Chemical Formulas Molecular and Ionic Substances

When these pure semiconductor elements have small amounts of certain other elements added to them (a process called doping), they become very good conductors of electricity. Semiconductors are the critical materials in solid-state electronic devices. (Chapter 13) 

A nonmetal is an element that does not exhibit the characteristics of a metal. Most of the nonmetals are gases (for example, chlorine and oxygen) or solids (for example, phosphorus and sulfur). The sohd nomnetals are usually hard, brittle substances. Bromine is the only liquid nonmetal. 

Consider the elements He, Ne, and Ar. Can you come up with a reason why they are in the same group in the periodic table  

Atomic theory has developed steadily since Dalton s time and has become the cornerstone of chemistry. It results in an enormous simphfication all of the millions of compounds we know today are composed of the atoms of just a few elements. Now we look more closely at how we describe the composition and structure of chemical substances in terms of atoms. 

The chemical formula of a substance is a notation that uses atomic symbols with numerical subscripts to convey the relative proportions of atoms of the different elements in the substance. Consider the formula of aluminum oxide, AI2O3. This means that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2 3. Consider the formula for sodium chloride, NaCl. When no subscript is written 

---

Additional information may be conveyed by different kinds of chemical formulas. To understand this, we need to look briefly at two main types of substances molecular and ionic. 

A formula unit of a substance is the group represented by the substance s chemical formula, that is, a group containing the kinds and numbers of atoms or ions listed in the chemical formula. Formula unit is a general term that can be used in reference to elements, molecular compounds, or ionic compounds. One formula unit of the noble gas neon, Ne, contains one neon atom. In this case, the formula unit is an atom. One formula unit of water, Ff20, contains two hydrogen atoms and one oxygen atom. In this case, the formula unit is a molecule. One formula unit of ammonium chloride,... 

Formula unit A group represented by a substance s chemical formula—that is, a group containing the kinds and numbers of atoms or ions listed in the chemical formula. It is a general term that can be used in reference to elements, molecular compounds, or ionic compounds. 

Given the name or chemical formula for a substance, (1) categorize the substance as a metallic element, carbon in the diamond form, another nonmetallic element, an ionic compound, a polar molecular compound with hydrogen bonds, a polar molecular compound without hydrogen bonds, or a nonpolar molecular compound (2) identify the type of particle that forms its fundamental structure and (3) identify the type of attraction holding its particles in the solid and liquid form. 

Chemical equations can be written to show whether dissolved substances are present in solution predominantly as ions or molecules. When the complete chemical formulas of all reactants and products are used, the equation is called a molecular equation. A complete ionic equation shows all dissolved strong electrolytes as their component ions. In a net ionic equation, those ions that go through the reaction unchanged (spectator ions) are omitted. 

Salts of the sulfite ion, SOs, react with adds in a way similar to that of carbonates, (a) Predict the chemical formula, and name the weak acid that forms when the sulfite ion reacts with adds, (b) The add formed in part (a) decomposes to form water and an insoluble gas. Predict the molecular formula, and name the gas formed, (c) Use a source book such as the CRC HaidbookcfChennshy and Physics to confirm that the substance in part (b) is a gas under normal room-temperature conditicms. (d) Write balanced net ionic equations of the reaction of HCl(aq) with (i) Na O aq), (ii) Ag2S03(s), (iii) KHS03(s), and (iv) ZrtSO iaq). 

Chemical Formulas Compoimds are represented by chemical formulas, which indicate the elements present in the compound and the relative number of atoms of each. These formulas represent the basic units that make up a compound. Pure substances can be categorized according to the basic units that compose them. Elements can be composed of atoms or molecules. Compounds can be molecular, in which case their basic units are molecules, or ionic, in which case their basic imits are ions. The formulas for many ionic compoimds can be written simply by knowing the elements in the compound. Chemical Formulas To understand compounds, we must understand their composition, which is represented by a chemical formula. The connection between the microscopic world and the macroscopic world hinges on the particles that compose matter. Since most matter is in the form of compounds, the properties of most matter depend on the molecules or ions that compose it. Molecular matter does what its molecules do ionic matter does what its ions do. The world we see and experience is governed by what these particles are doing. 

Just as atomic mass is the mass of an atom, molecular mass is the mass of a molecule. The analogous term formula mass is used for ionic and other nonmolecular substances. Molecular mass is the sum of the atomic masses of all atoms in the molecule. When referring to the large numbers of molecules or ions that take part in a visible chemical reaction, it s convenient to use a unit called the mole, abbreviated mol. One mole of any object, atom, molecule, or ion contains Avogadro s number of formula units, 6.022 x 1023. 

It is appropriate to start with the definition of isomerism. Different chemical substances (at least by some experimentally detmninable properties) are said to be isomers if and only if they have the same molecular formula. This definition restricts the notion of isomerism to compounds containing covalent bonds (for only then can one speak of molecules and molecular formulas), and possibly also other kinds of bcmds, such as ionic bonds. For example, urea and ammonium cyanate are isomers. Wohler s isomerization of the latter into the former substance (1828) is considered to represent the ezperimmtum cruci in disproving the vitalist theory. The above definition also impUes that two isomeric molecules consist of the same set of atoms bonded differently. 

---