Sample Questions:

1.(JUNE 1996 436): Calcium(Ca) atoms and chlorine (Cl) combine to form a compound.

What is the molecular formula of this compound?

Answer Ca+2 ( alkaline earth metal) and Cl-1 ( halogen)produce CaCl2. A calcium atom loses two electrons, one to each chlorine. The resulting bonds are ionic.
2.(JUNE 1995 436): The formula of a compound is Y₂Z₃. The element Y is in group IIIA (or 13) of the periodic table.

Explain why element Z has to be a member of group VIA.

Answer If it took two Y's(each +3) to balance out three z's, then 2(3) +3z =0. Z = -2, and that charge is what is assumed by oxygen's family.

3. (AUGUST 1994: 416) A molecule of a particular substance consists of two chlorine atoms bonded to a single magnesium atom.

What is the chemical name of this substance according to the rules for naming binary compounds?

Answer aluminum chloride

4. (JUNE 2000: 436) What is the correct formula of a compound formed by an element "Y" from group IVA and an element "Z" from group VIA?

Answer YZ2. Y has 4 valence electrons and needs 4 more. It has to share 4, two with each Z, which have 6 valence electrons and each needs two more.

 5. (JANUARY 2000: 430) Which compound does not follow the octet rule?

1. BeCl₂
2. PF₃
3. KP₃
4. Be₃P₂

Answer C, should be K3P

Notes:

Metals react with nonmetals to produce salts, also known as ionic compounds. When naming these compounds, we do not change the name of the metal ion ( an ion is a charged atom). But we do change the ending of the nometal ion to ide.

Examples:

• When sodium reacts with chlorine, it becomes sodium chloride.
• When calcium reacts with fluorine, it becomes the salt calcium fluoride.

To predict the formula of the resulting compound, consult the periodic table for the charge of the ion (alkali metals are +1, halogens, -1, etc) and use the appropriate number of each ion so that the total charge of the compound is 0.

Examples:

• What is the formula for potassium oxide? We need two K⁺¹ and A single O^-2 to get a sum of 0 charge, so the correct formula is K₂O.
• What is the formula for magnesium phosphide? We need three Mg⁺² and two P^-3 to get a sum of 0 charge, so the correct formula is Mg₃P₂.

Polyatomic Ions

Some atoms combine to form groups with an extra number of electrons or with a deficit of electrons. Because these charged atoms contain more than one atom, they are called polyatomic ions or radicals. (radicals is not the best name because they can be confused with free radicals which do not have a charge) Here are some important polyatomics.

Covalent Compounds (436 only)

Non metals don’t assume a charge when reacting with each other. Instead they share electrons in an attempt to fill their outermost shells. Lewis dot structures use a dot for each valence electron. For the simplest structures, we then try to arrange the dots ( without creating any new ones) so that each atom is satisfied. Hydrogens only want two electrons. Carbon, nitrogen, oxygen, and the halogens want eight electrons( this is known as the octet rule).

Examples:

H : H = H2

::O:: C: :O:: = CO2.