how do you know how the elements have bonded by looking at the formula alone?

[lboogy]

Hi everyone,

 

sorry for keep posting on here to ask questions, right now though if i come accross something i don't understand this is my best resource - i hope you all don't mind.

 

If you have a formula, say CaCO3 (just for example, my question isn't specific to this formula, it's a more hypothetical one) how can you tell how these elements have bonded? Is it covalent or ionic?

 

Also, I've seen H2O written with (l) after it denoting it is in liquid form. But then in the same formula I've seen other elements written with the (aq) to denote disolved in water. If something is written with the (aq) after it, but H2O is not part of the formula, does it mean that the hydrogen and oxygen from the water is not having an effect in the equation?

 

Also....what does (OH) mean after an element? I know OH- means it's an alkali, but what about when there is no negative sign there?

 

thanks in advance!

[Darkblade48]

For determining covalent vs. ionic bonds, the short answer is it's up to "experience". However, the longer answer is that you should look at electronegativities between the two bound elements. For example, if the difference in electronegativity is greater than 1.7, the bond is said to be 51% ionic. You can find tables of the percentage ionic character of a single chemical bond (usually on a periodic table).

 

Note that calcium carbonate is Ca(CO₃)₂

 

What do you mean by having an OH after an element? Perhaps like NaOH? This is a compound, known as sodium hydroxide. In water, it will dissociate to form Na+ and OH-, and as you have mentionned, the hydroxyl anion acts as a strong base.

[lboogy]

The example of the OH in brackets that I read was Ca(OH)2(s) - it's calcium hydroxide, but was wondering why the OH are in brackets and without a negative sign?

[YT2095]

because CaOH2 would look silly and ambiguous.

is it 2 lots of CaOH?

 

also it doesn`t need the negative sign when it bound, the compound is then treated as a Whole.

[YT2095]

maybe best if I try and explain it this way....

 

you need to look at the electronic charge, Ca has a charge of +2

Oxygen is a -2

Hydrogen is a +1

when an oxygen and a hydrogen join it`s like adding -2 and +1. the result is -1, (that`s why H2O is prefered, and CaO exists happily too)

 

so the OH is a -1 state, but Calcium is a +2.

so it will need 2 lots of the -1 OH to combine with the +2 Ca to balance out even

 

Ca 2+ and 2x OH -1 = Ca(OH)2 and all is nice and correct

[jdurg]

 

Note that calcium carbonate is Ca(CO₃)₂

 

 

Ummm....... I don't think so, unless the Carbonate ion suddenly went from having a -2 charge to having a -1 charge. CaCO₃ is the proper formula.

[YT2095]

(CO3)2 works for a an oxidation number of +1 such any group 1 element.

Ca is a group 2

 

well spotted JDurg.

[Darkblade48]

Ummm....... I don't think so, unless the Carbonate ion suddenly went from having a -2 charge to having a -1 charge. CaCO₃ is the proper formula.

 

Jeez you're right. I posted that at 7 am local time

 

I really should stop answering chemistry questions so early in the morning

[jdurg]

(CO3)2 works for a an oxidation number of +1 such any group 1 element.

Ca is a group 2

 

well spotted JDurg.

 

Actually, that is wrong as well YT. (CO3)2 is only viable if the cation has a charge of +4. For a group 1 element, it would be X2CO3 where X is equal to Li, Na, K, Rb, Cs, Fr.

[agaubr]

Usually the simple way to describe ionic compounds is to say they are composed of a metal and nonmetal element such as NaCl, CaCO3, etc. A covalent compound is composed of nonmetals such as CH4, O2, N2, etc.

 

When you add an ionic compound to water such as NaCl, table salt, you might describe this action as

 

NaCl (s) ---> Na+(aq) + Cl-(aq) indicating the salt is a solid and with the addition of the water becomes dissolved. However the dissolution cause the solid crystal matrix to being held together by electrostatic interaction of a positively charged sodium ion and a negatively charged chloride ion to come apart so that the ions are now dissolve din water. the ions are now surrounded by polar water molecules which can interact with the charged ions by a dipole-ion interaction which causes the ions to stay in solution. Therefore when an ionic salt dissolves it is no longer a slat but discrete ions in solution which have different properties and behave differently. So when you say I dissolved an ionic compound you have a very important statement and description of the system.

 

As for covalent compounds such sugar they dissolve as a unit and do not form charged systems and their properties are different. However, there are exceptions such as molecular HCl(g) in the gas phase becomes an H+(aq) and Cl-(aq) when in water. It is this property which allows for the acidity of the solution due to the formation of H+(aq).

 

agaubr