Notes, summaries, assignments, exams, and problems for Chemistry

The Nature of Matter

Matter is composed of atoms and empty space.

Molecules and crystals are associations of atoms, whether identical or different, always present in a fixed number or proportion.

Chemical elements are substances made of identical atoms.

Compounds are substances formed by atoms, molecules, or crystals of different types.

How Elements Appear in Nature

• Atomic: Atoms exist as isolated and widely separated particles.
• Molecular: Independent particles formed by identical atoms, usually two or three.
• Crystalline: Elements consisting of an endless number of identical atoms, closely packed and perfectly ordered.

How Compounds Appear in Nature

• Molecular: Comprised of independent particles formed by a fixed number of different atoms.
• Crystalline:

Fundamental Atomic Concepts

Atomic Number and Mass Number

Two fundamental properties of an atom are its atomic number and mass number.

• The Atomic Number (Z): This is the number of protons in the nucleus of an atom. For example, a neutral Sodium atom (Na) with atomic number 11 has 11 protons and 11 electrons.
• The Mass Number (A): This is the total number of protons and neutrons in the nucleus of an atom. For example, Sodium-23 (²³Na) has a mass number of 23.

The number of neutrons can be calculated by subtracting the atomic number from the mass number (A - Z).

The Atom, Molecule, and Orbitals

The Atom

In physics and chemistry, an atom is the smallest unit of a chemical element that retains its identity and properties. It cannot be divided further by... Continue reading "Atomic Structure and Electron Configuration Principles" »

Chemical Bonding and Atomic Configurations

How the elements reach a noble gas configuration gives rise to three types of chemical bonds between atoms: ionic, covalent, and metallic.

• Chemical bonds: All the forces that hold together atoms when they form molecules or crystals, as well as the forces that hold together the molecules when present in a liquid or solid.
• Octet rule (Gilbert N. Lewis): Atoms are bonded with each other to achieve eight electrons in their valence level.

The Nature of Ionic Bonding

Ionic bonds form when atoms combine to reach the noble gas configuration. Atoms lose electrons to reach the noble gas configuration or gain electrons to reach it. One electronegative atom will tend to gain electrons and form a negative ion (anion)... Continue reading "Chemical Bonding Principles and Atomic Structures" »

Atomic Model Evolution

Democritus's Atomic Theory

Democritus proposed that matter could not be divided into smaller pieces forever. He believed that there were tiny, eternal, invisible, and indivisible particles called atoms (meaning indivisible in Greek) that constituted all matter.

Dalton's Atomic Theory (1808)

• Matter is made of indivisible and indestructible particles called atoms.
• All atoms of an element are identical.
• Atoms of different elements combine in fixed proportions to form compounds.

Thomson's Atomic Model (1897)

J.J. Thomson experimentally verified the existence of the electron. He discovered that the electron's mass was much lower than that of atoms, suggesting that atoms possessed a negative charge and were divisible. His model described... Continue reading "Atomic Model Evolution and Chemical Bonding" »

Other Contributions to the New Worldview

Tycho Brahe: His astronomical observations were accurate enough in the long run to support heliocentric theses. His findings were inconsistent with the belief in Aristotelian spheres and the immutability of the cosmos. Brahe discovered a new star in the constellation Cassiopeia, destroying the notion that the supralunar world is free from generation and corruption; he also mapped the orbits of comets.

Johannes Kepler and Elliptical Orbits

Johannes Kepler, a disciple of Brahe, was an enthusiastic supporter of heliocentrism. He formulated a series of laws that moved away from the Aristotelian inheritance maintained by Copernicanism, specifically the circularity of orbits and the uniformity of their velocity.... Continue reading "Scientific Revolutions: Heliocentrism and Quantum Physics" »

Understanding Human Growth Curves and Body Types

4.1. The Emerging Growth Curve

Each variable of interest (weight, height, etc.) can be related to different ages, defining what is called the emerging growth curve. This curve is intended to gain insight into the process of the acquisition of some variables over the years.

4.2. The Velocity Curve: Measuring Physical Growth Rate

The velocity curve (or physical growth curve) shows the amount gained (rate) each year (e.g., in cm, g, or kg). In a typical curve of growth rate, specific moments are expressed:

• Slowdown
• Stability
• Acceleration
• Culmination point of growth (peak velocity)
• Final growth stage

4.3. Body Proportions and Differential Growth Rates

In the study of human growth, the body and its parts do... Continue reading "Human Growth and Development: Curves, Rates, and Somatotype Classification" »

Fundamental Chemical Principles: Acid-Base Theories

Arrhenius Theory: Ionic Dissociation

The Arrhenius theory postulates the existence of positive and negative ions in aqueous solutions of acids, bases, and salts (electrolytes) to explain their electrical conductivity.

Key Definitions in Arrhenius Theory

• Acid: A substance, electrically neutral, that in aqueous solution dissociates into $ ext{H}^+$ ions (protons) and negative ions.
• Base: A substance, electrically neutral, that in aqueous solution dissociates into $ ext{OH}^-$ ions (hydroxide or hydroxyl ions) and positive ions.

Neutralization Reaction (Arrhenius)

According to this theory, the neutralization reaction occurs between an acid and a base, yielding a salt and water.

Brønsted-Lowry Theory:

The Calvin Cycle

The Calvin cycle's objective is to fix carbon dioxide (CO2) using ribulose-1,5-bisphosphate. For every triose (3-carbon sugar), two are needed to form a glucose, requiring 3 ribulose and 3 CO2. A hexose (6-carbon sugar) requires 6 ribulose and 6 CO2.

Phases of the Calvin Cycle

1. Carbon Fixation: Ribulose-1,5-bisphosphate (5C) combines with CO2 to form a 6C compound. Enzyme: Ribulose bisphosphate carboxylase. This creates a highly unstable compound. Input: 1 CO2 per ribulose.
2. Reduction: The unstable 6C compound splits into two triose molecules (3C), specifically glyceraldehyde-3-phosphate. Input/Output: None.
3. Phosphorylation: Glyceraldehyde-3-phosphate is phosphorylated using ATP, transforming into 1,3-bisphosphoglycerate. Input: 1

Q: Explain in detail the interaction of hydrophobic or hydrophobic effect. Is it a chemical bond? Explain your answer. Discuss the importance of this phenomenon in the formation and stability of biological membranes.
Hydrophobic Effect: The tendency of nonpolar molecules to cluster in polar solvents like H₂O for energetic reasons. Nonpolar substances tend to associate with one another in aqueous solutions because this results in fewer water molecules being involved in the cage that surrounds them. This situation is thermodynamically favored as it leads to lower order due to fewer water molecules (i.e., the entropy is higher) compared to when each polar molecule is surrounded by individual water molecules.
It is a chemical bond since it is... Continue reading "Understanding Hydrophobic Effects and Stereoisomerism in Chemistry" »

Stellar Dynamics: From Formation to Evolution

Stellar Composition and Formation

Stars are celestial bodies primarily formed by hydrogen and helium atoms. The simplest atoms are formed from a small number of more complex elemental particles. These particles contain large amounts of elementary matter. As the universe expanded and cooled, elementary particles began to combine, forming atoms. Inside a star, hydrogen density is so high that pressures and temperatures reach intense conditions.

Nuclear Fusion in Stars

Under these extreme conditions, nuclear fusion reactions occur. In these reactions, hydrogen atoms combine to produce more complex atoms, such as helium, releasing a high amount of energy. This process is the fundamental energy source for... Continue reading "Stellar Dynamics and Earth's Spheres: A Cosmic Connection" »