Ch 2 Chemical Compounds and Bondings
Chemical bond: force of attraction between 2 atoms or 2 ions.
There are 2 main types of chemical bonds:
1. Ionic bond: a bond between a metallic atom (metal) and a non-metallic atom (non-metal) in which there is a complete transfer of electrons from the metal to the non-metal.
The compound which is formed is called an ionic compound.
Ex.: NaCl , MgCl2, Al2O3
To write the formula of an ionic compound we use the criss-cross method (we down cross multiply the charges without the sign, only the numbers of the charges)
Remark: if the charges are the same then they cancel each other in the formula so there will be one atom of the metal and one atom of the non-metal in the compound.
Exercise:
Write the formula of the compound which is formed between the following elements, and name each compound.
a) Rb and S:
b) Ca and Se:
c) Al and Br:
d) Na and N:
- Draw a Bohr diagram to show the transfer of electrons (loss / gain) in an ionic compound.
Example: Na2O (sodium oxide)
Remark: The ionic bond is also described as an electrostatic force of attraction between a positive ion and a negative ion (eg: Na+ Cl- ).
2. Covalent bond: a bond between a non-metal and another non-metal in which there is a sharing of electrons between the non-metallic atoms. The compound that is formed is called a covalent or molecular compound.
Molecule: 2 or more atoms (non-metallic) bonded together; the atoms can be of the same element such as O2 or of different elements such as HCl, CH4, H2O,…..
We show the sharing of electrons between non-metallic atoms by using the Lewis diagram. In addition we can make intersecting circles for the atoms to show the sharing.
Reminder: Lewis diagram of an atom shows only the valence electrons of that atom.
Most of the atoms follow the octet rule (there are very few exceptions), that is each atom will have eight electrons in the valence shell (same as noble gases) except hydrogen will have 2 electrons after sharing (same as helium).
Remark: 2 electrons that are not bonded to any other atom is called a lone pair (non-bonding pair) of electrons.
Exercise:
Draw lewis diagrams to show the sharing of electrons in each of the following compounds:
1. NF3 2. CH4 3. CO2
4. CCl4 5. CH2O
Remark:In drawing Lewis structure, we show the bonds between the atoms and we also show all lone pairs (if present) on any atom.
- Naming molecular compounds:
1: mono 2: di 3: tri 4: tetra 5: penta 6: hexa 7: hepta 8: octa 9: nona 10:deca
Example: PCl5 : phosphorus pentachloride
Remark: If the first element contains only one atom we don’t write mono before it; If the second element contains only one atom we have to write mono before it.
Ex.: NO : nitrogen monoxide CO: carbon monoxide
Exercise: Fill in the table below
Name
Chemical Formula
diphosphorus pentoxide
SO3
CO
Aluminum sulfide
Al2S3
SF6
Calcium oxide
Lithium nitride
Li3N
Remark: If the compound contains a transition metal, then we have to mention the type of charge of the transition metal by inserting a roman numeral in brackets (I, II, III, IV ….) after the symbol of the transition metal.
Example:
Name the following compounds:
- FeCl2 : Iron (II) chloride
- Cu(NO3)2 : Copper (II) nitrate
Remark: There are few transition metals that have only one type of charge such as zinc, nickel, and silver; in this case no roman numeral is required.
Zinc : Zn+2 Silver: Ag+ Nickel: Ni+2
- ZnSO4 : zinc sulfate
- AgNO3 : silver nitrate
- CuSO4 : copper (II) sulfate
- Co(NO3)3 : cobalt (III) nitrate
Exercise: Write the formula of the following compounds.
Calcium phosphate: Iron(III) hydroxide:
Sodium hydroxide: Manganese(II) hydroxide:
Barium sulfate: Zinc carbonate:
Ammonium nitrate:
Remark: We must enclose the polyatomic ion in brackets if the number after it is more than 1.
Note: If the polyatomic ion that ends with the prefix –ate decreases by one oxygen atom then the prefix changes to -ite.
If the prefix ending with – ite decreases by one oxygen atom then we precede the prefix by hypo, whereas if the prefix ending with – ate increases by one oxygen atom then we precede the prefix by per.
Example:
ClO3- is called chlorate; if we reduce one oxygen atom then the ion becomes ClO2- and is called chlorite, however if we increase by one oxygen atom then the ion becomes ClO4- and is called perchlorate;
and if the chlorite is reduced by one oxygen atom then the ion becomes ClO- and is called hypochlorite.
Exercise: Name the following compounds:
K2SO3 :
NaNO2 :
Mg(ClO4)2 :
LiBrO2 :
- Comparison Table between ionic and covalent (molecular) compounds:
property
Ionic Compounds
Covalent Compounds
State (at room temperature)
solids
Solids, liquids or gases
Melting point and boiling point
Very high
Usually low
Involvement of electrons
Loss and gain (transfer) of electrons
Sharing of electrons
Electric conductivity
When dissolved in water (in solution) , electric conductivity is high
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