Chemical Bonds
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CHEMISTRY UNIT 6
(Chemical Bonds)
Valence electrons are the electrons in an incomplete highest energy level
A chemical bond is the linkage between atoms produced by the transfer or sharing of electrons
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Electron transfer results in ionic bonding
Ex: Na and Cl
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Electron sharing produces covalent bonding
Ex: water H2O
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Finding Energy changes from Ionic bonding
Ex: The formation of Na and Cl ions from Na and Cl atoms. What is the total energy change? Is the reaction Endothermic or exothermic.
Na + Cl Na+ + Cl- (what happened)
1st: Na loses an electron [endothermic]
Na + ionization energy Na+ + e-
2nd: Cl gains an electron [exothermic]
Cl + e- Cl- + electron affinity
3rd: Na+ and Cl- bond to form Na+Cl- which we are
told gives off 189 kcal
So: 1 mole Na + 119 kcal 1 mole Na++ e-
1 mole Cl + e- 1 mole Cl- + 83 kcal
____1 mole Na++ 1 mole Cl-1 mole Na+Cl-+189 kcal
1 mole Na + 1 mole Cl 1 mole Na+Cl-+153 kcal
By canceling above, the total reaction is exothermic.
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A Molecular formula indicates the kinds of atoms in a compound formed and the ratio of these atoms
Ex: H6O3Na22Cl22
An Empirical formula indicates (1) the kinds of atoms in the compound formed (2) the simplest whole number ratio of the atoms in a compound
Ex: Na17Cl17 is a Molecular formula
NaCl is an Empirical formula
The Oxidation state of an element is represented by a signed number called the oxidation number
Oxidation number rules:
- The oxidation number of a free element is zero.
- The oxidation number of a mono-atomic ion is equal to its charge.
- The algebraic sum of the oxidation numbers of all the atoms in a compound is zero.
A chemical reaction in which an element attains a more positive oxidation state (loss of electrons) is called oxidation
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A chemical reaction in which an element attains a more negative oxidation state (gain of electrons) is called reduction
The substance that is reduced is called the oxidizing agent
The substance that is oxidized is called the reducing agent
Ex:In the reaction between sodium and sulfur
(Na0 + S0 Na2+1S-2)
Describes the following:
Sulfur reduced by sodium
Sodium oxidized by sulfur
Sulfur is the oxidizing agent
Sodium is the reducing agent
Positive ions are called cationsEx: Na+
Negative ions are called anionsEx: Cl-
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A covalent electron pair is two shared electrons that forms the bond between two atoms
Ex: water
A molecule is the smallest chemical unit of a substance that is capable of stable independent existence
Ex: one water molecule
Diatomic molecules consist of two atoms
Ex. H2 F2 Cl2 Br2 I2 O2 N2
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Bond energy is the energy required to break chemical bonds
Ex: Breaking up a H - H bond
A single covalent bondconsist of 1 pair of electrons being shared
Ex: Diatomic Flourine F2
A double covalent bond consists of 2 pairs of electrons being shared
Ex: Calcium Oxide
A triple covalent bond consists of 3 pairs (6) electrons being shared
Ex: Ethyne C2H2
In a molecule containing three atoms either the atoms are in a straight lineor they form a bent molecule
Ex. Straight: Be Cl2 Bent: H2O
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Ex: If Na is in with F and Cl. Which one will get to bond with Na?
Ex: In a water molecule, where do the electrons stay most of the time?
Percentage of Ionic character of a bond A-B, X is electronegativity
XA - XB
% I.C. = ------* 100%
XA
Ex: Na-Cl What is % ionic character?
When is a bond covalent or ionic; polar or non-polar
Bonds with more than 50% ionic character are ionic
Bonds with between 5% and 50% ionic character are polar covalent
Bonds with less than 5% ionic character are considered to be non-polar covalent
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Hybridization is the combining of two or more orbitals of nearly the same energy into new orbitals of equal energy Ex: CH4
Electronegativity is the degree of attraction an atom has for the shared electron in a covalent bond
In a non-polar covalent bond there is an equal attraction for the shared electron, which results in a balanced distribution of charge
Ex: H-H
In a polar covalent bond there is an unequal attraction for the shared electrons, which result in an unbalanced distribution of charge
Ex: H-F
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A non-polar molecule is composed of non-polar bonds or uniformly spaced, like polar bonds
Ex: H-H
Polar molecules have an unbalanced distribution of electrons, thus have two regions of different electric charges
Ex: Water
Resonance is a bonding situation that cannon be represented by a single formula
Ex: sulfur dioxide (SO2)
A polyatomic ion is a charged group of covalently bonded atoms
Ex: NH4+ ; NO3- ; PO4---