Matter, Chemical Trends, and Chemical Bonding

Significant Figures and Scientific Notation

Understanding how to properly record and interpret measurements is fundamental in chemistry. Significant figures reflect the precision of a measurement, and scientific notation is used to express very large or small numbers efficiently.

• Significant Figures: Digits in a measurement that are known with certainty plus one digit that is estimated. Rules include counting leading, trailing, and sandwiched zeroes.

• Scientific Notation: A method to express numbers as a product of a coefficient and a power of ten, e.g., .

• Rounding: When performing calculations, round the result to the correct number of significant figures based on the operation (multiplication/division: least number of sig figs; addition/subtraction: least number of decimal places).

Accuracy and Precision

These concepts are used to describe the quality of measurements in chemistry.

• Accuracy: How close a measurement is to the true value.

• Precision: How close repeated measurements are to each other.

• Percent Error:

• Percent Precision: Quantifies the reproducibility of measurements.

Structure of the Atom

Atoms are composed of protons, neutrons, and electrons, each with distinct properties and locations.

• Protons: Positive charge, located in the nucleus, mass ≈ 1 amu.

• Neutrons: Neutral charge, located in the nucleus, mass ≈ 1 amu. They contribute to the stability of the nucleus.

• Electrons: Negative charge, located in energy levels (shells) around the nucleus, mass ≈ 1/1836 amu.

Bohr-Rutherford Model and Energy Levels

The Bohr-Rutherford model describes electrons as occupying specific energy levels or shells around the nucleus.

• Valence Electrons: Electrons in the outermost shell; they determine chemical reactivity and bonding.

Isotopes and Atomic Notation

Isotopes are atoms of the same element with different numbers of neutrons. They are represented using atomic notation and have similar chemical properties but may differ physically.

• Atomic Notation: , where A = mass number, Z = atomic number, X = element symbol.

• Example: is an isotope of carbon.

• Radioisotopes: Unstable isotopes that decay over time, emitting radiation.

Calculating Average Atomic Mass

The average atomic mass of an element is calculated from the percent abundance and mass of its isotopes, often using data from a mass spectrum.

• Formula:

• Example: If chlorine has two isotopes, (75%) and (25%), then

The Periodic Table: Classification and Properties

The Periodic Table organizes elements by increasing atomic number and groups them by similar properties.

• Main Group Elements: Groups 1, 2, 13-18.

• Transition Elements: Groups 3-12.

• Inner Transition Elements: Lanthanides and actinides.

• Metals, Non-metals, Metalloids: Metals are typically left and center, non-metals are right, metalloids border the staircase.

• Group Names: Group 1: Alkali metals, Group 2: Alkaline earth metals, Group 17: Halogens, Group 18: Noble gases.

• Diatomic Elements: H2, N2, O2, F2, Cl2, Br2, I2.

Groups, Periods, and Ion Formation

Groups (columns) and periods (rows) indicate similar properties and electron configurations. Elements form ions to achieve stable electron arrangements.

• Stable Ion Charge: Determined by the group number (e.g., Group 1 forms +1 ions).

Forces in Atoms and Effective Nuclear Charge

Several forces act within atoms, including electrostatic attraction and repulsion. Shielding or screening affects the effective nuclear charge experienced by electrons.

• Effective Nuclear Charge (): The net positive charge experienced by valence electrons, calculated as , where Z is the atomic number and S is the shielding constant.

• Shielding: Inner electrons reduce the attraction between the nucleus and outer electrons.

Atomic Radius

Atomic radius is the distance from the nucleus to the outermost electron shell. It can be measured by various methods.

• Trend Down a Group: Increases due to additional energy levels.

• Trend Across a Period: Decreases due to increased effective nuclear charge.

• Cationic vs. Anionic Radius: Cations are smaller than their neutral atoms; anions are larger.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom in the gaseous state.

• Trend Down a Group: Decreases due to increased distance and shielding.

• Trend Across a Period: Increases due to higher effective nuclear charge.

• Equation:

Electron Affinity

Electron affinity is the energy change when an atom gains an electron.

• Trend Down a Group: Generally decreases.

• Trend Across a Period: Generally increases.

• Equation:

Electronegativity

Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond.

• Trend Down a Group: Decreases.

• Trend Across a Period: Increases.

• Measurement: Pauling scale is commonly used.

Summary Table: Periodic Trends

The following table summarizes the main periodic trends:

Additional info: Academic context was added to clarify definitions, trends, and formulas for self-contained study notes.