Chemistry Basics and Atomic Models

Nature of Chemistry and Atomic Structure

• Empirical knowledge is based on directly observed phenomena that can be experimented on.
• Theoretical knowledge is created to explain observations using ideas and rationalizes things not directly observable.
• Theories are constantly revised to align with current evidence.
• Theories aim to describe, explain, and predict phenomena in a concise manner,
• IUPAC (International Union of Pure and Applied Chemistry) was established in 1919 to regulate standards and procedures in chemistry.

Early Models of Matter

• Democritus proposed that matter can be divided into indivisible particles called atoms, which are in constant motion and are separated by empty space.
• Aristotle disagreed with Democritus, believing that matter is composed of four elements: earth, water, air, and fire.
• Dalton's model (pool ball model) stated that all matter is composed of tiny, indivisible spheres called atoms; atoms of the same element have identical properties; atoms of different elements have different properties; atoms combine in whole number ratios to form new compounds; atoms cannot be created, destroyed, or subdivided in chemical changes.
• Thomson's model (plum pudding model) proposed that an atom consists of positive and negative particles, with negatively charged particles embedded in a positively charged sphere.

Further Model Development

• Rutherford's model (nuclear model), based on the gold foil experiment, concluded that an atom has a small, dense, positively charged nucleus at its center and mostly empty space. Some electrons orbit this nucleus.

• Nagaoka's model (Saturnian model) proposed that an atom has a massive nucleus at its center, similar to a planet, surrounded by electrons orbiting it, much like Saturn's rings.

• Chadwick discovered the neutron, a heavy particle with no charge, which also resides in the atom's nucleus.

• Bohr's model suggested that electrons orbit the nucleus in specific energy levels, or shells. Electrons can transition between energy levels by absorbing or releasing specific amounts of energy. This model was primarily based on the discrete light emission pattern of hydrogen atoms. The energy levels are discrete and the shells are designated by the principal quantum number, n.

• Electrons can transition between levels, jumping to a higher level by absorbing energy and to a lower level by releasing it.

Assumptions of Bohr's Model

• Electrons can travel indefinitely within an energy level without losing energy.
• The greater the distance between the nucleus and the energy level, the more energy is required for an electron to travel within that energy level.
• Electrons cannot exist between energy levels.

Experimental Evidence for Bohr's Model: Line Spectra

• Line spectra are patterns of distinct lines corresponding to specific wavelengths of light emitted (or absorbed).
• The analysis of line spectra provides evidence for the existence of quantized energy levels in atoms.

Atomic Structure: Particle Characteristics

• Electrons (e⁻), orbiting the nucleus, have a negative charge and have a small mass.
• Protons (p⁺), in the nucleus, have a positive charge and are more massive than electrons.
• Neutrons (n⁰), in the nucleus, have no charge and are similar in mass to protons.

Atomic Number and Mass Number

• Atomic number (Z) is the number of protons in the nucleus of an atom, uniquely identifying each element.
• Mass number (A) is the total number of protons and neutrons in an atom's nucleus.
• N (neutron number) = A - Z

Relative Atomic Mass

• Relative atomic mass (A_r) is used to compare the mass of atoms to a standard atom, typically carbon-12.
• One unified atomic mass unit (u) is equal to 1/12 the mass of a carbon-12 atom.

Explore the nature of chemistry and the early models of matter, from empirical and theoretical knowledge to the foundational concepts of atomic structure. This quiz will guide you through revolutionary ideas from Democritus to Dalton. Test your understanding of how these theories laid the groundwork for modern chemistry.