Atomic Models, Chemical Bonds, and Scientific Method

Atomic Models

Dalton's Atomic Model

Dalton proposed that atoms are indivisible, compact spheres with specific weights, explaining chemical reactions. However, this model doesn't explain the electrical nature of matter.

Rutherford's Atomic Model

Rutherford's experiment involved firing positively charged alpha particles at a gold foil. Most particles passed through undeflected, some were diverted, and others bounced back, leading to a new understanding of atomic structure.

Chemical Bonds

Ionic Bonds

When a metal is in the presence of a non-metal, a transfer of electrons occurs, forming ions with opposite charges. These ions are held together by electrostatic attraction. The ionic bond forms between ions of opposite charges. Ionic valence refers to the number of electrons gained or lost.

Properties of Ionic Compounds:

• Solid at room temperature.
• High melting and boiling points.
• Hard and difficult to scratch or break.
• Form crystals.
• Generally dissolve in water.
• Do not conduct electricity in the solid state, but conduct in liquid and solution states.

Covalent Bonds

Molecular Substances: Atoms are joined by covalent bonds, and the attraction between molecules is weak (e.g., H₂).

Properties of Molecular Substances:

• Low melting and boiling points; often gases or liquids at room temperature.
• Do not dissolve in water.
• Do not conduct electricity.

Covalent Crystals: Covalent bonds extend in all directions, forming stable lattices.

Properties of Covalent Crystals:

• Solid, hard, and have high melting points at room temperature.
• Do not dissolve in water.
• Do not conduct electricity.

Metallic Bonds

In metallic bonds, positive metal ions share a "sea" of electrons. Atoms transfer their electrons to achieve a stable octet, becoming positive ions. These ions are arranged in a network where electrons move freely.

Properties of Metals:

• Solid at room temperature.
• Good conductors of heat and electricity.
• Ductile and malleable.
• Metallic luster.

Scientific Method

1. Problem Statement

Define the research problem, which can originate from new discoveries or contradictions in existing theories. Isolate the area of study and break down the problem into smaller, manageable parts.

2. Hypothesis Formulation

After defining the problem, formulate assumptions about the underlying causes and gather information through literature review. A hypothesis is a testable conjecture.

3. Hypothesis Testing

Hypotheses are tested through experiments. Design experiments and apparatus, control variables, and record and analyze results. Graphical representations can help identify patterns.

4. Establishment of Laws and Theories

Confirmed hypotheses become laws, often expressed mathematically. A coherent system of laws forms a theory.

Mass, Volume, and Density

Mass

Mass measures the amount of matter in an object. It's an intrinsic property and doesn't depend on shape or state. A closed system has constant mass.

Volume

Volume measures the space occupied by matter. It can depend on factors like pressure and temperature.

Density

Density is the ratio of mass to volume (d = m/v).

Properties of Matter

All objects are made of matter, which occupies space and has mass. A material is a specific type of matter.

General Properties: Properties like mass and volume don't provide information about the type of substance.

Specific Properties: Properties that depend on the type of substance, not its quantity or form.

States of Matter

Solid

Fixed volume and shape, incompressible, and do not flow. Can be crystalline (ordered particles) or amorphous (disordered particles).

Liquid

Fixed volume, but no fixed shape. Slightly compressible and flow.

Gas

Occupy the entire volume of their container.

Mixtures

Heterogeneous Mixtures

Composed of multiple substances that can be separated by physical methods like filtration, sedimentation, centrifugation, and selective dissolution.

Homogeneous Mixtures (Solutions)

Composed of multiple substances that are uniformly distributed. Components can be separated by methods involving state changes like evaporation, distillation, and chromatography.