Core v Valence Electrons

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1 Unit 2 Bonding

2 Core v Valence Electrons The core electrons (represented by the noble gas from the previous row) are those electrons held within the atom. These electrons are not involved in the bonding, but contribute to the shielding effect The valence electrons are those electrons in the outermost energy level of an atom. These electrons are involved in bonding. Can be determined by location in periodic table.

3 Bonding Between Atoms Ionic Bonds (+ and -) Covalent Bonds (sharing) Metallic Bonds (delocalized electron cloud) Network Covalent Bonds (diamond)

4

5 Ionic bonding Electrostatic attraction (in lattice) Metals/nonmetals (widely diff electronegativity) Electrons transferred Polyatomic ion possibly High melting points Do not conduct as solids But conduct when dissolved or melted

6 Covalent Bonding Share electrons Two or more nonmetals Similar electronegativities Low melting points Do not conduct in any phase

7 Metallic Bonding Metal cations surrounded by their mobile valence electrons (in lattice) Malleable and ductile Conducts electricity in all phases Melting points vary (generally lowers as you move down a group) Metallic character increases as you move down a group (due to shielding effect)

8

9 Alloys An alloy is a solid solution of two or more metals (could include a nonmetal or a metalloid). Made by melting the elements involved, mixing them together, then re-solidifying. The properties of the alloy are much better than any of the original components. Common alloys Brass (Zn and Cu) Bronze (Sn and Cu)

10 Network Covalent Bonding Atoms held together in lattice of covalent bonds (like one big molecule) Very hard Very high mp and bp Electrons are localized about the atoms

11 Let s Look at Electronegativity

12 Determining Type of Bond Simplest use periodic table metal/non-metal should exhibit ionic bond Non-metal/non-metal should be covalent bond Metals will exhibit metallic bond Find electronegativity difference Large difference (>1.67) generally means ionic Small to medium difference (<1.67) generally means covalent Plot electronegativity computations using Bond Triangle

13 Bond Triangle For years, chemists have determined bond type between atoms by using the idea of electronegativity difference. Large electronegativity difference = ionic Small to medium electronegativity difference = nonpolar or polar covalent The bond triangle also uses electronegativity values, but in a slightly different manner A comparison is made between the average of electronegativities and the difference in the electronegativites of the two elements involved.

14 Bond Triangle, cont. Types elements Ave EN Diff EN Bond type Metal/nonmetal ~ ~ 2.5 Usually large Ionic Metal/metal ~ 1 Very low (~0) metallic Nonmetal/nonmetal Usually > 2.5 Low covalent By plotting Ave EN v Diff EN, you get a triangle that is divided into four areas. Area A represents ionic bonding. Area B represents metallic bonding Area C represents covalent bonding Area SM represents semimetals This bond triangle is not absolute, but it more closely can predict the bond type between atoms than just using electronegativity differences.

15 Ionic Covalent Metallic

16 Covalent bonds and polarity Within the bond, one atom is usually more electronegative than another That atom pulls the electrons in the bond to himself Causes a dipole one side of the bond is more negative and the other side of the bond is more positive Not an ionic bond but a very polar bond

17 Lewis Dot Diagram steps 1. Count valence electrons 2. If charged, add or subtract the charge 3. Arrange symbols, make no trains 4. Put in shared pairs 5. Determine remaining electrons 6. Determine who wants octet Groups 1, 2, 3 do not want eight Groups 4-7 want eight (know who can expand octet) 7. Put in unshared electrons 8. Rearrange bonds as needed

18 Lewis dot diagrams can predict type of covalent bond between atoms (single, double, or triple) but it cannot imply the shape of the molecule Structural formulas will replace a bonded pair of electrons with a stick. Then by understanding the VSEPR theory, the shape can be implied on two dimensional paper

19 Dipole moment The dipole moment measures the polarity of a molecule (based on shape of molecule) The larger the dipole moment, the more polar the molecule The greater the charge at the ends of the dipole and the greater the distance between the charges, the greater the value of the dipole moment

20 Valence-shell electron pair repulsion theory The VSEPR theory states that in a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible. This repulsion is different for the following possibilities. Unshared-unshared repulsion is the most. Shared-shared repulsion is the least. Unshared-shared repulsion is midway between the two.

21 5 Basic Shapes This theory does explain a wide variety of molecular shapes. Example Name of Shape Bond Angle BeF 2 Linear 180º BF 3 Trigonal planar 120º CH 4 tetrahedral 109.5º NH 3 Trigonal pyramid (pyramidal) 107º H 2 O Bent triagomic (angular, bent) 104.5º

22 More complex shapes These shapes arise because certain atoms can expand their octets (which means they can have more than 8 electrons around them). The only atoms that can expand their octet are those who have a d sublevel available to them. Name of Shape Bond Angle Trigonal bipyramidal 90º & 120º See-saw 90º & <120º T-shaped 90º octahedral 90º Square pyramidal 90º Square planar 90º

23 σ versesπ Sigma bonds overlap end to end Pi bonds overlap above and below

24 Hybridization One of the theories used to explain molecular geometry (AP expects you to know bold ) sp - linear sp 2 trigonal planar, bent sp 3 tetrahedral, trigonal pyramid, bent sp 3 d trigonal bipyramidal, seesaw, T-shaped, linear sp 3 d 2 octahedral, square pyramidal, square planar

25 Molecular Orbital Theory In the molecular orbital theory, as two atomic orbitals approach each other they create two molecular orbitals. Bonding orbital of lower energy Antibonding orbital of higher energy Type of covalent bond (single, double, or triple) can be predicted using the formula bond order = # bonding electrons # antibondingelectrons 2

26 Resonance Some Lewis dot diagrams can have a variety of arrangements Ex. Carbonate ion CO 3 2- Resonance explains the disparity in bond lengths

27 Van der Waals forces Ion-dipole forces attraction between an ion and a polar molecule Dipole-dipole forces attraction between two polar molecules Dipole-induced dipole forces attraction between a polar and nonpolar molecule Dispersion forces attraction between two nonpolar species Hydrogen bonds (FON) -- attraction between molecules containing fluorine, oxygen, or nitrogen and hydrogen the hydrogen is electron starved holds hands with neighboring F,O,or N

28 H 2 O Three famous molecules NH 3 HF

29 Intermolecular Forces of Attraction A substance will normally exist in the solid, liquid, or gas phase depending upon the force of attraction between the particles A solid substance will have strong intermolecular forces of attraction (lots of snuggling) A gaseous substance will have weak intermolecular forces of attraction between the particles. A liquid substance has intermediate forces of attraction between its particles

30 Matter Matter is defined as anything that has mass and volume. The mass of an object is the amount of matter the object contains. The volume of an object is the space that it takes up. EVERYTHING is made up of matter.

31 (Pure) Substance A substance is matter that has a uniform and definite composition. A substance can be an element or a compound. Substances are also referred to as pure substances because they contain only one kind of matter. All samples of a substance have identical physical properties.

32 Three States of Matter

33 Solid, liquid, and gas A solid is matter that has a definite shape and a definite volume. The particles cannot move other than vibrate. A liquid is a form of matter that flows, has a fixed volume, and takes on the shape of its container. These particles can tumble over each other. A gas is a form of matter that takes on both the shape and the volume of its container. These particles are flying all over the place.

34 Mixture A mixture is a physical blend of two or more substances. These mixtures can be either homogeneous or heterogeneous. These mixtures contain phase(s). A phase is any part of a system that has uniform composition and properties.

35 Homogeneous Mixtures A homogeneous mixture is one that has a completely uniform composition; that is, has only one phase (that is, it looks the same throughout). Examples include salt water, Kool-Aid, tap water, air, brass Homogeneous mixtures are also known as solutions. Solutions can be solid, liquid, or gaseous. (brass, Kool-Aid, air)

36 Heterogeneous Mixture A heterogeneous mixture is one that is not uniform in composition; that is, has more than one phase. Examples include blood, M&M blizzard, most soups, granite, concrete

37 How to separate mixtures? Heterogeneous mixtures can sometimes be easily separated. Filtration Picking vegies out of soup. Homogeneous mixtures are somewhat more difficult to separate. One way to separate is by distillation Another way is by chromatography

38 Pure Substances Elements and Compounds Elements are the simplest forms of matter that can exist under normal lab conditions. Elements cannot be separated into simpler substances by chemical means Each element is represented by a one- or twoletter chemical symbol. (ex. Fe not FE) Compounds are a chemical combination of two or more elements. Compounds can be separated by chemical means.

39 Matter Substances Definite composition Physically separable Mixtures of Substances Variable Composition Homogeneous mixture (also known as solutions: examples such as air, tap water) Heterogeneous mixture (distinct phases: examples are soup, concrete, granite) Element (Examples: iron, sulfur, carbon, oxygen) Chemically separable Compound (Examples: water, iron II sulfide, aluminum sulfate)

40 Physical v Chemical Properties Physical properties are characteristics of a substance that can be observed without changing the composition of the substance Color, texture, mp, bp, density, etc Chemical properties are characteristics of a substance that cannot be observed until the substance changes into a new substance. Chemical property of water is that if an electric current is supplied, the water will decomposed into hydrogen gas and oxygen gas

41 Physical vs Chemical Change A physical change does not alter the substance. Change in shape; change in location; solution formation (w/o change in energy); phase chg A chemical change involves producing new substance(s) Change in energy; change in color/odor; solid produced; gas produced; change in surface of solid

42 Know the 6 phase changes Melting solid changes to liquid Freezing liquid changes to solid Boiling liquid changes to gas Condensing gas changes to liquid Subliming solid changes to gas Direct change does not melt first Depositing gas changes to solid Direct change does not condense first

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