c) If a sample of each were stirred into a beaker of water, which beaker would conduct electricity?

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1 Chapter 8: Bond Review Characteristics of ionic compounds: (a) have high melting and boiling points (b) exist as crystals and are therefore brittle and will cleave when struck (c) many are soluble in water (review solubility rules) (d) conduct electric current in their molten form and in their dissolved forms because in these two states, there are charged particles which can move. They will not conduct electricity in their solid state (charged particles are there, but they cannot move) nor in their gaseous phase.(charged particles are too far apart to conduct electricity). In ionic compounds, an electron (or more) is actually TRANSFERRED because one element is electronegative enough to actually take it AWAY. However, sometimes the more electronegative element is not "powerful" enough to actually take another atom's electron away. Then the electron of both atoms are placed between the two and are SHARED which forms a covalent bond. Covalent bonds are found in molecular compounds and in polyatomic ions Question 1: Suppose that you have two white powders. One of them is a sodium hydrogen carbonate (baking soda) and the other is a powdered sugar, dextrose. a) Write the chemical formula for each one below. Assume that the formula for dextrose is the same as that for glucose. Baking soda: Dextrose: b) Which one of these is an ionic compound? c) If a sample of each were stirred into a beaker of water, which beaker would conduct electricity? Characteristics of covalent compounds: (1) low melting and boiling points (2) rather unstable, most are not solids but rather rubbery, powdery, liquids or gases (3) does not conduct electricity in ANY form because it melts and dissolves into neutral molecules which although they can move, they do not have charge. (4) exist as true discrete molecules and not simply an array of positive and negative charges as if found in an ionic crystal. (5) many are insoluble in water; this is based on polar vs. non-polar substances. LIKE DISSOLVES LIKE 153

2 Covalent bonds are formed by sharing electrons between atoms. However, not all covalent bonds are the same--sometimes the electrons are shared equally (such as in H 2, O 2, Cl 2 ) and sometimes the electrons are shared closer to the more electronegative element. In NON-POLAR COVALENT BONDS, the electrons are shared equally or have an electronegativity difference of less than If there is no electronegativity difference at all, the bond is said to the "pure". Question 2 What type of bond is found in bromine? Question 3 What type of bond(s) are found in methane? In POLAR COVALENT BONDS the electrons are shared but with an electronegativity difference of 0.40 or more. When you determine that a bond is "polar", you indicate its polarity by drawing an arrow OVER the bond pointing to the atom which is the most electronegative. This shows that the electrons are being shared MUCH closer to the more electronegative element. This shift of electrons within a bond causes the bond to become "partially" or "slightly" positive on one end and "partially" or "slightly negative on the other. Since these are not true charges, we cannot use just a + or a - sign. Instead we use the lower-case Greek letter delta, to indicate "partially positive" or - for "partially negative". Question 4 What type of bond(s) are found in water? How many polar bonds and how many non-polar bonds are there? Question 5 How many polar bonds and how many non-polar bonds are there in C 2 Cl 6? Question 6 How many polar bonds, and how many non-polar bonds are there in C 2 F 4? 154

3 The presence of polar bonds in a molecule MAY OR MAY NOT cause the entire molecule to be polar. A POLAR MOLECULE means that one entire END of the molecule is partially positive and the other end is partially negative. A molecule is NOT POLAR if it has partially-negative outsides and a partially-positive inside. Something must be able to attach to BOTH poles. The presence of polar bonds in a compound does not mean that the whole particle will be polar. If a molecule has polar bonds symmetrically arranged around the central atom, the charge centers cancel themselves out and the particle is NOT polar. Question 7 How many polar bonds in carbon tetrachloride? Is the molecule itself polar? Question 8 How many polar bonds are there in carbon dioxide? Is the molecule itself polar? Question 9 How many polar bonds are there in ammonia (NH 3 )? Is the molecule itself polar? The following are good questions to ask yourself when deciding whether or not a molecule is polar: (1) Is there only ONE bond in the particle and that bond is polar? If so, the entire particle is POLAR. If the one bond is pure or non-polar, the particle will be non-polar. (2) Are there several identical bonds (either polar or non-polar) around a central atom which are symmetrically arranged due to there being no unshared electrons on the central atom to distort the symmetry? If so, the particle is NOT POLAR because it would result in part of the charge being "buried" on the central atom. (3) Are there several different peripheral atoms around a central atom? This WILL always result in a particle being POLAR (4) Are there any unshared pairs of electrons on a SINGLE central atom? This will always result in a POLAR molecule for the compounds we will study in first year chemistry.. 155

4 Sometimes, the central atom does not have enough bonding sites to bond all of the peripheral atoms using its own pure electrons. In order to achieve enough bonding sites, it is necessary for the central atom to hybridize (or mix) its electron orbitals. For example, consider the compound beryllium chloride, BeCl 2, a molecular compound. Look at the orbital diagram of the central atom. It has only a PAIR of electrons and 0 bonding sites. The central atom needs to be able to bond 2 chloride ions, and in order to do this, it splits the s pair and puts one of the electrons up into the p orbital. Now it has two bonding sites, but the electron which is available for bonding in each case is NOT a pure s nor a pure p. They are both mixed or hybridized much as you would mix one gallon of red paint and one gallon of white paint the result is 2 gallons of pink paint. The result of electron hybridization is 2 electrons which are called an sp hybrid and which have an averaged energy between that of a pure s and a pure p. BeCl 2 Now think about the compound boron trifluoride, BF 3, the central atom s orbital diagram has a pair of s electrons and a single p. It has only ONE bonding site and it needs 3 sites to bond all of the fluorides. The central atom splits the s pair and promotes one of the s electrons to an empty p orbital. As soon as one electron is promoted, all electrons lose their original identities. This is like mixing one gallon of red paint ( s electron) and two gallons of white (2 p electrons). The result will be a hybrid called the sp 2 hybrid electron which has the averaged energy of one s and two p electrons. BF 3 Consider the compound methane, CH 4. The carbon central atom s orbital diagram has a pair of s and 2 single p electrons with only two bonding sites. It needs four sites to bond all of the hydrogens to the central atom. It promotes one of the s electrons to the empty p orbital and hybridizes (mixes) the energies of one s and 3 p electrons to result in an sp 3 hybrid. CH 4 REMEMBER, only covalent particles with a central atom will hybridize its electron energies. In an ionic bond, the electrons are LOST and GAINED, and there is no hybridization. 156

5 The other important thing to remember is that, due to the shape of the orbital, only PURE p electrons can form a double bond. When a compound forms a resonating double bond or a fixed double bond, it must SAVE a p orbital as pure and not allow it to be hybridized. Question 10 What hybrid is found in the compound C 2 H 4? Question 11 What hybrid is found in carbon dioxide? (3) The third type of bond we will study is called the metallic bond, and it is found exclusively in pure metal elements. There is no reason why one neutral atom of a metal would "stick" to another neutral metal atom to form a solid piece of metal, yet they do, and the reason lies in the bonding which occurs. In a pure metal sample, each of the metallic atoms "donates" all of its valence electrons to "the cause" and forms a large negatively-charged cloud which is free to circulate throughout the entire sample. This negative cloud "cements" the positively-charged particles together and thus forms a very strong bond. Light reflecting off of the electron cloud (which moves at near the speed of light) is what gives all metals luster. The fact that there are charged particles WHICH CAN MOVE explains why all metals conduct electricity. Characteristics of metallic bond substances (1) conduct electricity in their solid and liquid (molten) forms (2) have luster (1) have extremely high melting and boiling points (even higher than ionic compounds) Question 12 Draw a representation of the metallic bond of sodium. (b) of iron? Do you think hardness of a metal has anything to do with the size of the electron cloud? 157