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Ocr f321 23 may 2013~official discussion thread

Reply 180

Shanze S

Original post by yodawg321

Just scroll underneath the questions. The mark scheme is there

Oh right , thank you

Reply 181

Liberty.

10

Original post by yodawg321

Question

Explain the decrease in atomic radius across each period?
Predict and explain whether a sodium ion is larger or smaller than a sodium atom.
Why does the boiling point increase from Sodium to Aluminium?

Atomic Radius Decrease...
->Electrons added to same shell, no extra electron shielding
->Greater nuclear charge, with increase of protons and electrons
->Greater nuclear attraction between nucleus and outer shell electrons, pulling them closer... thus decreasing the atomic radius.

Na ion compared with Na atom
-> larger, referring to atomic radius.
-> Na atom has 11 electrons, 1 electron in outer shell.
-> Na+ has 10 electron, 8 in outer shell
-> Na+ has less electron shielding.
-> Na+ will have greater nuclear attraction for outer shell electrons.
-> Same nuclear charge.
-> Thus Na+ will have smaller atomic radius, so Na atom is larger.

Na -> Al
-> Across same period, both form strong metallic bonds.
-> Decrease in atomic radius across period, Al has smaller radius
-> No difference in electron shielding, electrons added to same shell
-> Al has greater nuclear charge, due to addition of protons and electrons
-> Al had greater nuclear attraction for it's outer shell electrons
-> thus more energy is required to break Al metallic bonds.

Please correct if I've gone wrong somewhere, but this is my thought process to answering those type of questions :smile:

EDIT: Good point from above for Na -> Al, greater nuclear attraction means greater electrostatic attraction between delocalised electrons and positive Al3+ ions within metallic lattice.

(edited 10 years ago)

Reply 182

Liberty.

10

Do we have to know about the history of the 'model of an atom'? :tongue:

Reply 183

gettingcreamy

what do u guys think might come up in tomorrows exam?

Reply 184

HelenPaddock

Two questions;
Can someone please explain how bond angles of 104.5 are made?
Also, in the Jan 2013 paper on the ionisation energy question 3c, why is there a big jump between the 6 and 7th?
Thanks

Reply 185

yodawg321

12

Original post by Liberty.

Atomic Radius Decrease...
->Electrons added to same shell, no extra electron shielding
->Greater nuclear charge, with increase of protons and electrons
->Greater nuclear attraction between nucleus and outer shell electrons, pulling them closer... thus decreasing the atomic radius.

Na ion compared with Na atom
-> larger, referring to atomic radius.
-> Na atom has 11 electrons, 1 electron in outer shell.
-> Na+ has 10 electron, 8 in outer shell
-> Na+ has less electron shielding.
-> Na+ will have greater nuclear attraction for outer shell electrons.
-> Same nuclear charge.
-> Thus Na+ will have smaller atomic radius, so Na atom is larger.

Na -> Al
-> Across same period, both form strong metallic bonds.
-> Decrease in atomic radius across period, Al has smaller radius
-> No difference in electron shielding, electrons added to same shell
-> Al has greater nuclear charge, due to addition of protons and electrons
-> Al had greater nuclear attraction for it's outer shell electrons
-> thus more energy is required to break Al metallic bonds.

Please correct if I've gone wrong somewhere, but this is my thought process to answering those type of questions :smile:

EDIT: Good point from above for Na -> Al, greater nuclear attraction means greater electrostatic attraction between delocalised electrons and positive Al3+ ions within metallic lattice.

Sodium Ion will be smaller because it will turn into a +1 ion.
This will result in smaller shells than Na atom.
So there will be more nuclear attraction as the remaining electrons will be attracted to the nucleus more.

and for the Na-Al bit you basically got that right.

All you have to say is that

Al has more delocalised electrons.
Al has more ionic charge
So there is a greater nuclear attraction
More energy needed to break the strong metallic bonds.

Reply 186

yodawg321

12

Original post by HelenPaddock

Two questions;
Can someone please explain how bond angles of 104.5 are made?
Also, in the Jan 2013 paper on the ionisation energy question 3c, why is there a big jump between the 6 and 7th?
Thanks

So the first bond angle is 120, thank you know that.

The second bond angle is 104.5 because you have to look at the central atom.
You can see that it has 2 bonded pairs and 2 Lone pairs
2 Lone pairs will make the shape Non-Linear aka 104.5 degrees.

Reply 187

username1107833

2

Original post by HelenPaddock

Two questions;
Can someone please explain how bond angles of 104.5 are made?
Also, in the Jan 2013 paper on the ionisation energy question 3c, why is there a big jump between the 6 and 7th?
Thanks

Okay, oxygen has 8 electrons. 2 in it's 1st shell, and 6 in its 2nd shell. The 6th ionisation energy you are still removing electrons from the 2nd shell. But when you reach the 7th, you've already taken all of the 6 electrons in the 2nd shell, so you start removing electrons from the 1st shell. Since the 1st shell is closer to the nucleus it experiences more electrostatic attraction, and less electron shielding since it's the first shell.

Reply 188

username1107833

2

Original post by Liberty.

Do we have to know about the history of the 'model of an atom'? :tongue:

No I don't think so, it isn't on the specification.

Reply 189

HelenPaddock

Ahh makes sense, thank you

Reply 190

joshturnbull123

Original post by sladyy96

can someone please explain hydrogen bonding?

Hydrogen Bonding:
Basically another type of intermolecular force that can only occurs between:
-δ+ Hydrogen atom on one molecule
-and the lone pair of electrons on the δ- atom of the neighbouring molecule

As long as you can draw it you'll be fine... For example with water draw two molecules of H20 and make sure you put on the δ charges and then draw a dotted line from the δ+ H on one to the lone pair of electrons on the O atom on the other molecule!

Reply 191

sladyy96

Original post by joshturnbull123

Hydrogen Bonding:
Basically another type of intermolecular force that can only occurs between:
-δ+ Hydrogen atom on one molecule
-and the lone pair of electrons on the δ- atom of the neighbouring molecule

As long as you can draw it you'll be fine... For example with water draw two molecules of H20 and make sure you put on the δ charges and then draw a dotted line from the δ+ H on one to the lone pair of electrons on the O atom on the other molecule!

great, thank you!! :smile:

Reply 192

Freddy-Francis

9

Original post by sladyy96

can someone please explain hydrogen bonding?

Sure. A strong dipole-dipole attraction between an electron defecient hydrogen atom on one molecule and a lone pair of electrons on a highly electronegative atom on another molecule :biggrin:

Reply 193

sladyy96

Original post by HelenPaddock

Two questions;
Can someone please explain how bond angles of 104.5 are made?
Also, in the Jan 2013 paper on the ionisation energy question 3c, why is there a big jump between the 6 and 7th?
Thanks

Not sure if this answers your question directly, but the way I look at it is according to the number of regions of electron densities...
6 regions: 6bonds(octahedral)
4 regions: 4bonds(tetrahedral); 3bonds and 1lonepair(pyramidal); 2bonds and 2lonepairs(non-linear)
3 regions: 3bonds (trigonal planar); 2bonds and 1lonepair (non-linear)
2 regions: 2bonds (linear)

if you just remember the angles for octahedral, tetrahedral, trigonal planar, and linear, you can work out the others. Everytime there's a lone pair instead of a bond, the angle decreases by 2.5deg.
For example, for the shapes with 4 electron density regions:
4 bonds is 109.5deg; 3bonds and 1lonepair is 107deg; 2bonds and 2lonepairs is 104.5deg.

Reply 194

Freddy-Francis

9

How are Van Der Waals formed?

Reply 195

megan1096

Which shapes are symmetrical and which are asymmetrical? Can't picture them in my head :frown:

Reply 196

Freddy-Francis

9

Original post by sladyy96

Not sure if this answers your question directly, but the way I look at it is according to the number of regions of electron densities...
6 regions: 6bonds(octahedral)
4 regions: 4bonds(tetrahedral); 3bonds and 1lonepair(pyramidal); 2bonds and 2lonepairs(non-linear)
3 regions: 3bonds (trigonal planar); 2bonds and 1lonepair (non-linear)
2 regions: 2bonds (linear)

if you just remember the angles for octahedral, tetrahedral, trigonal planar, and linear, you can work out the others. Everytime there's a lone pair instead of a bond, the angle decreases by 2.5deg.
For example, for the shapes with 4 electron density regions:
4 bonds is 109.5deg; 3bonds and 1lonepair is 107deg; 2bonds and 2lonepairs is 104.5deg.

Do we have to know the 5 bonding regions. Triagnol Bipyramid. There are two angles for it.. 90deg at equator and axis, 120deg at equator

Reply 197

yodawg321

12

Original post by Freddy-Francis

Do we have to know the 5 bonding regions. Triagnol Bipyramid. There are two angles for it.. 90deg at equator and axis, 120deg at equator

No we don't need to know that

Reply 198

Freddy-Francis

9

Original post by megan1096

Which shapes are symmetrical and which are asymmetrical? Can't picture them in my head :frown:

Symmetrical - Dipoles cancel out each other. The molecules are Non-Polar.
asymmetrical - Dipoles dont cancel out but adds together to make a molecule with a larger dipole moment.