Chem Regents Formulas & Equations

CHEMICAL FORMULAS EXPLAINED

SYMBOLS: each element has been assigned a one, two, or three letter symbol for its identification. The first letter is ALWAYS CAPITALIZED, any additional letters are always lower case.

DIATOMIC MOLECULES: When you write the symbols of un-combined elements, almost all are written without a subscript – they are monatomic. Some elements exist in nature as two identical atoms covalently bonded into DIATOMIC MOLECULES. They are:  Br2I2N2Cl2H2O2F2    Be sure that whenever you write the formulas for these you write them as diatoms.

CHEMICAL FORMULAS: compounds are composed of combinations of elements chemically combined in definite proportions by weight (mass).  a) Formulas: use chemical symbols and numbers to show both   qualitativeand quantitative information about a substance. 1) Qualitative:  information that cannot be counted or measured (what elements are present)   2) Quantitative: information that can be either counted or measured (how much is present)   b) Types of formulas: Two basic types of formulas provide different types of information about a compound.  1) Empirical Formula: the simplest integer ration in which atoms combine to form a compound CH2  *Formulas for ionic substances are empirical formulas. 2) Molecular Formula: the actual ratio of atoms in a molecule C4H8

ATOMS, COMPOUNDS AND IONS a) Atoms and compounds are electrically neutral which means that  there are equal numbers of positive and negative charges    b) Ions are not neutral and may be either positively or negatively charged. An ion that contains more protons than electrons has an overall  positive charge and is called a(n)  cation. An ion that contains less protons than electrons has an overall  negative charge and is called a(n)  anion.    c) Polyatomic Ions (PAI): a group of atoms covalently bonded together possessing a charge. Parentheses are used to enclose PAIwhen there is more than one of the ions in a unit of a compound. The subscript written after the parentheses tells how many of the ions are present in the compound. **The subscript refers to each of the elements in the ion. Example:   (NH4)+               (PO3)-   d) 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. These water molecules are loosely held to the compound.   Example:    Barium Chloride dihydrate   BaCl2 • 2H2O *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. 

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Energy: There are 2 types of reactions dealing with energy 1.                Endothermic: (inside) (heat) ·        Chemical reactions that absorb energy or heat ·        It feels cold to the touch ·        You’ll see energy added on the left hand side of the equations i.e. ice packs   2.                Exothermic (outside) (heat) ·        Chemical reactions that release energy ·        They feel hot to the touch ·        You’ll see energy on the right hand side of the equations i.e. heat packs

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