6.1 Shapes of molecules and ions Flashcards

1
Q

What is the electron pair repulsion theory

A

Model used for explaining and predicting the shapes of molecules and polyatomic ions

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2
Q

What determines the shape of a molecule or ion

A

The electron pairs surrounding a central atom
They repel one another so 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 electron pairs result in different shapes

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3
Q

Example - Methane molecule CH4

A

Symmetrical with four 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 three-dimensional space

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4
Q

What shape is a methane molecule

A

Tetrahedral shape with four equal H-C-H bond angles of 109.5 degrees

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5
Q

How do we show 3D shapes to visualise structures

A

Using lines and wedges

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6
Q

What does a solid line represent

A

A bond in the plane of the paper

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7
Q

What does a solid wedge show

A

Comes out of the plane of the paper

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8
Q

What does a dotted wedge show

A

Going into the plane of the paper

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9
Q

What are wedges useful for representing

A

Bonds in organic molecules

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10
Q

A lone pair of electrons is slightly _____ to the central atom, and occupies _____ space, than a bonded pair. Therefore, a lone pair repels _____ strongly than a bonding pair

A

Closer
More
More

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11
Q

Relative repulsions between lone pairs and bonding pairs

A

bonded-pair/bonded-pair < bonded-pair/lone-par < lone-pair/lone-pair

INCREASING REPULSION ->

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12
Q

Things to remember about molecular shapes from four electron pairs

A

The four electron pairs around the central atom repel one another as far apart as possible into a tetrahedral arrangement
Lone pairs repel more strongly than bonded pairs
Therefore lone pairs repel bonded pairs slightly closer together, decreasing the bond angle - the angle between the bonded pairs of electrons
The bond angle is reduced by about 2.5 degrees for each lone pair.

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13
Q

What can each multiple bond in a molecule containing multiple bonds be treated as

A

A bonding region

For example - CO2 has 2

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14
Q

Things to remember about molecular shapes from other numbers of electron pairs

A

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

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15
Q

What shape does Boron trifluoride have

A

A trigonal planar shape with equal bond angles of 120 degrees

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16
Q

What shape does Sulfur hexafluoride have

A

An octahedral shape with equal bond angles of 90 degrees

17
Q

What can we do with electron-pair repulsion theory

A

Explain and predict the shapes of ions

18
Q

Shapes of ammonium

A

Four bonded pairs surrounding the central nitrogen atom
Has the same number of bonded pairs of electrons around the central atom as a methane molecule
Has the same tetrahedral shape and bond angles as a methane molecule.

19
Q

What shape do CO3^2- and NO3- ions have

A

Same as Boron trifluoride, trigonal planar shape

They have three regions of electron density

20
Q

How many centres of electron density does So4^2- have, and what shape does it have

A

Four centres of electron density around the central sulfur atoms and have the same shape as a methane molecule