Atoms and Atomic Structure

Definition and Structure of the Atom

An atom is the smallest unit of an element that retains the properties of that element. Atoms are composed of subatomic particles arranged in a specific structure.

1. Nucleus: The central region of the atom containing protons and neutrons.

2. Proton: A positively charged particle located inside the nucleus.

3. Neutron: A particle with no charge, also found in the nucleus.

4. Electron: A negatively charged particle that orbits the nucleus in energy levels.

5. Example: The carbon atom has 6 protons, 6 neutrons, and 6 electrons.

   Energy Levels and Electron Configuration

6. Electrons occupy specific regions around the nucleus called energy levels or shells. Each energy level can hold a limited number of electrons:

   • The first energy level can hold up to 2 electrons.

   • The second energy level can hold up to 8 electrons.

   • The third energy level can hold up to 18 electrons.

For an atom to be stable, its energy levels must be filled according to the number of electrons present.

Example: An atom with 8 electrons will have 2 in the first level and 6 in the second level.

Electron Configuration: The arrangement of electrons in an atom's energy levels. For example, Neon (10 electrons): 2 in the first level, 8 in the second level (stable configuration).

Elements and the Periodic Table

Definition of Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical means. There are about 90 naturally occurring elements, all listed on the Periodic Table.

• Only 25 elements are essential for living things.

• Four elements—carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)—make up about 96% of the mass of a human body.

Atomic Number and Atomic Mass

• Atomic Number: The number of protons in the nucleus of an atom. It also equals the number of electrons in a neutral atom.

• Atomic Mass: The sum of protons and neutrons in the nucleus.

Formula:

Example: Carbon has atomic number 6 and atomic mass 12, so it has 6 neutrons.

Ions and Isotopes

Ions

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge. The number of protons does not change.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

Example: has 11 protons and 10 electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons).

Chemical Bonds and Compounds

Compounds and Molecules

A compound is a substance made of two or more different elements bonded together. A molecule is a group of atoms held together by covalent bonds.

• Example of Compound: Sodium chloride (NaCl), water (H2O).

• Example of Molecule: Oxygen (O2).

Covalent Bonds

Covalent bonds form when two atoms share electrons. These are common in organic compounds.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., water).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., hydrogen gas).

Ionic Bonds

Ionic bonds form when atoms transfer electrons, resulting in oppositely charged ions that attract each other.

• Example:

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and a highly electronegative atom (such as oxygen) in another molecule. These bonds are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the uneven distribution of electrons between oxygen and hydrogen atoms. Oxygen has a stronger pull on electrons, making the oxygen side slightly negative and the hydrogen side slightly positive.

Cohesion and Adhesion

• Cohesion: Attraction between molecules of the same substance (e.g., water molecules stick together).

• Adhesion: Attraction between molecules of different substances (e.g., water and glass).

Example: Meniscus formation in a graduated cylinder and capillary action.

High Specific Heat Capacity

Water can absorb large amounts of heat due to its hydrogen bonds, helping regulate temperature in organisms and environments.

Example: Sweating leads to evaporative cooling.

Versatile Solvent

Water's polarity makes it an excellent solvent, able to dissolve many substances.

Solutions, Acids, and Bases

Solutions

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is often the solvent in biological systems.

• Example: Iced tea mix (solute) dissolved in water (solvent).

pH Scale

The pH scale measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) in a solution. The scale ranges from 0 (acidic) to 14 (basic), with 7 being neutral.

• Acid: More H+ ions, pH below 7 (e.g., soda, pH 3).

• Base: More OH- ions, pH above 7 (e.g., hair remover, pH 13).

• Neutral: Equal H+ and OH- ions (e.g., pure water, pH 7).

Example Equations:

Chemical Equations

Reactants, Products, and Coefficients

Chemical equations represent the transformation of reactants into products. Coefficients indicate the number of molecules involved.

Example:

• Reactants: Substances that undergo change (left side).

• Products: Substances formed (right side).

• Coefficients: Numbers before compounds, indicating quantity (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers within chemical formulas indicating the number of atoms (e.g., O2 means 2 oxygen atoms).

Summary Table: Atomic Particles

Summary Table: Types of Chemical Bonds

Additional info: Some details about electron configuration and chemical equations were expanded for clarity and completeness.