Unit 1_241204_113804

Introduction to Science and Chemistry

Definition of Science and Chemistry

• Science: The study of the physical and natural world through observation and experimentation. It encompasses multiple disciplines including physics, biology, chemistry, and earth sciences, each focusing on different aspects of the universe.

• Chemistry: A branch of science that examines matter, its properties, composition, structure, and the changes it undergoes during chemical reactions. Chemistry is known as the "central science" because it connects physics with other natural sciences.

Matter and Atoms

• Matter: Anything that has mass and occupies space. Matter is composed of atoms, which are the fundamental building blocks of all substances.

  • Atoms: The smallest unit of matter that retains all chemical properties of an element.

Atomic Theory

Dalton's Atomic Theory

• Matter is composed of extremely small particles called atoms.

• An element consists of only one type of atom. For example, the element oxygen is made up solely of oxygen atoms.

• Atoms of one element differ in properties from atoms of all other elements, thus distinguishing one element from another.

• A compound consists of atoms of two or more elements combined in fixed, whole-number ratios, like water (H₂O), which is composed of two hydrogen atoms and one oxygen atom.

• Atoms are neither created nor destroyed during a chemical change, upholding the principle of mass conservation.

Laws Related to Dalton's Atomic Model

• Law of Conservation of Matter: Matter cannot be created or destroyed; it can only change forms.

• Law of Constant Composition: A given compound always contains the same proportion of elements by mass, regardless of its source.

• Law of Multiple Proportions: When two elements form a series of compounds, the ratios of the masses of the second element that combine with a fixed mass of the first element can always be expressed in small whole numbers. This law highlights the discrete nature of chemical compounds.

Advances in Atomic Theory

Post-19th Century Developments

• Experiments conducted in the late 19th and early 20th centuries revealed that atoms are not indivisible, leading to deeper explorations into their substructure.

• J.J. Thomson: Discovered the electron using the cathode ray tube and proposed the Plum Pudding Model of the atom, where electrons were embedded in a positively charged 'soup'.

Notable Experiments

• R.A. Millikan: Conducted the oil drop experiment in 1909 to determine the charge of the electron, providing crucial evidence for the quantization of electric charge.

Modern Atomic Model

Nuclear Model

• Proposed by Ernest Rutherford following his alpha scattering experiment, this model introduced the concept of a dense spherical nucleus at the center of the atom.

• This model rejected the Plum Pudding Model's assumptions about the atom's structure, emphasizing a concentrated positive charge within the nucleus.

Structure of the Nuclear Atom

• Particles and Their Properties:

  • Electron: Mass = 9.11 x 10^-31 kg, Charge = -1 (1.6 x 10^-19 C), orbits around the nucleus.

  • Proton: Mass = 1.67 x 10^-27 kg, Charge = +1, located in the nucleus and defining the atomic number of an element.

  • Neutron: Mass = 1.67 x 10^-27 kg, Charge = 0, also found in the nucleus, contributing to atomic mass but not charge.

Atomic Symbols

• Atomic symbols are defined as:

  • Mass number (A) = protons + neutrons

  • Atomic number (Z) = protons (also equal to electrons in a neutral atom).

  • Example: Magnesium (Mg) has an atomic number of 12 and a mass number of 24, indicating it has 12 protons and 12 neutrons.

Isotopes and Ions

• Isotopes: Atoms of the same element that differ in the number of neutrons; they have the same number of protons but different mass numbers, influencing their stability and radioactive properties.

• Ions: Atoms or groups of atoms that carry a net charge due to the loss or gain of electrons. Cations are positively charged (loss of electrons) while anions are negatively charged (gain of electrons).

Periodic Table Structure

Organization

• The periodic table is organized into columns (groups) and rows (periods) representing elements with similar chemical and physical properties.

Characteristics of Elements

• Each element is defined by its unique atomic number and atomic mass, influencing its reactivity and stability.

Element Trends

• Atomic Size Trends:

  • Down a Group: Atomic size increases due to the addition of electron shells.

  • Across a Period: Atomic size decreases due to increasing nuclear charge pulling electrons closer to the nucleus.

• Ionization Energy Trends:

  • Down a Group: Ionization energy decreases as outer electrons are farther from the nucleus and more shielded.

  • Across a Period: Ionization energy increases as the nuclear charge increases, requiring more energy to remove an electron.

Electronic Configuration and Principles

• Aufbau Principle: Electrons fill atomic orbitals from lowest to highest energy levels, ensuring the most stable configurations.

• Pauli's Exclusion Principle: States that no two electrons in an atom can have the same set of four quantum numbers, leading to unique electron configurations.

• Hund's Rule: For orbitals of the same energy (degenerate orbitals), the lowest energy configuration has the maximum number of unpaired electrons, minimizing electron-electron repulsion.

Chemical Properties of Elements

• Flame Color Test: Different elements produce characteristic colors when burned, a result of electron transitions; e.g., lithium yields red, sodium gives yellow, and copper produces green.

• Reactivity Trends: Alkali metals are highly reactive, particularly with water and halogens, forming strong bonds and releasing significant energy.

Conclusion

• Chemistry encompasses the study of atoms and their interactions, which leads to insights into the behavior of matter and energy. Understanding atomic structure is fundamental to grasping broader chemical concepts, paving the way for developments in both theoretical and applied chemistry.