Determining Chemical Formulas

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Transcript Determining Chemical Formulas

Chemical Practice

Using ion models to determine chemical formulas

How do the models work?

• Designed to show electron movement.

• POSITIVE ions are trying to lose electrons so the model has an “arrow” showing electrons leaving.

+1 Arrow shows one electron leaving

• NEGATIVE ions are trying to gain electrons so the model can “accept” electrons.

Spaces show how many electrons the ion is trying to gain -2

EXAMPLES:

Potassium (K) and Chlorine (Cl)

**Write the ions first.

K +1 Cl -1

**Find the puzzle pieces representing each ion.

+1 -1

**One molecule of the compound is whatever it takes to complete the puzzle.

One of each element makes up the compound.

**Write the compound formula:

KCl

Magnesium (Mg) and Chlorine (Cl) • Find the ions: • Find ion models:

Mg +2 Cl -1

-1 +2 • Put puzzle pieces together to find ONE molecule of compound: One POSITIVE ion • Write the formula: +2

MgCl 2

-1 -1 Two NEGATIVE ions

Another way to determine the formula … • Write the ions in order: POSITIVE ion before NEGATIVE ion.

Mg +2 Cl -1

• Cross over just the numbers of the charge.

Mg + 2 Cl 1 Mg 1 Cl 2

• And, since the “1” is understood: MgCl 2

Hydrogen (H) and Phosphate (PO 4 ) • PO 4  polyatomic ion {These can be found on the back of your periodic table.} • Find each ion: • Put ion pieces together: • NOTE: You can only use the ions that make up the compound.

• Write the formula:

Polyatomic ion

= ion made of more than one atom

H +1 PO 4 -3 H

+1 +1 +1

3 PO 4

-3

Again, you could determine the formula using the cross over formula … • Write the ions in positive  negative order.

H +1 PO 4 -3

• Cross over just the numbers of the charges.

H +1 PO 4 -3 H 3 PO 4 1 • Again, the “1” is understood and should not be included in the formula:

H 3 PO 4

Ammonium (NH 4 ) and Phosphate (PO 4 ) • Write the ions: • Combine the puzzle pieces:

NH 4 +1

PO 4 -3

+1 -3 +1 • How do you write more than one polyatomic ion?

• Write compound formula: •Use parenthesis to show more than one polyatomic ion in a compound.

(NH 4 ) 3 PO 4

You could also use the cross-over method for finding the formula: • Write the ions in positive  negative order.

charges.

NH 4 +1 PO 4 -3

• Cross over just the numbers of the NH 4 +1 PO 4 -3 (NH 4 ) 3 PO 4 1 • Again, the “1” is understood and should not be included in the formula:

(NH 4 ) 3 PO 4

Aluminum (Al) and Sulfate (SO

)

• Write the ions: • Combine the puzzle pieces:

Al +3 SO 4 -2

-2 +3 • Remember that you can only use the ions that are part of the compound.

+3 -2 -2 • Write the formula:

Al 2 (SO 4 ) 3

Again, you could use the cross over method for finding the formula: • Write the ions in positive  order: negative

Al +3 SO 4 -2

• Cross over just the numbers of the charges.

Al + 3 SO 4 2 Al 2 (SO 4 ) 3

Calcium (Ca) and Sulfate (SO

)

• Write the ions: • Combine the puzzle pieces:

Ca +2 SO 4 -2

+2 -2 • Notice that the molecule is complete with one of each ion.

• Write the formula:

CaSO 4

What if you use the cross-over method?

• Write the ions in positive  order: negative