AP Chemistry Chapter 3: Atomic Structure, Bonding, and Chemical Reactions

Atomic Structure and Element Identification

Photoelectron Spectroscopy (PES) and Element Identification

Photoelectron Spectroscopy (PES) is a technique used to determine the electronic structure of atoms. By analyzing the energy required to remove electrons from a sample, PES can help identify elements based on their unique electron configurations.

• PES Spectrum: Shows peaks corresponding to electrons in different energy levels.

• Element Identification: The number and position of peaks can be matched to known electron configurations to determine the element.

• Example: A PES spectrum with two peaks at low binding energy and six at higher energy suggests an element with 2 electrons in the 1s orbital and 6 in the 2s/2p orbitals, such as oxygen.

Law of Definite Proportions and Percent Composition

Calculating Percent Composition

The Law of Definite Proportions states that a chemical compound always contains the same proportion of elements by mass. Percent composition is the percentage by mass of each element in a compound.

• Formula: $\text{Percent Composition} = \frac{\text{Mass of Element in 1 mol of Compound}}{\text{Molar Mass of Compound}} \times 100\%$

• Example: In water ($H_2O$), the percent composition of hydrogen is: $\frac{2.02\,g}{18.02\,g} \times 100\% = 11.2\%$

Chemical Bonding

Predicting Bond Types

The type of bond formed between atoms depends on the classification of the elements involved:

• Ionic Bonds: Formed between metals and nonmetals. Electrons are transferred from the metal to the nonmetal.

• Covalent Bonds: Formed between nonmetals. Electrons are shared between atoms.

• Example: Sodium chloride ($NaCl$) is ionic; water ($H_2O$) is covalent.

Naming and Writing Formulas for Ionic Compounds

• Criss-Cross Method: Use the charges of ions to determine the subscripts in the formula.

• Naming Rules: Name the cation first, then the anion (with -ide ending).

• Example: $Ca^{2+}$ and $Cl^-$ combine to form $CaCl_2$ (calcium chloride).

Naming and Writing Formulas for Covalent Compounds

• Prefixes: Use prefixes (mono-, di-, tri-, etc.) to indicate the number of each atom.

• Example: $CO_2$ is carbon dioxide; $N_2O_5$ is dinitrogen pentoxide.

Empirical and Molecular Formulas

Determining Empirical and Molecular Formulas from Percent Composition

The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula shows the actual number of atoms of each element in a molecule.

• Steps:

  1. Convert percent composition to grams (assume 100 g sample).

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get the ratio.

  4. Multiply to get whole numbers if necessary.

• Example: A compound with 40% C, 6.7% H, and 53.3% O has an empirical formula of $CH_2O$.

Combustion Analysis

Combustion analysis is used to determine the empirical and molecular formulas of organic compounds by measuring the amounts of $CO_2$ and $H_2O$ produced.

• Process: Burn a known mass of compound, measure $CO_2$ and $H_2O$ produced, calculate moles of C and H, and determine O by difference.

• Example: If combustion of 1.00 g of a compound produces 1.47 g $CO_2$ and 0.60 g $H_2O$, calculate moles of C and H, then find the empirical formula.

Redox Reactions and Oxidation Numbers

Using Oxidation Numbers to Identify Redox Reactions

Oxidation numbers are assigned to atoms to track electron transfer in reactions. A redox reaction involves a change in oxidation numbers.

• Rules: Elements in their standard state have oxidation number 0; oxygen is usually -2; hydrogen is +1; sum of oxidation numbers equals the charge of the species.

• Identifying Redox: If any atom changes oxidation number, a redox reaction has occurred.

• Example: In $2Na + Cl_2 \rightarrow 2NaCl$, Na goes from 0 to +1, Cl from 0 to -1.

Balancing Chemical Equations

Balancing by Adjusting Coefficients

Balancing chemical equations ensures the law of conservation of mass is obeyed. Only coefficients (not subscripts) are changed to balance atoms on both sides.

• Steps:

  1. Write the unbalanced equation.

  2. Balance atoms one element at a time, usually starting with the most complex molecule.

  3. Check all elements and adjust as needed.

• Example: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$

Translating and Classifying Chemical Equations

Formula Rules for Ionic and Covalent Compounds

• Ionic Compounds: Use charges to determine formula; balance charges to zero.

• Covalent Compounds: Use prefixes to indicate number of atoms.

Classification of Chemical Reactions

Chemical reactions can be classified based on the nature of reactants and products:

Type	General Form	Description	Example
Synthesis	$A + B \rightarrow AB$	Two or more substances combine to form one product.	$2H_2 + O_2 \rightarrow 2H_2O$
Decomposition	$AB \rightarrow A + B$	One compound breaks down into two or more substances.	$2H_2O \rightarrow 2H_2 + O_2$
Single Replacement	$A + BC \rightarrow AC + B$	An element replaces another in a compound.	$Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Double Replacement	$AB + CD \rightarrow AD + CB$	Two compounds exchange ions.	$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$
Combustion	$C_xH_y + O_2 \rightarrow CO_2 + H_2O$	Hydrocarbon reacts with oxygen to produce carbon dioxide and water.	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Additional info: Some content and examples have been expanded for clarity and completeness based on standard AP/General Chemistry curriculum.