Atoms and Elements

Subatomic Particles

Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons. Each has distinct properties that determine the structure and behavior of atoms.

• Protons: Positively charged particles found in the nucleus. Relative mass ≈ 1 amu; charge = +1.

• Neutrons: Neutral particles also located in the nucleus. Relative mass ≈ 1 amu; charge = 0.

• Electrons: Negatively charged particles found in orbitals around the nucleus. Relative mass ≈ 0.0005 amu; charge = -1.

Example: A carbon atom (C) has 6 protons, 6 neutrons (in its most common isotope), and 6 electrons.

Isotopes

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.

• Example: Carbon-12 (12C), Carbon-13 (13C), and Carbon-14 (14C) are isotopes of carbon.

Atomic Symbols

Atomic symbols represent the composition of atoms and ions, showing atomic number (Z), mass number (A), and charge.

• General format: AZXcharge

• Example: 2311Na+ represents a sodium ion with 11 protons, 12 neutrons, and a +1 charge.

Atomic Mass Unit (amu) and Average Atomic Mass

The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom. The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

• Formula:

• Example: Chlorine has two main isotopes: Cl-35 and Cl-37. If Cl-35 is 75% and Cl-37 is 25%, then amu.

Chemical Formulas and the Periodic Table

Molecular and Empirical Formulas

Molecular formulas show the exact number of each type of atom in a molecule. Empirical formulas show the simplest whole-number ratio of atoms.

• Example: Glucose: Molecular formula = C6H12O6; Empirical formula = CH2O

The Periodic Law and Table Organization

The periodic law states that the properties of elements are periodic functions of their atomic numbers. The periodic table is organized into periods (rows) and groups (columns) based on recurring chemical properties.

• Periods: Horizontal rows (numbered 1–7)

• Groups: Vertical columns (numbered 1–18); elements in the same group have similar properties.

Matter and Its Properties

States of Matter

Matter is anything that has mass and occupies space. It exists in three primary physical states:

• Solid: Definite shape and volume

• Liquid: Definite volume, takes shape of container

• Gas: No definite shape or volume; expands to fill container

Physical vs. Chemical Properties and Changes

• Physical properties: Can be observed without changing the substance's identity (e.g., melting point, density).

• Chemical properties: Describe a substance's ability to undergo chemical changes (e.g., flammability, reactivity).

• Physical change: Alters form but not composition (e.g., melting ice).

• Chemical change: Alters composition, producing new substances (e.g., rusting iron).

Extensive vs. Intensive Properties

• Extensive properties: Depend on the amount of matter (e.g., mass, volume).

• Intensive properties: Independent of amount (e.g., density, boiling point).

Measurement and Calculations

Measurement Process

Every measurement consists of three parts: a number, a unit, and an indication of uncertainty (significant figures).

• Uncertainty: All measurements have some degree of uncertainty, reflected in the last significant digit.

Significant Figures and Rounding

• Addition/Subtraction: Result has the same number of decimal places as the measurement with the fewest decimal places.

• Multiplication/Division: Result has the same number of significant figures as the measurement with the fewest significant figures.

Unit Conversions and Dimensional Analysis

Unit conversions use dimensional analysis to convert between units of length, mass, volume, and temperature.

• Example: To convert 25.0 cm to meters:

• Temperature conversions:

  • Celsius to Kelvin:

  • Celsius to Fahrenheit:

Mass, Moles, and Number of Particles

• Mole: The amount of substance containing Avogadro's number () of particles.

• Conversions:

  • Mass to moles:

  • Moles to number of particles:

Electromagnetic Radiation and Atomic Structure

Properties of Electromagnetic Waves

• Wavelength (\(\lambda\)): Distance between two consecutive peaks.

• Frequency (\(\nu\)): Number of cycles per second (Hz).

• Amplitude: Height of the wave from the center line.

• Speed of light (c):

• Relationship:

Energy of Electromagnetic Radiation

• Planck's equation: where

• Relationship between energy, wavelength, and frequency:

Bohr Model of the Hydrogen Atom

The Bohr model describes electrons in quantized orbits around the nucleus, explaining the line spectra of hydrogen.

• Ground state: Lowest energy level

• Excited state: Higher energy levels

• Energy transitions: Electrons absorb or emit photons when moving between levels.

• Bohr equation:

Photon Emission and Absorption

• When an electron drops from a higher to a lower energy level, a photon is emitted with energy equal to the difference between the two levels.

• Example: Calculate the wavelength of light emitted when an electron in hydrogen falls from to using the Bohr equation and .

Quantum Numbers

Four quantum numbers describe the properties of electrons in atoms:

• Principal (n): Energy level (n = 1, 2, 3, ...)

• Angular momentum (l): Shape of orbital (l = 0 to n-1)

• Magnetic (ml): Orientation of orbital (ml = -l to +l)

• Spin (ms): Electron spin (+1/2 or -1/2)

Classification of Elements and Chemical Bonds

Metals, Nonmetals, and Metalloids

• Metals: Good conductors, malleable, ductile, shiny; found on the left and center of the periodic table.

• Nonmetals: Poor conductors, brittle, dull; found on the right side of the periodic table.

• Metalloids: Properties intermediate between metals and nonmetals; found along the staircase line.

Ionic, Covalent, and Metallic Bonds

• Ionic bonds: Formed between metals and nonmetals via transfer of electrons.

• Covalent bonds: Formed between nonmetals via sharing of electrons.

• Metallic bonds: Involve delocalized electrons shared among metal atoms.