We start by writing symbols that contain the correct number of valence electrons for the atoms in the molecule. We then combine electrons to form covalent bonds until we come up with a Lewis structure in which all of the elements with the exception of the hydrogen atoms have an octet of valence electrons. Let's apply the trial and error approach to generating the Lewis structure of carbon dioxide, CO2. We start by determining the number of valence electrons on each atom from the electron configurations of the elements.
Explain the roles of subscripts and coefficients in chemical equations. Balance a chemical equation when given the unbalance equation.
Explain the role of the Law of Conservation of Mass in a chemical reaction. Even though chemical compounds are broken up and new compounds are formed during a chemical reaction, atoms in the reactants do not disappear nor do new atoms appear to form the products. In chemical reactions, atoms are never created or destroyed.
The same atoms that were present in the reactants are present in the products - they are merely reorganized into different arrangements. In a complete chemical equation, the two sides of the equation must be present on the reactant and the product sides of the equation.
Coefficients and Subscripts There are two types of numbers that appear in chemical equations. There are subscripts, which are part of the chemical formulas of the reactants and products and there are coefficients that are placed in front of the formulas to indicate how many molecules of that substance is used or produced.
You cannot change subscripts in a chemical formula to balance a chemical equation; you can change only the coefficients. Changing subscripts changes the ratios of atoms in the molecule and the resulting chemical properties.
For example, water H2O and hydrogen peroxide H2O2 are chemically distinct substances. The subscripts are part of the formulas and once the formulas for the reactants and products are determined, the subscripts may not be changed.
The coefficients indicate the number of each substance involved in the reaction and may be changed in order to balance the equation.
The equation above indicates that one mole of solid copper is reacting with two moles of aqueous silver nitrate to produce one mole of aqueous copper II nitrate and two atoms of solid silver.
Steps in Balancing a Chemical Equation Identify the most complex substance. Beginning with that substance, choose an element s that appears in only one reactant and one product, if possible. Adjust the coefficients to obtain the same number of atoms of this element s on both sides. Balance polyatomic ions if present on both sides of the chemical equation as a unit.
Balance the remaining atoms, usually ending with the least complex substance and using fractional coefficients if necessary.
If a fractional coefficient has been used, multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients. Count the numbers of atoms of each kind on both sides of the equation to be sure that the chemical equation is balanced.Use the step-by-step procedure to write two plausible Lewis electron structures for SCN −.
Calculate the formal charge on each atom using Equation Predict which structure is preferred based on the formal charge on each atom and its electronegativity relative to the other atoms present. After writing the structure, the entire structure should then be placed in brackets with the charge on the outside of the brackets at the upper right corner.
Example: Write the Lewis structure for the ammonium ion (NH 4 +).
Answer: Hydrogen atoms are always placed on the outside of the molecule, so nitrogen should be the central atom. Balanced chemical equation: A chemical equation in which the number of each type of atom is equal on the two sides of the equation.
Subscripts: Part of the chemical formulas of the reactants and products that indicate the number of atoms of the preceding element.
Write both an equations using Lewis Structures and a balanced chemical equations for the following reactions: Silicon atoms and Chlorine molecules Lithium atoms and Bromine molecules Ethylene molecules (C2H4) and Chlorine molecules; the products is C2H4Cl2.
Writing Lewis Formulas: The Octet Rule zThe octet rule states that representative elements usually attain stable noble gas electron configurations in most of their compounds.
zLewis dot formulas are based on the octet rule. zWe need to distinguish between bonding (or shared) electrons and nonbonding (or unshared or lone pairs) of electrons.
The fact that SO 2 is a resonance hybrid of two Lewis structures is indicated by writing a double-headed arrow between these Lewis structures, as shown in the figure above. Practice Problem 4: Write the Lewis structures for the acetate ion, CH 3 CO 2 -.