Chapter 2: Atomic Structure-Ionic and Covalent Compounds: Writing formulas of Ionic Compounds

Ionic compounds which are generally composed of a metallic element and a nonmetallic element, contain ions, which carry either a positive or negative charge. Covalent compounds contain neutral molecules, which are not electrically charged.

An ion is a positively or negatively charged atom or molecule. Ions are formed when an atom gains or loses an electron. The alkali metals like to lose an electron. Sodium is an example. As the neutral atom sodium has 11 protons and 11 electrons. The lose of one electron give the ion 10 electrons and Na becomes Na+ . All of the alkali metals lose an electron when they combine with other elements.

Alkaline earth metals lose two electrons forming doubly charged cations, i.e. Mg2+. It is interesting to note that for the monatomic atoms the number of the electrons lost or gain can be correlated with the number of electrons in the nearest noble gas. When sodium loses one electron it has as many electrons as the noble gas neon. In general metals lose electrons, forming cations, which have as many electrons as the preceeding noble gas element.

Chlorine, Cl, will gain one electron, forming Cl-, to have as many as argon. Nonmetals generally gain electrons, forming anions, to have as many electrons as a noble gas element. Substances with formulas containing metallic elements and nonmetallic elements are called ionic compounds.

Writing formulas consists of balancing the charges on the cation and anion to produce a formula with a balance of positive and negative charge. Sodium and chlorine form a compound sodium chloride with a formula of NaCl. Sodium likes to lose one electron and chlorine likes to gain one electron. A formula of NaCl balances Na + and Cl -. A compound containing calcium and bromine would have a formula of CaBr2.

Polyatomic ions are characterized as ions, charged species, with more than one type of atom. Some examples include NO3-, SO42-, OH-,and NH4+. What would be the formula of a compound containing aluminum and nitrate? Aluminum, as a metal, loses electrons to have as many electrons as the nearest noble gas. In this case aluminum forms a stable 3+ cation. Nitrate, NO3-, combines with Al3+. The formula of the compound is Al(NO3)3. The parenthese are needed to clearly indicate the presence of three nitrate ions.

Remember the charge on an ion is a result of the lose or gain of electrons, metals lose electrons to form cations, nonmetals gain electrons to form anions. Metals generally lose a number of electrons equal to their group number. Group IA elements lose one electron, Group IIIA elements lose three electrons. Nonmetals gain electrons equal to the group number minus 8. Chlorine, in Group VIIA, gains 8 - 7 = 1 electron. Oxygen gains two electrons. The formula for ionic compounds is determined by balancing the positive charge of the cations and the negative charge of the anion to form an electrically neutral compound.

What is the formula for a compound containing calcium and phosphate? Ca loses two electrons to form Ca2+. Phosphate has the formula PO43-. Manipulating the charges to achieve electroneutrality means three calcium ions balance two phosphate ions and the formula is Ca3(PO4)2.

Here is a site that discusses writing formulas.

Have you wondered about the chemical formula of lava?

Here is an alphabetic list of chemical names with the corresponding formula.

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