Unit 3: Chemical Bonds Ch. 8. IB Chemistry SL Ms. Kiely Coral Gables Senior High

Transcription

1 Unit 3: Chemical Bonds Ch. 8 IB Chemistry SL Ms. Kiely Coral Gables Senior High

2 BELL RINGER: Draw the Lewis structures of the following and identify the types of bonds present. 1) H₂O QUIZ NEXT CLASS! 2) CO₂ 3) HCN Tips: The least electronegative atom is usually the central atom. Carbon, C, is always the central atom. Hydrogen, H, is NEVER the central atom.

3 Answers: 1) H₂O 2) CO₂ 1) Two single covalent bonds 2) Two double covalent bonds 3) HCN 3) One single covalent bond; one triple covalent bond

4 The types of covalent bonds that are present in a molecule depend on how many electrons are being shared between two atoms at a time! A single bond forms when one electron pair is shared between two atoms; two electrons are shared by each atom; each atom shares one electron. A double bond forms when two electron pairs are shared between two atoms; each atom shares two electrons. A triple bond forms when three electron pairs are shared between two atoms; each atom shares three electrons.

5 LEWIS STRUCTURES In order to determine how nonmetal atoms bond and form molecules, we must draw its Lewis structure. A Lewis structure is a drawing of a molecule using dots, crosses, and/or lines to represent valence electrons and how they are shared between atoms. For example, let s build the Lewis structure of chlorine, Cl₂. Both are accurate ways of drawing the Lewis structure of chlorine. These structures show the chlorine atoms are bonded together via a single covalent bond.

6 Each chlorine atom shares its 1 lone valence electron with the other in order to achieve the octet rule. This causes each chlorine to reach their goal of having the stable electron configuration of the noble gas, Argon (Ar). The three pairs of valence electrons surrounding the Cl symbol that are not involved in the single covalent bond between the chlorine atoms, are called non-bonding pairs or lone pairs.

7 BTW, did you notice the chlorine has the molecular formula Cl₂? This is because all nonmetals exist as molecules! Meaning, all nonmetal atoms on the periodic table- IF THEY ARE NOT BONDED TO ANOTHER ELEMENT- must bond to themselves in order to exist stably. The ones you must memorize are the following non-metals, which exist diatomically: I₂ H₂ave N₂o Br₂ight O₂r Cl₂ear F₂uture

8 LEWIS STRUCTURES In order to determine how nonmetal atoms bond and form molecules, we must draw its Lewis structure. A Lewis structure is a drawing of a molecule using dots, crosses, and/or lines to represent valence electrons and how they are shared between atoms. For example, let s build the Lewis structure of chlorine, Cl₂. Both are accurate ways of drawing the Lewis structure of chlorine. These structures show the chlorine atoms are bonded together via a single covalent bond.

9 STEPS TO DRAWING LEWIS STRUCTURES Step 1: Count the total number of valence electrons in the molecule. Step 2: Draw the electron dot structure of each atom in the molecule in the order that the atoms are linked to each other (the atoms with the lowest electronegativity atom is the central atom. Carbon is always in the center! Hydrogen is always on the side!) Step 3: Between two atoms at a time, bond single valence electrons with each other (use a line to link them or just place them between the two atoms). DO NOT BREAK LONE PAIRS OF ELECTRONS. ONLY BOND SINGLE VALENCE ELECTRONS TOGETHER. Step 4: Check that each atom has achieved the octet rule (keep in mind hydrogen only wants two valence electrons). Make sure the total amount of valence electrons equals the initial amount you counted for the molecule.

10 Coordinate Covalent Bond In the last slides, we have put together molecules and molecular ions that are held together with single, double, or triple covalent bonds. In these types of bonds each atom shares the same amount of electrons with the other atom that it is bonding to. Coordinate covalent bonds are another type of covalent bond. In coordinate covalent bonds the shared electrons only come from ONE of the atoms, meaning the other atom is not able to share back. An arrow is used sometimes to show a coordinate covalent bond. Both of the electrons in the pair originate from the same atom. The other atom gains two electrons it was in need of; however, the pair still belongs to the original owner and so it is still an example of sharing. Carbon monoxide, CO

11 Practice: Draw the Lewis structures of the following and identify the types of bonds present. a. NO₃ b. NO+ When drawing the Lewis structure of a molecular polyatomic ion, do not forget to put a square bracket around the structure with the charge shown clearly outside the bracket. c. NO₂ d. O₃ e. N₂H₄ -make sure to add in an electron for each negative charge associated with a negatively charged polyatomic ion. -make sure to subtract an electron for each positive charge associated with a positively charged polyatomic ion. -put the Lewis structure in brackets with the charge shown outside.

12 a. NO₃ b. NO+ c. NO₂ d. O₃ e. N₂H₄ Answers When drawing the Lewis structure of an ion, do not forget to put a square bracket around the structure with the charge shown clearly outside the bracket.

13 Octet Rule is not always followed Incomplete octet: Some molecules are exceptions to the octet rule. Small atoms such as beryllium (Be) and the metalloid Boron, (B), form stable molecules in which the central atom has fewer than eight electrons in its valence shell. Molecules with incomplete octets are said to be electron deficient. Examples: Beryllium dichloride BeCl₂, Beryllium dichloride BeF₂, Boron trifluoride BF₃, and boron trichloride BCl₃ -Boron (B) is a metalloid. -Beryllium (Be): is a metal, so BeCl₂ and BeF₂ seem like they should be ionic at first. This is an exception due to how their orbitals overlap when bonding! They end up bonding covalently.

14 Octet Rule is not always followed Electron deficient molecules tend to bond with atoms or other molecules that have a lone pair on them. The atom or molecule with the lone pair can share its lone pair via a coordinate covalent bond to the electron deficient molecule, forming a coordinate compound. Barium chloride, BCl₃, is actually a very important catalyst used in several synthetic reactions due to its ability to accept electrons. Below is an image of BCl₃ becoming stable by accepting the electron pair of the nitrogen atom of ammonia, NH₃. Coordinate covalent bond!

15 Reflection - Quiz preparation Draw the Lewis structure that shows the bonding between: -water and boron trifluoride -ammonia and boron trichloride Draw the Lewis structure for: -phosphorous trichloride -ammonium -sulfate -sulfite