1.4 Bonding Flashcards
(21 cards)
What is electronegativity?
A measure of the electron-attracting power of an atom in a covalent bond.
What is the trend of electronegativity?
Increases across a period and decreases down a group.
what is a polar bond? and when does it occur?
when the atoms within the covalent bond have different electronegativities; the bonding electrons are pulled towards the more electronegative atom. This atom will take up a slightly negative charge and the other atom becomes slightly positive.
what causes a permanent dipole?
In a polar bond, the difference in electronegativity between the two atoms causes a permanent dipole. The greater the difference in electronegativity, the more polar the bond.
what are intermolecular forces?
Intermolecular forces are the weak bonding holding the molecules together
what are intramolecular forces?
the strong bonding between the atoms in a molecule.
What are/when do permanent dipole- permanent dipole interactions occur?
If the dipoles arrange themselves so that the negative region of one molecule is close to the positive region of another molecule, there will be an attraction between them.
These are called permanent dipole-dipole interactions and are an example of van der Waals forces.
how do induced dipole-induced dipole bonds happen?
When do induced dipoles occur?
A dipole can still be formed in non-polar molecules. This is due to the constant movement of electrons around the nuclei where sometimes more electrons are concentrated on one side of the atom at any one time, causing a temporary dipole.
The δ+ end of the molecule can pull the electron cloud of a neighbouring molecule towards it, giving the left side of that molecule a δ- charge. This means it induces a temporary dipole in the neighbouring molecule. The two dipoles are attracted to each other.
The second dipole induces a dipole in a third molecule etc. It is a domino effect.
what effect does the amount of electrons have on an induced dipole?
The strength of the bond increases as the amount of electrons increase.
discuss hydrogen bonding:
These are the strongest of the intermolecular forces. They occur between molecules containing hydrogen atoms bonded to small, very electronegative elements which have lone pairs – fluorine, oxygen or nitrogen. The highly polarising δ+ hydrogen atom attracts a lone pair of electrons from a small, highly electronegative atom in another molecule. The δ+ hydrogen atom is sandwiched between two electronegative atoms. It is covalently bonded to one and hydrogen-bonded to the other.
what elements can hydrogen bond with hydrogen?
fluorine
oxygen
nitrogen
briefly explain the difference between molecules that can and cannot hydrogen bond in terms of solubility:
The most significant intermolecular forces between water molecules are hydrogen bonds. Covalent compounds with hydrogen bonds between molecules, such as ammonia, can form hydrogen bonds with water molecules. These compounds will therefore be soluble. Compounds such as methane that only form van der Waals forces between molecules can’t form hydrogen bonds with water, and are therefore insoluble.
What is the VSEPR theory?
- The shape of a molecule or molecular ion is determined by the number of electron pairs in the valence (outer) shell around the central atom.
- All electron pairs repel each other (since electrons are all negatively charged). The shape that is formed is one that enables the electron pairs to keep as far away from each other as possible so that repulsion is minimised.
- Bonding pairs are spread out between the two bonding atoms, but lone pairs stay close to the central atom. As a result, lone pairs repel more than bonding pairs. This leads to the following sequence of repulsion for electron pairs: lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion.
how do you find out the amount of electron pairs?
You can do this in two ways, either by drawing a dot and cross diagram, or by following the steps below.
- Write down the number of electrons in the outer shell of the central atom. (This will be the same as the periodic table group number.)
- Add one electron for each bond being formed. (The formula will give the number of bonds.)
- Allow for any ion charge on the central atom. (If the ion has a charge of 1-, then add one electron. For a 1+ charge, deduct one electron.)
- Divide the total number of electrons by 2 to find the number of electron pairs.
- Compare the number of electron pairs with the number of bonds to find the number of bonding pairs and lone pairs. (Use the formula to find the number of bonding pairs.)
For a compound with double bonds, remember for step 2 that each double bond donates two electrons, and for step 5 a double bond counts as two bonds.
If a molecule has two electron pairs in the valence shell of the central atom, what is its shape and bond angles?
The shape of the molecule is linear. The two bonding pairs of electrons arrange themselves at 180˚ to each other because that is as far apart as they can get.
If a molecule has three electron pairs in the valence shell of the central atom, what is its shape and bond angles?
If there are three bonding pairs of electrons, they arrange themselves as far apart as possible, so they all lie in the same plane at 120˚ to each other. The shape of the molecule is trigonal planar.
If a molecule has four electron pairs in the valence shell of the central atom, what is its shape and bond angles?
The electron pairs all repel each other equally. All the bond angles are 109.5˚ and the shape of the molecule is tetrahedral.
If a molecule has three electron pairs and 1 lone pair in the valence shell of the central atom, what is its shape and bond angles?
Again, the electron pairs arrange themselves in a tetrahedral shape, but there is more repulsion between a lone pair and a bonding pair than between two bonding pairs. This forces the bonding pairs closer together, slightly reducing the bond angle between them to 107˚. The shape of the molecule is trigonal pyramidal.
If a molecule has two electron pairs and two lone pairs in the valence shell of the central atom, what is its shape and bond angles?
This also takes a tetrahedral arrangement, but the lone pair-lone pair repulsion forces the bonding pairs even closer and reduces the angle between them to 104.5˚. The shape is described as bent or V-shaped.
You do not have to know the bond angles in pyramidal or bent shape molecules, only that they are less than in tetrahedral molecules.
If a molecule has five electron pairs in the valence shell of the central atom, what is its shape and bond angles?
Repulsion between the bonding pairs means that three of the chlorine atoms are in a plane at 120˚ to each other and the other two chlorine atoms are at right angles to this plane. The shape is described as trigonal bipyramidal.
You only need to know the name of this shape. You will not be required to draw it or know the bond angles.
If a molecule has six electron pairs in the valence shell of the central atom, what is its shape and bond angles?
There are 12 electrons, consisting of 6 bonding pairs, around the central atom. They arrange themselves entirely at 90˚ in a shape described as octahedral.
