Topic Overview

Chemists in every country of the world speak many different languages. Despite their language differences, it is necessary for them to communicate with each other in a clear and concise manner. Chemists have agreed to a universal language of symbols to identify the elements of the earth, and a system of formulas and equations to explain how the elements interact. In this topic you will explore how the symbols are used to identify the different elements and how they can be combined into formulas.

Chemical Symbols and Formulas

While the names of the elements are often different in various languages of the world, it is important that a person in any country can quickly and accurately determine which element is being referred to.

Chemical Symbols A system for universal shorthand to identify the elements has been agreed upon. Each element has been assigned a unique one-, two-, or three-letter symbol for its identification. The first letter of a symbol is capitalized. If there are any other letters, they are lower case.

Some of the more common elements have single-letter symbols, such as O for oxygen, N for nitrogen and H for hydrogen. Other elements have symbols with two letters. Only recently discovered elements that don’t yet have permanent names are given three-letter symbols. These elements are given systematic names that represent their atomic number until a name can be agreed upon by the International Union of Pure and Applied Chemistry (IUPAC).

Nomenclature of the First 20 Elements of the Periodic Table

Subscript When writing the symbols of uncombined elements, almost all are written as monatomic, that is, without a subscript. A subscript is a number to the right and slightly below an element symbol that tells the number of atoms present. A subscript is not written if only one atom is present. Therefore, the symbol for iron is Fe, neon is Ne, and carbon is C.

Chemical Formulas Compounds are composed of combinations of elements chemically combined in definite proportions by weight. Formulas use chemical symbols and numbers to show both qualitative and quantitative information about a substance. Qualitative information relates to things that cannot be counted or measured, such as the number of atoms of each element present in a unit of the compound.

Types of Formulas Two basic types of formulas provide different types of info about a compound. Empirical formulas include all types of compounds. Molecular formulas are important when considering compounds formed from atoms sharing electrons.

Empirical Formula An empirical formula represent the simplest integer ratio in which atoms combine to form a compound. Ionic substances do not form discrete units or molecules, but rather an array of ions. Formulas of ionic substances are empirical.

Molecular Formula Covalently bonded substances form discrete units called molecules. In some cases, such as H₂O, the empirical formula not only represents the simplest ratio, but it also represents the actual ratio of the atoms in a molecule of water. In other cases, the molecular formula may be a multiple of the empirical formula. For example, the molecular formula of glucose is C₆H₁₂O₆, which is six times the empirical formula which is CH₂O.

Atoms, Ions, and Compounds

It's easy to interpret a formula for an element or a compound, but it's a bit more complicated to write the formula for a compound. How do you know what elements form the compound and in what proportion? To understand how elements form compounds, an understanding of atoms and ions is essential.

Atoms and compounds are electrically neutral; that is, they do not have a net charge. Both atoms and compounds contain positively charged protons and negatively charged electrons, but there are equal numbers of positive and negative charges, producing a neutral atom or compound. Ions, however, are not neutral and may be either positively or negatively charged. An ion that contains more protons than electrons will be positively charged, while an ion with more electrons than protons will have a negative charge. Positively charged ions attract negatively charged ions in a ratio that produces a neutral compound.

Ionic Charges The charge of an ion is indicated by a superscript following the symbol of the ion. When the ion has a charge of either 1+ or 1-, the number 1 is omitted, and only the sign of the charge is shown. Thus the sodium ion with a charge of 1+ is written as Na+, and chlorine with a charge of1- is Cl. The symbols of all other ions show both the size and sign of the charge. An aluminum ion is written as A13+, and an oxygen ion is shown as O²-. Polyatomic lons A polyatomic ion is a group of atoms covalently bonded together, possessing a charge. Selected Polyatomic Ions in the Reference Tables for Physical Setting/Chemistry is a list of common polyatomic ions and their charges.

Coefficients

You know what information a subscript provides in a chemical formula. However, sometimes there is a number called a coefficient written in front of a formula. The coefficient tells how many units of the formula a represent, and it applies to the entire formula. To determine the number of atoms present, consider the formula without the coefficient, and then multiply each value by the coefficient to find the total of each type of atom. For example, 2H₂O means that there are two molecules of water. These two molecules contain four hydrogen atoms and two oxygen atoms.

Hydrates

When water from some ionic solutions evaporates, the solute forms a crystal lattice that binds water within the structure. Such a compound is called a hydrate. These crystals have a definite number of water molecules for each unit of the compound. Barium chloride (BaCl2) traps two water molecules as shown by the formula of the hydrate, BaCl2-2H2O. Copper sulfate (CuSO4) has five water molecules and a formula of CuSO4.5H2O. Alum (NaAl(SO4)2) has 12water molecules attached, NaAl(SO4)2 12H2O. The anhydrous (not hydrated)compound can be obtained by heating the crystals to drive off the water in a chemical reaction, the water in a hydrate does not react. However, it adds mass to the compound. For example, 10.0 g of a truly dry crystal of copper(II) sulfate contains more CuSO, than 10.0 g of the hydrated crystal, which contains both CuSO4 and H₂O. If a certain amount of a material is made from a hydrated crystal, the mass of water must be considered in determining how much of the compound must be used.

Nomenclature

All compounds must be electrically neutral, that is, the sum of the charge must equal zero. Common oxidation states for each element are listed in the upper right hand corner of each element's box in the periodic table. For many elements, the oxidation state is equal to the charge on the ion. Elements from Group 1 have an oxidation number of +1 and always have a charge of 1+ in compounds. All Group 2 elements have 2+ charges in compounds. Group 3 elements usually have a 3+ charge.

Equalizing Charges Compounds achieve neutrality by having an equal number of positive an negative charges. When a sodium ion (Na+) and a chloride ion (CI) combine, they will do so in a 1:1 ratio. The resulting formula will be NaCl as such a ratio produces a neutral compound.

In the case of a combination of Mg2+ with CI, a 1:1 ratio would not produce a neutral compound. To achieve neutrality there must be two Cl ions for each Mg2+. The correct formula will be MgCl2. When ions ha unequal and opposite charges, a simple technique will produce the correct formula. Simply write the charge of one ion as the subscript of the other. Transfer the number only, not the sign. Notice that this procedure automatically balances the positive and negative charges, producing neutral formulas.

Vocabulary

Analysis - The process of breaking down a substance or a mixture to determine its composition or properties.

Chemical Change - A process where one or more substances undergo a chemical reaction, resulting in the formation of new substances with different chemical compositions.

Coefficient - A number in a chemical equation representing the stoichiometric relationship between reactants and products.

Decomposition - A type of chemical reaction where a compound breaks down into simpler substances or elements.

Diatomic Molecule - A molecule composed of two atoms of the same or different elements chemically bonded together.

Double Replacement - A chemical reaction where ions in two compounds exchange places to form two new compounds.

Empirical Formula - The simplest whole-number ratio of atoms in a compound.

Endothermic - A reaction or process that absorbs heat from its surroundings.

Exothermic - A reaction or process that releases heat into its surroundings.

Formula - A symbolic representation of a compound showing the types and numbers of atoms present.

Molecular Formula - A chemical formula that indicates the actual number of each type of atom in a molecule of a compound.

Molecule - A group of two or more atoms held together by chemical bonds.

Physical Change - A change in which the substance's physical state or appearance changes without altering its chemical composition.

Polyatomic Ion - An ion composed of two or more atoms covalently bonded together and carrying a net electric charge.

Product - A substance formed in a chemical reaction.

Qualitative - Descriptive information about properties or characteristics without numerical measurements.

Quantitative - Information about quantities or amounts, usually involving numerical measurements.

Reactant - A substance consumed in a chemical reaction to produce products.

Single Replacement - A chemical reaction where one element replaces another in a compound.

Subscript - A small number written at the lower right of a chemical symbol indicating the number of atoms of that element in a molecule.

Symbol - An abbreviation or representation of an element used in chemical formulas.

Synthesis - A chemical reaction where two or more substances combine to form a more complex product.