2Forming Chemical Bonds The force that holds two atoms together is called a chemical bond .The valence electrons are the electrons involved in forming chemical bonds.Elements tend to react to acquire eight electrons. This is call a stable octet.Noble gases (group VIIIA/18) have this structure (octet) and are inert (does not form bonds).Atoms can gain, lose, or share electrons to reach an octet.
3Forming IonsPositive ions (cations) are formed when atoms lose one or more valence electrons. [Usually these atoms are metals.]Reactivity of metals (cations) are based on how easily they lose electrons. [ Ionization energy]Negative ions (anions) are formed when atoms gain one or more valence electrons. [These atoms tend to be non-metals.]Electronegativity is the ability to attract or gain electrons.
5Forming Ionic BondsIonic bonds: Complete transfer of electrons between atoms (difference of electronegativity of 1.6 or greater).Two neutral atoms will form ions. The resulting compound is called an ionic compound.
6Properties of Ionic Compounds Form crystal lattice bonding structure.Example: NaCl (sodium chloride)High melting point and boiling point due to strong electrostatic charge between the atoms (cation is attracted to anion after the transfer of electrons).Hard, rigid and brittle solids.Conducts electricity in liquid state or dissolved in water only.
7Forming Covalent Bonds Covalent bonds: Sharing of electrons between atoms (difference of electronegativity of less than 1.6).Occurs usually between elements close to each other on the periodic table (mostly non-metals).The resulting compound is called a molecule.
8Properties of Covalent Compounds Have definite and predictable shapes.Low melting and boiling points.Relatively soft solids.Can exist as solids, liquids or gases.
9Complete your Venn diagrams Requirements:5 for ionic only5 for covalent only1 similarity
10Comparison of Bonding Types ioniccovalent, Metal and nonmetal2 nonmetalsionsmoleculesmolten saltsconductivesharing ofelectronsvalenceelectronstransfer of electronsnon-conductivehigh mplow mpDEN > 1.6DEN < 1.6
11The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.
12Electronegativity and Bond Character The character and type of a chemical bond can be predicted using the electronegativity difference of the elements that are bonded.Electronegativity is a measure of how strong the atom is pulling on electrons that it is sharing in a bond with another atom.For identical atoms, which have an electronegativity difference of zero, the electrons are shared equally and the bond is considered nonpolar covalent, which is a pure covalent bond.
13Determining Number of Covalent Bonds To determine how many bonds exist in a molecule, use the following formulaN - A = # bonds2-- Where N is the # of needed electrons, which is 8 for all elements but H, which is 2.-- Where A is the # of available electrons, which is the number of valence electrons.
14Lewis Dot Structures A system of “drawing” bonds Shows how valence electrons are arrangedDots represent valence electronsPairs of dots represent bonding pairs of electrons.Show the reaction between sodium and chlorine:Show the reaction between calcium and bromine:
15Drawing Lewis Structures 1.Determine the # of bonds needed2.Draw a skeleton structure.3.Connect the atoms with the # of bonds. (determined in step 1)4.Finish by making sure all atoms in the structure have an octet of electrons.5.Remember, if the structure has a positive charge, it has fewer available electrons, and if it has a negative charge it has more available electrons.
16Lewis Structure Practice Draw Lewis Dot Structures for the following molecules and polyatomics:CO N O Cl H2O SO4 -2
17Exceptions to the Octet Rule There are two elements which don’t want an octet of electrons and those are Be (wants 4) and B (wants 6).Third-row and heavier elements often satisfy the octet rule but can exceed the octet rule using their empty valence d orbitals, especially when surrounded by highly electronegative atoms such as chlorine, bromine, and oxygen.
18Exceptions to the Octet Rule The exceptions are easy to recognize-- when you calculate the # of bonds, the formula will give you an answer that makes no sense. If this happens, do the following:Connect all of the atoms to the central element with one bond.Give all of the surrounding elements an octet of electrons.Count the number of electrons you have put in the structure so far.- If it equals the available # of electrons you calculated, you’re done.- If it is less than than the available # of electrons you calculates, placethe needed number of electrons around the central element.
19Exceptions to the Octet Rule Draw Lewis Dot Structures for the following molecules:BF PCl SF XeF2
20Carbon BondingCarbon follows the rules previously discussed, but it has a few unique properties.Carbon is always the central atom in its compounds and it tends to bond to itself.Carbon always has four bonds.Anytime you have a choice for a skeleton structure, always go for the most symmetrical option.
21Carbon Bonding Draw Lewis Dot Structures for the following molecules: C2H C3H C2H2
22ResonanceResonance occurs when you have a double bond which can be placed in more than one location.More than one valid Lewis structure can be written for a molecule. Examples: O3, NO3-, NO2-, SO22-, CO32-
24Molecular ShapeValence Share Electron-Pair Repulsion (VSEPR) model allows us to predict the molecular shape by assuming that the repulsive forces of electron pairs cause them to be as far apart as possible from each other.Only the valence electron pairs are considered in determining the geometry.
25Effect of the number of electron pairs around the central atom 4 charge clouds, tetrahedral2 charge clouds, linear3 charge clouds, trigonal planar
26PREDICTING EXPECTED GEOMETRY ACCORDING TO VSEPR THEORY Lewis dot structure determines the total # of electrons around the central atom.Multiple bonds (double and triple) count as one.The number of bonding and nonbonding electron pairs around the central atom determines the geometry of electron pairs and the molecular geometry.Lone e pairs affect geometry more than bonding pairs.The shape is referred to as bent if there are lone pairs on the central atom.