Atoms and Atomic Structure

Definition of an Atom

An atom is the smallest particle of an element that retains the properties of that element. Atoms are the fundamental building blocks of matter.

Subatomic Particles and Atomic Arrangement

• Nucleus: The dense central core of the atom, containing protons and neutrons.

• Proton (p+): A positively charged particle found inside the nucleus.

• Neutron (n0): A particle with no charge, also located in the nucleus.

• Electron (e-): A negatively charged particle that orbits the nucleus in energy levels.

Energy Levels (Electron Shells)

Electrons travel around the nucleus in specific regions called energy levels or shells. Each energy level can hold a certain maximum 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 (but is often stable with 8 in basic chemistry).

All energy levels being used must be filled for an element to be stable (octet rule).

Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second. This is the electron configuration for oxygen.

Electron Configuration Examples

• 10 electrons: 2 in the first level, 8 in the second (Neon, stable configuration).

• 16 electrons: 2 in the first, 8 in the second, 6 in the third (Sulfur, not a full outer shell, so not as stable as noble gases).

Elements and the Periodic Table

Definition of an Element

An element is a 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.

• 96% of the mass of a human is composed of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O).

Atomic Number and Atomic Mass

• Atomic Number: The number of protons in the nucleus of an atom. It is unique for each element and is also equal to the number of electrons in a neutral atom.

• Atomic Mass (Mass Number): The total number of protons and neutrons in the nucleus.

Example: Carbon has an atomic number of 6 (6 protons) and a typical atomic mass of 12 (6 protons + 6 neutrons).

Determining Protons, Neutrons, and Electrons

• Protons (p+): Equal to the atomic number.

• Electrons (e-): Equal to the atomic number in a neutral atom.

• Neutrons (n0): Atomic mass minus atomic number.

Example: For carbon (atomic number 6, atomic mass 12): Neutrons = 12 - 6 = 6

Ions

Ions are charged particles formed when atoms gain or lose electrons. The number of protons does not change.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

Example: Na+ has 11 protons and 10 electrons.

Isotopes

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

• Example: Carbon-12 (6p/6n), Carbon-13 (6p/7n), Carbon-14 (6p/8n)

Chemical Bonds and Compounds

How Elements Combine

• Compound: A substance made of two or more different elements bonded together (e.g., NaCl, H2O).

• Molecule: A group of atoms held together by covalent bonds (e.g., O2).

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., H2, ethane).

Ionic Bonds

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

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (already covalently bonded to a highly electronegative atom) and another electronegative atom. They are important in holding water molecules together and stabilizing large molecules like proteins.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the uneven distribution of electrons. Oxygen has a stronger pull on electrons, making the oxygen side slightly negative and the hydrogen side slightly positive.

Hydrogen Bonding in Water

Hydrogen bonds cause water molecules to attract each other, leading to unique properties:

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

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

• Capillary Action: The ability of water to move up narrow tubes against gravity.

Other Properties of Water

• High Specific Heat Capacity: Water can absorb large amounts of heat due to hydrogen bonding, helping regulate temperature.

• Evaporative Cooling: As water evaporates (e.g., sweating), it removes heat from surfaces.

• Versatile Solvent: Water dissolves many substances due to its polarity, making it the "universal solvent." In solutions, water is almost always the solvent.

Solutions, Acids, and Bases

Solutions

• Solution: A homogeneous mixture of two or more substances.

• Solute: The substance being dissolved (e.g., iced tea mix).

• Solvent: The substance doing the dissolving (e.g., water).

pH Scale (Acids and Bases)

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

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

• Base: More OH- ions, pH greater than 7 (e.g., Nair, pH 13).

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

Example: HCl in water forms H+ and Cl-; NaOH in water forms Na+ and OH-.

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The general format is:

$$ \text{Reactants} \rightarrow \text{Products} $$

• Coefficients: Numbers before compounds/elements indicating the number of molecules or atoms involved (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Small numbers within formulas indicating the number of atoms in a molecule (e.g., H2O has 2 hydrogens and 1 oxygen).

Example:

$$ 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 $$

Here, 6 molecules of carbon dioxide and 6 molecules of water react to form 1 molecule of glucose and 6 molecules of oxygen.

Summary Table: Subatomic Particles

Key Point: The number of protons defines the element; electrons determine chemical behavior; neutrons affect isotopic mass.