 , etc [Exceptions: etc]. C O
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Formal Charge C . . . . Consider CO molecule where combines with O . . . . . . case structure C “feels” O “feels” share 1 electron each 5 e 7 e share 2 electrons each 6 e 8 e Share 3 electrons each 7 e 9 e In none of the above cases both atoms “feel” octet of electrons. Contd. . O . C . ::: .. O . . . . . C . . . : .. . O . . C . :: So they prefer to share as, where both atoms feel octet of elctrons . . O C : :::
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Formal Charge (contd) C has two paired electrons (1 lone pair) and three shared electrons (6x1/2). This assigns a total of five electrons to C, one more than 4 valence electrons. O has two paired electrons (1 lone pair) and three shared electrons. This assigns a total of five electrons to O, one less than 6 valence electrons. Thus it appears as though C gained one electron and O lost one electron. So, Formal charge on C = -1 Formal charge on O = +1 Contd… . .
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Chapt-3B - Formation of binary ionic compounds ighly...

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