How do atoms of magnesium react with molecules in air to form compounds? Magnesium ribbon burns in air with an extremely bright white light, giving off a large amount of energy, and white smoke with is mostly magnesium oxide in very fine particles. The magnesium ribbon easily crumbles into a white powder which is a mixture of magnesium oxide (about 90%) and magnesium nitride (about 10%). since air is a mixture of oxygen and nitrogen gases, both elements react with the magnesium metal. Graphic in a new window

Magnesium Oxide: Determine the formula of a compound formed by the reaction of magnesium and oxygen.

Solution: First examine the electron arrangement of the magnesium and oxygen atoms.  Symbol  Atomic No.  Bohr diagram  Group No.  Lewis Dots  Mg  12  2 - 8 - 2  2  2  O  8  2 - 6  6  6 Write the Lewis symbols for each atom. See graphic on the left. Determine the numbers of electrons which the atoms will lose and gain by applying the Octet Rule. Mg loses two electrons to have an octet. Oxygen gains two electrons to have an octet.  Mg Atom   Mg Ion  O Atom   O Ion  12 p+    12 p+  8 p+    8 p+  12 e-  lose 2 e-  10 e -  8 e-  gain 2 e-  10 e-  0 charge    +2 charge  0 charge    -2 charge The ionic bond between ions results from the electrostatic attraction of opposite charges. The final formula of magnesium oxide is MgO.

Magnesium Nitride: Determine the formula of a compound formed by the reaction of magnesium and nitrogen.

Solution: First examine the electron arrangement of the magnesium and nitrogen atoms.  Symbol  Atomic No.  Bohr diagram  Group No.  Lewis Dots  Mg  12  2 - 8 - 2  2  2  N  7  2 - 5 5  5 Write the Lewis symbols for each atom. See graphic on the left. Determine the numbers of electrons which the atoms will lose and gain by applying the Octet Rule. Mg loses two electrons to have an octet. Nitrogen gains three electrons to have an octet.  Mg Atom   Mg Ion  N Atom   N Ion  12 p+    12 p+  7 p+    7 p+  12 e-  lose 2 e-  10 e -  7 e-  gain 3 e-  10 e-  0 charge    +2 charge  0 charge    -3 charge The ionic bond between ions results from the electrostatic attraction of opposite charges. The final formula of magnesium nitride is Mg3N2.

Why does magnesium nitride have subscripts in the formulas? The Octet Rule is the basis for the predictions about the charges on ions. Ionic compounds are formed as the result of the formation of positive and negative ions. Electrons are actually transferred from one atom to another to form rare gas electron structures for each ion. The atom which forms a positive ion loses electrons to the atom which gains electrons to form a negative ion. A compound is not stable unless the number of electrons which are lost equals those gained.

SUMMARY PRINCIPLE OF IONIC COMPOUNDS An ionic compound is formed by the complete transfer of electrons from a metal to a nonmetal and the resulting ions have achieved an octet. The protons do not change. Metal atoms in Groups 1-3 lose electrons to non-metal atoms with 5-7 electrons missing in the outer level. Non-metals gain 1-4 electrons to complete an octet.

OCTET RULE: Elemental atoms generally lose, gain, or share electrons with other atoms in order to achieve the same electron structure as the nearest rare gas with eight electrons in the outer level. The proper application of the Octet Rule provides valuable assistance in predicting and explaining various aspects of chemical formulas.