Unit 1) 8) Shapes Of Molecules And Polyatomic Ions

Question 1

What is a covalent bond

Answer 1

• is formed when atomic orbitals overlap to form a molecular orbital : normally occurs when both orbitals are half filled before they overlap
• atoms share electrons to form covalent bond
  (two non metal atoms share a pair of electrons)

Question 2

What is a dative covalent bond

Answer 2

• when an atom provides both the electrons that form the bond (once it has formed it is identical to all other covalent bonds )

Question 3

Explain an example of a dative covalent bond and give other examples

Answer 3

• when an ammonium ion is formed in solution
• when an ammonia molecule picks up a hydrogen ion. The hydrogen ion has no electrons therefore cannot contribute an electron to the covalent bond. Both the electrons co,E from the lone pair on the nitrogen atom in the ammonia molecule
• carbon monoxide and ozone (O3) are other examples of molecules that contain a dative covalent bond

Question 4

What is the shape of a covalent ion or molecule dependent upon

Answer 4

• the shape of a covalent ion or molecule is dependent upon the repulsion between the electrons around a central atom

Question 5

What is valence shell electron repulsion (VSEPR)

Answer 5

• the repulsion between electrons around a central atom

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Question 6

How do calculate number of electron pairs / shape of electron pairs
This method only works for those molecules with hydrogen

Answer 6

• VSEPR (valence shell electron pair repulsion) theory can be used to predict the shapes of molecules and polyatomic ions.

The number of electron pairs surrounding a central atom can be found by:

• taking the total number of valence (outer) electrons on the central atom and adding one for each atom attached
• adding an electron for every negative charge
• removing an electron for every positive charge
• dividing the total number of electrons by two to give the number of electron pairs

Question 7

What is the arrangement of electron pairs and angle between electron pairs If the number of electron pairs is 2,3,4,5,6

Answer 7

2 - linear - 180 degrees between electron pairs
3 - trigonal planar - 120 degrees
4 - tetrahedral - 109.5
5 - trigonal bipyramidal - 120 degrees and 90 degrees
6 - octahedral

Question 8

What are electron pairs - what do they do

Answer 8

Electron pairs are charged clouds around an atom and they repel each other as far as possible

Question 9

Electrons can either be in a ________ pair or a ______ (non bonding ) pair

Answer 9

Electrons can either be in a bonding pair or a lone (non bonding ) pair

Question 10

How is the shape of an ion or molecule determined

Answer 10

By the electron pairs around the central atom : This includes the number of bonding pairs and the number of non bonding (lone) pairs of electrons

Question 11

Why do lone pairs have greater repulsion

Answer 11

• lone pairs are held closer to the central atom so they have greater repulsion than the bonding pairs of electron

Question 12

Electron pairs are _______ charged and _______ each other. They are arranged to minimise _________ and maximise _________.

Answer 12

Electron pairs are negatively charged and repel each other. They are arranged to minimise repulsion and maximise separation.

Question 13

What order to electron pair repulsion decrease in strength in

Answer 13

non-bonding pair/non-bonding pair , non-bonding pair/bonding pair , bonding pair/bonding pair
- so lone pairs of electrons repel lone pairs of electrons more than they repel bonding pairs of electrons

Question 14

• when working out the shape of a molecule or ion, you must work out the number of bonding pairs and lone pairs
• how can the arrangement of electrons in a covalently bonded molecule be shown

Answer 14

• by electron dot diagrams

Lewis structure

Question 15

Rules for Lewis for Lewis dot structure

Answer 15

1) arrangement
- atom with lowest electronegativity is usually central atom eg C,N,S,P
- sometimes halogen is central such as in oxoacids such as HCLO4 , but is more commonly terminal since it normally only forms one bond like hydrogen
- hydrogen is always terminal
- oxygen binds twice
- nitrogen bonds 3 times
- carbon bonds 4 times

2) total number of valence electrons within molecule
- in neutral molecule this is sum of the valence electrons
- for - ions add number of electrons equivalent to size of negative charge
- for + ions subtract electrons equivalent to size of positive charge

3) drawing
- place 8 electrons around atoms starting with the most electronegative (usually outer ones)
- if central atom ends up with less than 8 valence electrons , move one or more of the lone pairs of electrons from the terminal atoms into a position in between the terminal atom and central atom to form a multiple bond

Question 16

What is the octet rule and what are exceptions

Answer 16

covalent species adopt a structure in which each atom is surrounded by 8 electrons or 2 for hydrogen
- 2nd row element C, N , O, F observe the octet rule : except
- : B which has 3 valence electrons and forms 3 bonds eg BCl3
- : Be which has 2 valence electrons and forms 2 bonds BeCl2
They achieve less than 8 valence electrons and are veryyy reactive (aren’t stable )
Also : 3rd row elements are heavier elements and can exceed the octet rule by using empty d orbitals

Question 17

What is a Lewis base

Answer 17

A substance which is capable of donating an unshared pair of electrons to form a covalent bond

Question 18

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Answer 18

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Question 19

Look in notes for :

Answer 19

• examples of shapes And table to summarise everything