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what are cations and anions and why do they form and stacks of other questions

Help with chemisrty final review :)?
1. what are cations and anions and why do they form?

2. define covalent bond and tell what types od elements form from them.

3. define ionic bond and tell that types of elements forn from them

4. define polar covalent bond and tell what makes a bond polar.

5. what is a formula unit?

6. use VESPR theory to predict the shape of methane (CH4) molecule.

7. use VESPR theory to predict the shape of water molecule.

8. give the chemical formula for calcium sulfate.

9. give the chemical formula for carbonic acid.

10. name the reaction type : Cu + AgNO3 --> CuNO3 + Ag

kingchemist Mon, 05/18/2009 - 14:55

Too many questions!
A cation is a positive ion eg Na+. It is more stable electronically than a sodium atom (2,8 instead of 2,8,1 for the sodium atom.) Anions are negative ions eg O = 2,6, O2- = 2,8 - again the ion is more stable electronically and has the same electron arrangement as a noble gas

A covalent bond is the strong attraction between atoms for shared electrons. Non-metal atoms usually form covalent bond eg H-H, H-O-H (H2O, H-Cl)

An ionic bond occurs when an atom (usually a metal) loses an electron(s) to make a cation, and this electron(s) is gained by a non-metal atom which becomes an anion. The ionic bond is the strong attraction between the ions. EgĀ  Na ----> Na+ + e- and Cl + e- ----> Cl-
Then Na+ + Cl- ----> Na+Cl- which is an ionic compound.

A polar covalent bond is again the strong attraction between atoms for shared electrons. However, some atoms are better at attracting electrons than other atoms eg in HCl, the chlorine atom is better at attracting electrons than the H atom so the two shared electrons spend more of their time closer to Cl than H. This give the Cl atom a slight negative charge delta-) while the hydrogen become slightly positive as it has less than its shared of the electrons (delta+) eg delta+ H-Cl delta-. The bond is therefore polar (charged)

The formula unit is the simplest way of representing the formula eg a sodium chloride crystal contains many sodium ions and equally many chloride ions, but is represented by the formula unit Na+Cl-

In methane, CH4 there are four C-H covalent bonds. The each are made of two electrons and the four bonds repel each other and end up about 109o from each other pointing to the corners of a tetrahedron
In water there are two O-H bonds which repel each other but there are also two lone pairs (non-bonded pairs) of electrons which also repel. The result is a non-linear or bent molecule with an H-O-H bond angle of 105o. The lone pairs point to give an overall tetrahedral structure though not a perfect tetrahedron as in methane. If you click on the structures I have attached you will see the shapes more clearly

CaSO4

H2CO3

Displacement

Phewwwwwwwwww - I hope my effort is worth it!

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