Matter, Chemical Trends, and Chemical Bonding

Classification of Matter

Matter can be classified based on its composition and uniformity. Understanding these classifications is fundamental to studying chemistry.

• Homogeneous substances: Materials with a uniform composition throughout (e.g., saltwater, air).

• Heterogeneous substances: Materials with a non-uniform composition (e.g., salad, granite).

• Pure substances: Consist of only one type of particle; can be elements (e.g., gold) or compounds (e.g., water).

• Mixtures: Combinations of two or more substances that retain their individual properties; can be homogeneous or heterogeneous.

Example: Salt dissolved in water forms a homogeneous mixture, while sand in water forms a heterogeneous mixture.

States of Matter and Interparticle Forces

The physical state of a substance (solid, liquid, gas) is determined by the arrangement, energy, and forces between its particles.

• Solids: Particles are closely packed in a fixed arrangement; strong interparticle forces; low energy.

• Liquids: Particles are close but can move past each other; moderate interparticle forces; moderate energy.

• Gases: Particles are far apart and move freely; weak interparticle forces; high energy.

Example: Ice (solid), water (liquid), and steam (gas) are all forms of H2O with different particle arrangements and energies.

Physical and Chemical Properties and Changes

Properties and changes of matter are classified as physical or chemical based on whether the composition of the substance changes.

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

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

• Physical changes: Changes that do not alter the chemical composition (e.g., melting, dissolving).

• Chemical changes: Changes that result in the formation of new substances (e.g., rusting, combustion).

Evidence of chemical change: Color change, formation of a precipitate, gas production, energy change (light/heat).

Separation of Mixtures

Mixtures can be separated using physical methods based on differences in properties of their components.

• Filtration: Separates solids from liquids.

• Distillation: Separates substances based on boiling points.

• Chromatography: Separates based on movement through a medium.

• Decanting: Separates liquid from solid by pouring off the liquid.

Example: Salt can be separated from water by evaporation.

Significant Figures and Measurement Uncertainty

Measurements in chemistry are reported with significant figures to reflect the precision of the measuring instrument. All measurements have inherent uncertainty.

• Significant figures: Digits in a measurement that are known with certainty plus one estimated digit.

• Rules for significant figures: Nonzero digits are always significant; zeros between nonzero digits are significant; leading zeros are not significant; trailing zeros are significant if there is a decimal point.

• Rounding: When performing calculations, the result should be rounded to reflect the least precise measurement.

• Uncertainty: The last digit in a measurement is uncertain; uncertainty can be expressed as ± value.

Example: 12.30 g has four significant figures; 0.00450 mL has three significant figures.

Atomic Structure and Subatomic Particles

Atoms are composed of subatomic particles: protons, neutrons, and electrons. The discovery of these particles led to the modern understanding of atomic structure.

• Protons: Positively charged particles in the nucleus; determine atomic number.

• Neutrons: Neutral particles in the nucleus; contribute to atomic mass.

• Electrons: Negatively charged particles in orbitals around the nucleus.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number, Mass Number, and Isotopes

The atomic number, mass number, and isotopic abundance are key to understanding the identity and mass of elements.

• Atomic number (Z): Number of protons in the nucleus; defines the element.

• Mass number (A): Total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Relative atomic mass (Ar): Weighted average of the masses of an element's isotopes.

Equation for relative atomic mass:

Example: Chlorine has two main isotopes: Cl and Cl. The relative atomic mass is calculated using their masses and abundances.

The Periodic Table and Periodic Trends

The periodic table organizes elements by increasing atomic number and similar chemical properties. Periodic trends arise from electron arrangement and atomic structure.

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows; properties change progressively across a period.

• Major element types: Metals, nonmetals, metalloids; each has characteristic properties and locations.

Periodic trends:

• Atomic radius: Generally decreases across a period and increases down a group.

• Ionization energy: Energy required to remove an electron; increases across a period, decreases down a group.

• Electron affinity: Energy change when an atom gains an electron; generally becomes more negative across a period.

• Electronegativity: Tendency of an atom to attract electrons in a bond; increases across a period, decreases down a group.

Example: Fluorine is the most electronegative element; cesium has a large atomic radius.

Summary Table: Classification of Matter