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chapter 4- Chemical Bonds (Part 3) Flipbook PDF
chapter 4- Chemical Bonds (Part 3)
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LESSON LEARNING OUTCOMES By the end of this lesson, students should be able to: Discuss Lewis structures and use Lewis symbols to:
➢Represent most main-group elements of the periodic table ➢Write equations for the reaction between anions and cations
LEWIS STRUCTURE ❖
❖
❖
Lewis structure (also known as Lewis dot diagrams, Lewis dot formulas, Lewis dot structures, and electron dot structures) is a very simplified representation of the valence shell electrons in a molecule. It is used to show how the electrons are arranged around individual atoms in a molecule. Electrons are shown as "dots" or for bonding electrons as a line between the two atoms. Knowing the number of valence electrons allows us to draw Lewis symbols for the elements and Lewis structure for compounds.
Lewis structure of ionic compound
Lewis structure of molecular compound
How to Draw Lewis Dot Symbol? Write down the element's symbol. Find the number of valence electron (based of the group #). 3) Draw Valence Electrons around the symbol (one at a time until all sides are filled, then pair). 1) 2)
Example: Lewis symbol for Carbon Carbon = 4 valence electrons
Draw the Lewis Symbols for the first 20 elements in the Periodic Table.
How to draw the Lewis Structures for ions of elements??
Lewis Structures for Ions of Elements
Atoms will gain or lose electrons in order to achieve a stable, Noble Gas (Group 18), electronic configuration. Negative ions (anions) are formed when an atom gains electrons. Positive ions (cations) are formed when an atom loses electrons. The chemical symbol for the element is surrounded by the number of valence electrons present in the ion. The whole structure is then placed within square brackets, with a superscript to indicate the charge on the ion. Example:
Oxygen would need 2 more electrons to be stable. It now has a 2- charge.
Al had 3 valence electrons and loses them all. It will have a 3+ charge.
LEWIS STRUCTURE FOR IONIC COMPOUND ❖ For
Lewis Structures of ionic bonds the atoms are not joined but draw next to each other.
❖ The
overall charge on the compound must equal zero, that is, the number of electrons lost by one atom must equal the number of electrons gained by the other atom.
EXAMPLE 1 – IONIC COMPOUND Lithium + Fluorine → Lithium Fluoride Step 1: Lithium atom loses one electron to form the cation Li+ Step 2: Fluorine atom gains one electron to form the anion FStep 3: Write the equation and draw the structure
Lithium Fluoride
EXAMPLE 2 – IONIC COMPOUND Magnesium + Bromine → Magnesium Bromide Step 1:
Magnesium atom loses two electron to form the cation Mg2+
Step 2: Bromine atom gains one electron (7 valence ē) to form the anion F-. Therefore, in order to gain 2ē from Mg, 2 atoms of bromine is needed.
Step 3: Write the equation and draw the structure
EXAMPLE 3 - IONIC COMPOUND Potassium + Sulphur → Potassium Sulphide K(+1) S(-2) K₂S
or
Draw Lewis structure to show the formation of ionic compounds in the following reactions. a) b) c) d) e) f) g)
Potassium + oxygen → Magnesium + chlorine → Lithium + oxygen → Calcium + sulfur → Potassium + bromine → Aluminium + oxygen → Aluminium + chlorine →
LEARNING CHECK 1 The element X has one electron and the element Y has six electrons in their outermost shells respectively. a) What is the formula of the compound formed between the elements X and Y? b) Draw the Lewis structure to show the formation of the compound.
LEWIS STRUCTURE FOR COVALENT COMPOUNDS Hydrogen forms stable molecules when it shares two electrons. Two electrons fill Hydrogen’s valence shell. ❑ Second row non-metals Carbon through Fluorine from stable molecules when surrounded by eight electrons – the Octet Rule. ❑ Valence electrons in covalent bonds can either be bonding pairs, if involved directly in the bond or lone pairs if not involved in the bond. ❑
LEWIS STRUCTURE RULES FOR COVALENT COMPOUNDS 1) 2) 3) 4)
5)
Decide how many valence electrons are possessed by each atom in the molecule. Arrange the electrons so that each atom contributes one electron to a single bond between each atom. Count the electrons around each atom: are the octets complete? If so, your Lewis dot structure is complete. If the octets are incomplete, and more electrons remain to be shared, move one electron per bond per atom to make another bond. Repeat steps 3 and 4 as needed until all octets are full.
EXAMPLE 1 – COVALENT COMPOUND Draw the formation of PH₃.
Step 1: P = 5 valence eˉ
H = 1 valence e-
Step 2: Draw one pair of electrons per bond. Step 3: Arrange the remaining electrons according to octet rule.
EXAMPLE 2 – COVALENT COMPOUND Draw the formation of CS₂.
Step 1: C = 4 valence eˉ
S = 6 valence e-
Step 2: Draw one pair of electrons per bond.
7 electrons
7 electrons
6 electrons Step 3: Move the electron for double bond
EXAMPLE 3 – COVALENT COMPOUND Draw the formation of C₂H₂. Step 1: C = 4 valence eˉ
H = 1 valence e-
Step 2: Draw one pair of electrons per bond.
Step 3: Move the electron for double bond Step 4: Move the electron for triple bond
1. A. B. C.
Which is the correct Lewis structure for NOCl?
2. A. B. C.
D.
Which is the correct Lewis structure for N₂H₂?
3.
Which is the correct Lewis structure for CH₃OH? A.
B.
C.
D.
4.
Draw Lewis structure for the following molecules. f) C2H4 a) H2O b)
NH3
g)
O₂
c)
HF
h)
CF₄
d)
CO2
i)
N₂
e)
HCN
Exceptions to the Octet Rule There are three main types of exceptions to the octet rule:
Molecules in which an atom has less than an octet (incomplete octet) b) Molecules in which an atom has more than an octet (expanded octet) c) Molecules with an odd number of electrons a)