i am really confused

i am really confused where the numbers and charges come from when writing out an ionic compound. For example for chloride ion it is written Cl-, where does the negative come from? Or for sulfate ion it is SO4^2-, where do the four and two come from on the periodic table?
Help me please, i am trying to major in chemistry and i am already stumped in my first class in college.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Cl is atomic number 17; that means, in its neutral state, it is 1 electron short of the ideal 18 of argon, so the chloride ion captures the missing electron (which is where the negative (the charge of a single electron). S is 16, two short the sulfur atom tries to compensate by bonding with 4 atoms of O; the net result is SO4 which carries a minus 2 charge; the valence properties are indicated by the columns in which the element occurs. Column 1 are metals, typically monovalent (+1); column 2 are bivalent metals (+2), as metals they downsize to achieve their ideal by shedding electrons, thus the positive charge of the unbalanced proton; column 17, the halogens are non-metals (which capture electrons hence the negative charges) and typically -1; column 16 are bivalent. Each of the columns has a typical valence.

Did you have chemistry in high school? There are lots of basic chemistry texts that explain this, and the Encyclopaedia Britannica article on the periodic law will give you this information incase it was left out of your high school chem class.

I'm slightly aghast that a students in America are starting degree level chemistry courses not knowing material like this!? (we got taught these things in the UK at age 12!)

But anyways, just to add to Martin's info.

the subscript numbers (the ones on the bottom) don't "come from the periodic table" as such - they just tell you how many atoms of that element are present. (If there's just one atom present, we don't both to write a subscript 1)

The superscript numbers tell you the charge on the ion. (the whole ion)
if it's just a single positive or a single negative, you only write a + or a -
If it's more than that, you write 2+, 3+ etc
or 2-, 3- etc...

Each negative charge means that one electron has been gained.
Or each positive charge means that one electron has been lost.

I share Feline's shock; I was raised in the US and I understood both the period law and the concept of valences and therefore how the formulae were derived before I took high school chemistry where one learns about Avogadro's constant; I sincerely hope that this is a fluke and not the product of some misplaced "New Chem." I say this because I had a rather traditional phylogenetic biology and was horrified to find that my "peers" in California's "advanced" classes never learned about Linnaean classification and had no concept of the evolution of life, though they knew than any mortal would want to about the Krebs Cycle, similarly, my California high school chem class focused much more on thermodynamics than on chemistry, but it still covered a good deal about actual chemical processes.

Lest anyone think that this is the rant of an educational reactionary, I am also the product of the New Math of the Yale Mathematics Study Group, and have always thought that the traditional arithmetic to algebra to geometry to algebra to calculus a colossal misapplication of mathematical theory; nevertheless, there is a prevailing problem in American education, the graduate thesis syndrome, that people who are supposed to be teaching beginners distain elementary principles and the acquisition of rote facts for "exciting things"; but those things can only be exciting to minds that have been thoroughly schooled in fundamentals.

Until we get people who are willing to teach fundamentals rather than the concept mongers, we will find students who are fundamentally ignorant of the basis of their knowledge. Such "knowledge" as they possess cannot be built upon or changed. I know that the bluegreen algae of my earliest biology is no longer accepted, but because I was taught that they have a cellulose membrane with chlorophyll dispersed throughout the cytoplasm and no organized nucleus but dispersed DNA and I was also taught that green algae have cellulose cell walls and and organized nucleus containing the DNA and a chloroplast and that bacteria have a cellulose cell wall and no organized nucleus, I can readily understand and accept the 5 Kingdoms revision placing the bluegreen algae in the kingdom bacteria.

US does teach this typically in the first year in middle school, but most students forget by the time high school chemistry rolls by.

Wait, wait, wait.
Heaven did not tell us that he or she is an American.
Like me, I'm a Chinese. If not I tell you, should you think that I'm ALSO an American?

Addendum: The Cl- ion is taught in China at the age of 15. However, those taking non-Science are not required to learn it.

Because Deryck's location is given as Hong Kong and because of phrases like "should you think ..." rather than "would you think..." we can tell the writer is not American; Heaven's English is idiomatically American.

Martin,
Sorry of the confusion. At the time of writing, I hadn't put HK into the location yet.

I remember staring at the peroidic table when I was 12. We learned about elements etc...........And wondered about the weird elements like Y, Pd, Eu, Gd etc that you never hear about...what are they used for? Now I'm a user of those rarities.

The real problem lies with the high school level teachers. It is my FIRM belief that if you teach high school chemistry, physics, biology, Math etc, you should have MAJORED in that subject in college.

My first year 2nd semester high school chemistry teacher use to wake me up during class, cause she couldn't figure out how to balance out a simple reaction. It was her first semester teaching chemistry, as she has taught 7th grade basic science for 10 years. How was I suppose to be enriched and excited about this stuff if my teacher didn't have answers to my simple questions. Same thing goes for my phsycis teacher. Hell I knew more than both of them after just 1 year in engineering physics and chemistry in college.

However by the time most people gave up on chemistry, I stuck with it, and was fortunate enough to have a great 2nd year chemistry teacher in High School. She had all the answers, and guidance towards my career using chemistry. Maybe cause she worked as a chemist for 20 years before retiring to teach.

It is also my firm belief that most people in the US could care less about science....esepcially my generation (I'm 24). Most went along with the computer boom and are now suffering from a saturated prorammers market. Then the other 90% of college bound students went on to major in some business field, or Marketing. I think its like 1 to 5% that dive into science/engineering in the US. In China its closer to 20%. Its the science people that create new jobs and new sectors (semiconductors anyone?). Heck, a college down the road graduated 2500 marketing majors last may. I hope they are having fun flipping burgers.

I'm not going to complain too much, as I'm job hunting right now, and have gotten 3 great offers in just 1 month. I'll be graduating with 21 undergraduates and 8 graduate students in material science this december(our ceramic engineering and metalurgical engineering just merged).

Yes, knowing the subject you teach is an awfully good idea; unfortunately, at least here in California, teachers are required to attend education classes to get a teaching credential. One last rant on this topic and I'm done.

The reigning philosophy is that if you spend your graduate years learning chalk holding 1 & 2 and bulletin board making you can become a teacher and put all of your students into permanent study groups where they will teach each other. :roll:

The entire content of the departments of education (often inflated to schools or even colleges!!!) could be handled in a weekend seminar (with breaks for pizza) and then after a few beers it would all be forgotten and education would improve a hundred-fold.

WebElements: the periodic table on the WWW [http://www.webelements.com/]

Copyright 1993-20010 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.