1. How Elements Form Compounds
Ionic Bonding
How Elements Form Compounds

2. I CAN describe chemical bonding and determine an ionic chemical formula.

3. Chemical Bonding
Chemical Bonding is the joining of two or more atoms to form a compound.
Bonding occurs when atoms transfer electrons between them, or, in some cases, move close together and share electrons between them.

4. Valence Electrons
During bonding, the only electrons involved are those at the outer edge of the atom’s electron cloud.
These are called VALENCE ELECTRONS.
Atoms can have between 1 and 8 Valence Electrons.

5. Atoms become IONS
When an atom gains or loses electrons by transfer there is a change in its overall charge, the atom is transformed into another form called an
ION

6. POSITIVE IONS are called CATIONS and are formed from METALS on the LEFT SIDE of the periodic table.
NEGATIVE IONS are called ANIONS and are formed from NON-METALS on the RIGHT SIDE of the periodic table.
We will only look at the SIX MAIN GROUPS for right now.

8. IONIC BONDING
IONIC BONDING occurs between two IONS with OPPOSITE CHARGES.
Remember OPPOSITES ATTRACT!

9. Characteristics of IONIC Compounds
Generally consist of a METAL CATION and a NON-METAL ANION.
High melting points
Almost always solid at room temperature
Brittle
Generally not very flammable
Dissolve in water [electrolytes]

10. Forming an IONIC Compound
The simplest IONIC COMPOUNDS are called BINARY IONIC compounds.
They consist of only TWO DIFFERENT kinds of atoms…..one a POSITIVE ION from the metal side of the PT and the other a NEGATIVE ION from the non-metal side of the PT.
Together their CHARGES add up to equal ZERO.

11. TO DETERMINE AN IONIC FORMULA FOLLOW THESE STEPS
Find the elements in the Periodic Table and write down their symbols.
Use the element’s location on the Periodic Table to find the charge of its ION…go to the TOP of the column and get the charge.
If they have EQUAL BUT OPPOSITE CHARGES (for
example a +2 and a -2), then the formula is simply
ONE ATOM OF EACH since they add up to equal
ZERO.
4. If the charges are not equal but opposite (such as
+3 and -2) CRISS-CROSS the charges (NO SIGNS-
numbers only).
Write the FORMULA. A formula contains CHEMICAL SYMBOLS that
tell what elements are in the compound and SMALL NUMBERS called
SUBSCRIPTS which tell how many of each kind of atom are in the
compound.

12. EXAMPLE PROBLEM ( +1 ) ( -1 ) = 0 K Br
What would be the formula of a compound of Potassium and Bromine?
1. Find both elements on the PT, get their SYMBOL and
CHARGE:
( +1 ) ( -1 ) = 0
K Br

13. Formula is KBr (+1 ) + ( -1 ) = 0
2. If they are EQUAL BUT OPPOSITE then
they cancel each other out, so the
formula is ONE ATOM of each element.
Positive element always goes first!
(+1 ) ( -1 ) = 0
K Br
Formula is KBr

14. PROBLEM
What is the formula for an ionic compound of Magnesium and Chlorine?

15. Mg Cl Mg1Cl2 = MgCl2 (+2 ) + (-1 ) = 0
Determine the symbol and charge of both elements see if they are equal. If not, CRISS-CROSS the numbers!
(+2 ) (-1 ) = 0
Mg Cl
Mg1Cl2 = MgCl2
Remember ONE is not usually written as a subscript!

16. Problem
What is the formula of an ionic compound of Aluminum and Oxygen?
(+3) + ( -2 )
Al O

17. (+3) + (-2 ) = 0
Al O
So the formula is: Al2O3

18. Practice Problems
Find the formula for a compound made with each pair of atoms:
1. Sodium and Sulfur
2. Calcium and Chlorine
3. Potassium and Phosphorus
4. Aluminum and Nitrogen
5. Magnesium and Fluorine