Chemistry people HELP ME AAAAAAGH!!!!!

[BoffinMum]

I am trying to help DS1 with his iGCSE revision for triple science and we are stuck on these bits of the Chemistry papers:

Valencies
Properties of graphite
Properties of lithium fluoride
Oxidation and reduction
Isotopes

We have a text book here but both of us are more than slightly confused and losing the will to live.

I was rubbish at this in 1984 and even more rubbish now.

HHHHHHEEEEEELP!!!!

What are the questions?

Tell him to ask his teacher! He cannot be doing the exam right now! Most schools do revision classes at he appropriate time so you may be a tad early to be getting involved in old papers right now.

Give us some specific questions and we will help

OK, here we go. First area of confusion.

Two isotopes of helium are 3/2 He and 4/2 He (can't type with subscript here, sorry).

Which two diagrams show the arrangement of particles in these two isotopes?

Then you have a multiple choice with 3/2 He and 4/2 He written over two columns, and ABCD down the side. There is a key - e=electron, p=proton, n=neutron, nucleus. There are diagrams of the ?particles, but essentially the information we derive from this is:

      3/2 He     4/2 He

A 3e 2p 1n 4e 2p 2n
B 3e 1p 2n 4e 2p 2n
C 2e 2p 1n 2e 2p 2n
D 2e 3p 2e 4p

The information I don't get is the significance of the 3/2 He and 4/2 He and how it relates to this table.

The Biology teacher, Chemistry teacher and Physics teacher have set three past papers each and this needs to be done before he goes back on Tuesday or he gets a detention.

He is about to go into Year 11 BTW.

As far as the 3/2 and 4/2 means, the bottom number us the atomic number (also known as proton number) which means how many protons in the element. The top number is the Mass number, the number if neutrons plus protons in the nucleus. The atomic number is the same as the number of electrons.

Next question:

Draw a diagram which shows the formula of lithium oxide, the charges on the ions and the arrangement of the valency electrons around the negative ion.
Use x to represent an electron from an atom of oxygen.
Use o to represent an electron from an atom of lithium.

My problem - I don't even know how to look this up!!

Question 2: google 'dot cross diagram lithium oxide'.. BBC bitesize covers it...

Right, we understand mass numbers and atomic numbers now and have completed two outstanding questions!!

OK, we don't understand the dot and cross diagrams but at least we know what to ask the teacher for help with, and we have filled in the Lithium one on the sheet. Thank you.

Next question.

We have a table to do with some particles.

Particle Protons Electrons Neutrons
A 15 15 16
B 15 18 16
C 15 15 17

What is the evidence in the table for the following?

Particle A is an atom
They are all particles of the same element
Particle B is negative ion
Particles A and C are isotopes

What is the electronic structure of particle A?
What is the valency of the element?
Is the element a metal or a non mental? Give a reason for your choice.

We both have no clue on this one.

A cross is a proton
A dot is a neutron
And a minus is an electron

It represents the charge. Dots and crosses should be in the middle and minuses in rings around it

A/B/C all have the same number of protons, aka the same atomic number, hence are the same element. If it has equal number of protons and electrons it's an atom (neutral total charge) if not then it's an ion, either positive or negative depending on the balance

A&C are both atoms of the same element with different numbers of neutrons in their nucleus, in other words, isotopes.

(But I can't do the valency/metal thing cos A level chemistry was twenty five years ago, oh god that's a long time)

1. It has the same number of electrons and protons, therefore has a neutral charge.
2. They all have the same number if protons. If they had different proton numbers they would be different elements.
3. It has 3 more electrons than protons. Electrons have a negative charge therefore the overall charge is 3-. Which is negative.
4. An isotope has the same protons but a different number of neutrons, as is the case for A and C.

Now we are attempting a dot and cross diagram for selenium chloride, but we didn't know much about this compound. We looked it up on Wiki, etc, and it seems to have 34 electrons. But a) we are still not quite sure about the process of constructing the diagrams when they are for a compound, and b) we are also doubtful they would pick something with a very complicated structure for a GCSE level exam. Advice welcome.

Now a question on oxidation and reduction.

We have a table showing elements in the second period of the Periodic Table and some of their oxidation states in their most common compounds.

element Li Be B C N O F Ne
number of outer electrons 1 2 3 4 5 6 7 8
oxidation state +1+2+3+4-3-2-1 0

A

1. What does it mean when the only oxidation state of an element is zero?
2. Explain why some elements have positive oxidation states but others have negative ones.
3. Select two elements in the table which exist as diatomic molecules of the type x

B Beryllium hydroxide, a white solid, is an amphoteric hydroxide.

1. Name another metal which has an amphoteric hydroxide.
2. Suggest what you would observe when an excess of aqueous sodium hydroxide is added gradually to aqueous beryllium sulphate.

C.

1. Give the formulae of lithium fluoride and nitrogen fluoride (DS has put LiF and NF here)
2. Predict two differences in their properties.
3. Explain why these two fluorides have different properties. (DS has said helpfully that it is because one has more lithium in and one has more nitrogen in. duh.)

Right, drags dot and cross diagrams out of back of memory.

For your Se
Electrons go in shells. So, round the middle, where you have written how many neutrons and protons are in the nucleus, draw 4 rings.
The first shell can hold 2 electrons, so put 2 'e' or '-' (e is easier to see!).
The next shell can hold 8 electrons, so add 8 'e's to the second ring.
The third shell can hold 18 electrons. so add 18 'e's/
You've now added 28 electrons, so add the remaining 6 electons to the outer ring.

Chlorine: 17 electrons
2 in first shell
8 in second shell
7 in third shell.

I'm worrying myself now, as I know Cl only needs 1 electron, making third shell full at 8.

Ask DS about s shells, p shells and d shells.

Will think more when in the shower, and come back after some wiki.

Try BBC bitesize for dot cross diagrams and properties and stuff. They won't have the iGCSE spec, but you should find it in additional science in at least one of the specs there. Or Doc Brown, v good for chemistry.

This worries me! Doing a child's homework on Mumsnet! Clearly this will mask the fact this child cannot do this homework. Surely there is an urgent discussion to be had with the teacher, detention or not! I would NEVER consider getting help for a child in his way. It is cheating and defeats the purpose of the homework for THE CHILD!

Well, if he understands it once he's looked at the info, that's a good thing. If he doesn't, he needs to 'fess up to his teacher.

agrees with BlackMogul

I also have qualms. What exactly are you trying to achieve here Boff? If it's understanding the concepts yourself do you can help him get to grips then OK I guess, but it sounds like you need a proper plan.