Presentation on theme: "Valence electrons Electrons in the highest occupied energy level of an atom. These are the electrons that determine the element’s properties."— Presentation transcript:
1Valence electronsElectrons in the highest occupied energy level of an atom.These are the electrons that determine the element’s properties.
2Electron dot structures Diagrams that show the valence electrons as dots.The core electrons and the nucleus are included in the symbol of the element
3Octet RuleIn forming compounds, atoms tend to achieve noble gas configuration.8 electrons in the highest energy level.
4IONS Atoms or groups of atoms that have a positive or negative charge. Cations - positive ion resulting from loss of electrons.Anions - negative ions resulting from gain of electrons.
5Formula Unit Represents an ionic compound. Lowest whole number ratio of ions in the compound.
6Formula unitsIonic compounds form as repeating links in a crystal matrix.Each cation is bound to each neighboring anionThe ions are “locked in place”NaCl is the smallest ratio that indicates this matrix
7Ionic BondBonds resulting from the electrostatic attraction between oppositely charged ions. In an ionic compound the net ionic charge is 0.
8Ionic Compounds Metal + Non-metal Polyatomic cation+Non-metal Metal+ Polyatomic anionSolid at room temperatureHigh melting point >300°C
9Ionic compounds NaCl Na2SO4 CaCO3Crystalline solids that have high melting points.They are often soluble in waterThey conduct electricity when in solution, or when molten.
10Predicting the formula Identify the charge of the cationIdentify the charge of the anionMake a “T” tableAdd ions until the positive charge equals the negative charge
11Predicting the formula Polyatomic ions are groups of atoms that stay together, they are treated like “super atoms” The entire group carries the charge.Table 9.3 p.257NH4+
16Compounds with transition metals Transition metals can have more than one charge.You may have more than one possible compound: FeO, or Fe2O3Make tables & work backwards to determine cation chargeIndicate charge with a roman numeralREMEMBER THE TABLE MUST BE BALANCED !!!!!Fe+?O-2iron(II) oxideFe+?O-2iron(III) oxide
17Some Ions we need to just Know Silver is always +1Zinc is always +2Cadmium is always +2Do not use a roman numeral with theseIron may be +2 or +3Tin may be +2 or +4Lead may be +2 or +4More in table 9.2 p.255
34Law of Definite Proportions In samples of any chemical compound, the masses of the elements are always in the same proportions.
35Law of Multiple Proportions When two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.
36Isoelectronic ions Ions containing the same numbers of electrons. Generally for isoelectronic ions size decreases as nuclear charge increases.
37Bond energy The energy required to break a bond. Table 8.4 p.365 Bond length- the distance where energy is minimum.Table 8.5 p.365
38Coulomb’s Law E=2.31x10-19Jxnm Q1Q2 r E= energy in joules r= distance between ion centers in nmQ1&Q2= ion charges
39Molecular Compounds All non-metals covalently bonded. Solid, liquid or gasLow melting point <300°CSmallest representative particle is a molecule.
40Non-polar covalent bond A covalent bond in which the electrons are shared equally. The two atoms have nearly the same electronegativities
41Polar covalent bondA covalent bond in which the electrons are not shared equally. The more electronegative atom will pull more of the electrons toward itself.
42Polar moleculeOne end of the molecule has a slightly positive charge and one has a slightly negative charge.This is called a dipole.Depends on the shape.
43Lattice energy Lattice Energy=k(Q1Q2) r The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid.Lattice Energy=k(Q1Q2)r
44Use the following to calculate DH°f of BaCl2(s). Lattice energy= kJ/mol1st ionization Ba= 503kJ/mol2nd ionization Ba= 965kJ/molElectron affinity Cl=-348kJ/molBond energy Cl2=239kJ/molDH sublimation Ba=178kJ/mol
45Bond Energies & Enthalpy DH=SD(bonds broken)-SD(bonds formed)S=sum of termsD=bond energy per mol of bonds, always positive.
46Localized Electron Bonding Model A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms.Lewis StructuresVSEPR Theory
47Writing Lewis Structures Sum the valence electrons from all the atoms.Use a line to show a pair of electrons between each pair of bound atoms (Bonding Pairs)
48Writing Lewis Structures Arrange the remaining electrons to satisfy the duet rule for hydrogen and the octet rule for the 2nd row elements. (Lone Pairs)Double or triple bonds may be needed.
49Comments on Octet RuleC,N,O,F obey octet rule.B and Be often have less than 8 electrons. Very reactive.2nd row never exceed rule.3rd row and up often obey octet rule but may exceed it., due to d orbitals.
50When writing lewis structures satisfy the octet rule for the atoms first. Place any remaining electrons on the elements that have available d orbitals.
51Resonance structuresOccur when it is possible to have two or more valid electron dot structures for the same molecule or ion.SO3, SO2
52Formal Charge (FC)A method to decide which of many possible non-equivalent Lewis structures is most likely to occur.Atoms in molecules try to achieve FC as close to 0 as possible.
53FC=(# valence e- on free atom) -(# valence e- assigned to the atom in the molecule). (Valence e-)assigned = (# lone pair electrons) + 1/2(#shared electrons)
54VSEPR theory Valence-shell-electron-pair repulsion theory. Because electron pairs repel molecular shape adjusts so the valence electron pairs are as far apart as possible.
55Hybrid orbitalsIn hybridization several atomic orbitals mix to form the same number of equivalent hybrid orbitals
56Sigma bondsFormed along the axis that joins the atomic nuclei when two atomic nuclei combine to form a molecular orbital.
57Pi bondElectron in pi bonds are found in sausage shaped regions above and below, or in front and behind the bond axis.
58Paramagnetic molecules Show an attraction to an external magnetic field.Molecules contain one or more unpaired electrons.
59Diamagnetic Molecules Molecule is repelled by an external magnetic force.Associated with paired electrons
60ElectronegativityThe ability of an atom in a compound to draw electrons to itself.Pauling electonegativity values Table 14.2 p.405Large electronegativity differences correspond to ionic bonds