Properties of Matter

Intensive vs. Extensive Properties

Understanding the distinction between intensive and extensive properties is essential for characterizing substances in chemistry.

• Intensive Properties: These do not depend on the amount of substance present. Examples include density, temperature, composition, mass per atom, and phase of matter.

• Extensive Properties: These depend on the quantity of substance. Examples include number of atoms, mass, volume, energy, and heat.

• All numerical values are measured except for those that are countable.

Example: The mass of a sample is extensive, while its density is intensive.

Measurement in Chemistry

Uncertainty, Accuracy, and Precision

All measurements in chemistry have inherent uncertainty, which must be considered when reporting results.

• Accuracy: Indicates how close a measured value is to the true or accepted value.

• Precision: Indicates how close repeated measurements are to one another.

• Systematic Error: Consistent, repeatable error associated with faulty equipment or bias.

• Random Error: Error that varies unpredictably from one measurement to another.

Example: If a scale consistently reads 0.5 g too high, it has a systematic error. If readings fluctuate randomly, it is random error.

Significant Figures in Measurement

Significant figures reflect the precision of a measured value. The number of significant figures in a calculation depends on the measurement and the mathematical operation performed.

• When multiplying or dividing, the result should have the same number of significant figures as the measurement with the fewest significant figures.

• When adding or subtracting, the result should have the same number of decimal places as the measurement with the fewest decimal places.

Example: (rounded to two significant figures).

SI Units and Metric Prefixes

Seven Fundamental SI Units

The International System of Units (SI) defines seven base units for scientific measurement:

• Meter (m): Length

• Kilogram (kg): Mass

• Second (s): Time

• Ampere (A): Electric current

• Kelvin (K): Temperature

• Mole (mol): Amount of substance

• Candela (cd): Luminous intensity

Metric Prefixes to Memorize

Metric prefixes are used to express multiples or fractions of SI units.

Development of Atomic Theory

Key Scientific Contributions

The atomic theory has evolved through the work of several scientists, each contributing important discoveries.

• Democritus: Proposed that all matter is composed of small, indivisible particles called atomos.

• Aristotle: Suggested that different types of atoms account for the properties of substances (earth, air, fire, water).

• Dalton: Established modern atomic theory:

  • Matter is composed of extremely small particles called atoms.

  • Atoms of a given element are identical in mass and properties.

  • Atoms of different elements have different properties.

  • Atoms combine in simple whole-number ratios to form compounds.

  • Atoms are rearranged in chemical reactions, but not created or destroyed.

Experimental Evidence and Revisions to Atomic Theory

• J.J. Thomson (1897):

  • Discovered the electron using cathode ray experiments.

  • Showed that cathode rays are streams of negatively charged particles (electrons).

  • All electrons are identical and have a negative charge.

  • Revision: Demonstrated that atoms are divisible and contain subatomic particles.

• Robert Millikan (1909):

  • Measured the charge of the electron using the oil drop experiment.

  • Created a mist of oil droplets and used electric fields to determine the charge.

  • Revision: Provided the first accurate value for the electron's charge, refining atomic models.

• Ernest Rutherford (1911):

  • Conducted the "gold foil" experiment, discovering the nucleus.

  • Showed that atoms have a small, dense, positively charged nucleus containing protons.

  • Revision: Replaced the "plum pudding" model with the nuclear model of the atom.

Additional info: These experiments laid the foundation for the modern understanding of atomic structure, including the existence of electrons, protons, and the nucleus.