# Basic question

#### Jchem

Hello all, I'm re-learning chemistry on my own and have a quick question that I cant find the answer to in my book.

Its about writing formulas.

For example, aluminum chloride. I know that Al has a valence number of +3, and I know that Cl has a valence number of -1.

So applying the "crossover rule" the answer is AlCl3.

Now the only reason I know the valence numbers of Aluminum and Chlorine is because I got them from a Valence number table with about 10 different elements.

My question is, how do I figure out the valence number for all the other elements?

thanks

#### kclo4x

well i that staring from helium over to boron you can count helium at 0 and fluorine at one and so on an count it columes as the # to but if Al has the valance of three that would prove me other wise
b=5 al=3???
c=4 si=4
n=3 p=3
o=2 s=2
fl=1 cl=1

#### The Bob

kclo4x said:
well i that staring from helium over to boron you can count helium at 0 and fluorine at one and so on an count it columes as the # to but if Al has the valance of three that would prove me other wise
b=5 al=3???
c=4 si=4
n=3 p=3
o=2 s=2
fl=1 cl=1
Why would boron be 5??? It has two inner shell electrons and 3 outer shell electrons (the valance ones). It has a proton number of 5 but Aluminium has a proton number of 13 yet it has a valance of 3.

Jchem said:
My question is, how do I figure out the valence number for all the other elements?
The groups in the periodic table will give you a fair number of the valence electrons e.g. Sodium has 1 outer shell electron, Potassium has one as well and they form Sodium/Potassium Chloride with Chlorine (outer shell of 7). Group 2 have 2 valance electrons etc. (although consider that it is easier for group 6 and 7 to gain electrons).

However you must also wish to know about the transition metals. All I have to say is that these can be predicted but it really does take knowledge of oxidation states and the like. Copper can form compounds where where 2 of the other element is need, simply one is needed or two coppers are needed. This is oxidation state use.

Also a knowledge of ions is needed (e.g. OH- has a charge of minus one. You may have been able to work this out due to oxygen being a 2- ion and hydorgen being a +1 ion. However SO42- has a charge of minus 2. This is another workable one but it is not obvious and some people do put them seperately, which can be confusing).

I am not sure how to answer the question to the stand you want it because it does need some understanding first. I think it might be wise if I post some questions and you try answering them:

What is the molecular formulae for:
(a) Sodium oxyide?
(b) Potassium fluoride?
(c) Calcium oxide?
(d) Magnesium chloride?
(e) Methane (Carbon and Hydrogen)?
(f) Calcium carbonate?
(g) Potassium hydroxide?
(h) Hydrochloric Acid?
(i) Sulphuric Acid?
(j) Uranium Hexafluride (just for fun)

This will allow you to see if you understand the basis of compounds and, thus, the valance of elements. Then this can progress to the transition metals when the basis is there.

The Bob (2004 ©)

P.S. I apologise if this post seems unneccesary or patronising. This was not the point at all.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving