6.1 Shapes of molecules and ions Flashcards
What is the charge of an electron and what does this cause to occur?
Electrons are negatively charged and therefore they repel one another
What is the electron-pair theory?
It is a model used in chemistry to explain and predict the shape of molecules and polyatomic ions.
- the electron pairs surrounding a central atom determine the shape of the molecule or ion.
- The electron pairs repel one another so that they are arranged as far apart as possible
- the arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
- Different numbers of electrons pairs result in different shapes
describe the shape of a molecule of methane, CH4
It is symmetrical with 4 C-H covalent bonds
Four bonded pairs of electrons surround the central carbon atom
The four electron pairs repel one another as far apart as possible in the 3D space.
The result is a tetrahedral shape with four equal C-H bond angles of 109.5o
What does a solid line represent?
A bond in the plane of the paper
HWat does a solid wedge reppresent?
The bond coming out of the paper
What does a dotted wedge represent?
The bond goes into the paper
What is the difference between the position of the lone pair and the bonded pair od electrons?
The lone pair of electrons are slightly closer to the central atom. As a result it repels more strongly.
Which of the three repels the most and which repels the least:
Bonded-pair/Lone-pair
Lone-pair/Lone-pair
Bonded-pair/Bonded-pair
Repels the most:
Lone-pair/Lone-pair
Repels the least:
Bonded-pair/Bonded-pair
Describe the structure of a tetrahedral arrangement
The four electron pairs around the central atoms repel one another as far apart as possible
Becuase lone pairs repel more than bonded pairs of electrons, lone pairs repel bonded pairs slightly closer together decreasing the bond angle
What is a bond angle?
It is the angle between the bonded pairs of electrons
What degree does the bond angle decrease pair extra lone pair added?
2.5o
What are the bond pairs, lone pairs, bond angle and electron pairs/regions for a tetrahedral arrangement?
Bonded pairs:
4
Lone pairs:
0
Bond angle:
109.5o
Electron pairs/regions:
4
What are the bond pairs, lone pairs and bond angle for a pyramidal arrangement?
Bonded pairs:
3
Lone pairs:
1
Bond angle:
107
What are the bond pairs, lone pairs and bond angle for a non-linear arrangement?
Bonded pairs:
2
Lone pairs:
2
Bond angle:
104.5o
What si the bond angle od a linear arrangement?
180o
What is the shape of a carbon dioxide molecule and why?
It is linear because the 2 bonded regions repel one another as far apart as possible. This give CO2 a linear shape with 3 atoms aligned in a straight line
What is the principle of electron-pair repulsion theory?
Electron pairs around the central atom repel each other as far apart as possible
The greater the number of electron pairs, the smaller the bond angle
lone pairs of electrons repel more strongly than bonded pairs of electrons
Give an example and explain the shape of a molecule with three electron pairs (include the bond angle)
Boron trifluoride (BF3) has only 3 bonded pairs around the central Boron atom. Electron-pair repulsion gives a trigonal planar shape with equal bond angles of 120o
Give an example and explain the shape of a molecule with six electron pairs (include the bond angle)
Sulfide hexafluoride (SF6) has 6 bonded pairs of electrons around the central sulfur atom. Electron-pair repulsion gives an octahedral shape with equal bond angles of 90o
What are the electron pairs and bond angle of a linear shape?
Electron pairs/regions:
2
Bond angle:
180o
What are the electron pairs and bond angle of a trigonal planar shape?
Electron pairs/regions:
3
Bond angle:
120o
What are the electron pairs and bond angle of a tetrahedral shape?
Electron pairs/regions:
4
Bond angle:
109.5o
What are the electron pairs and bond angle of an octahedral shape?
Electron pairs/regions:
6
Bond angle:
90o
How is an octahedral shape obtained?
By joining together six corners occupied in a fluorine atom
Describe the structure, shape and bond angle of an ammonium ion.
the ammonium ion (NH4+), ahs four bonded pairs surrounding the central nitrogen atom.
An NH4+ ion has the same number od bonded pairs of electrons around the central atoms as a methane molecule
It ahs the same tetrahedral shape and bond angle of 109.5o as a methane molecule
Describe the structure, shape and bond angle of a CO3 2- ion.
It has three regions of electron density surrounding the centre carbon atom.
It has a trigonal planar shape with an angle of 120o
Describe the structure, shape and bond angle of a NO3 - ion.
It has three regions of electron density surrounding the central nitrogen atom.
It has a trigonal planar shape with an angle of 120o
Describe the structure, shape and bond angle of a SO4 2- ion.
It has four centres of electron density around the central sulphur atoms
It has a tetrahedral shape with a bond of 109.5o