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 and Electron Configuration

Energy Levels

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 2 electrons.

• The second energy level can hold 8 electrons.

• The third energy level can hold 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 for main group elements).

Electron Configuration Examples

• Example 1: An atom with 8 electrons: 2 in the first energy level, 6 in the second. This is the electron configuration for oxygen. The atom is stable if its outermost shell is full.

• Example 2: An atom with 10 electrons: 2 in the first, 8 in the second (neon). This configuration is stable (noble gas configuration).

• Example 3: An atom with 16 electrons: 2 in the first, 8 in the second, 6 in the third (sulfur). The third shell is not full, so it is less stable than a noble gas.

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 (Z): The number of protons in the nucleus of an atom. It is unique for each element and determines the element's identity.

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

Determining Subatomic Particles:

• Number of protons = atomic number

• Number of electrons = atomic number (in a neutral atom)

• Number of neutrons = atomic mass - atomic number

Example: Carbon has atomic number 6 and atomic mass 12. Number of protons = 6, electrons = 6, neutrons = 12 - 6 = 6.

Ions and Isotopes

Ions

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

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of 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).

How Elements Combine: Compounds and Molecules

Compounds

A compound is a substance made of two or more different elements bonded together in fixed proportions.

• Example: Salt (NaCl), Water (H2O)

Molecules

A molecule is a group of atoms held together by covalent bonds.

• Example: Oxygen molecule (O2)

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Example: Water (H2O)

• Found in organic compounds

Polar and Nonpolar Covalent Bonds

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

• Nonpolar 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 and large biological molecules together.

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 form between the positively charged hydrogen of one water molecule and the negatively charged oxygen of another.

• These bonds are responsible for many of water's unique properties.

Key Properties of Water

• 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, causing a meniscus and capillary action).

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

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

• Versatile Solvent: Water dissolves many substances due to its polarity.

Solutions, Acids, and Bases

Solutions

A solution is a homogeneous mixture of two or more substances. It consists of a solute (the substance dissolved) and a solvent (the substance doing the dissolving).

• Example: Iced tea mix (solute) dissolved in water (solvent). Water is often called the "universal solvent".

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: Substance with more H+ ions, pH less than 7 (e.g., soda, pH 3.0).

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

• Neutral: Equal amounts of H+ and OH- ions (e.g., pure water, pH 7.0).

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The substances on the left are reactants, and those on the right are products.

• Chemical Equation Example:

• Chemical coefficients indicate the number of molecules or atoms involved (e.g., 6CO2 means 6 molecules of carbon dioxide).

• Subscripts indicate the number of atoms in a molecule (e.g., H2O has 2 hydrogen atoms and 1 oxygen atom).

Table: Comparison of Bonds

Additional info: Some explanations and examples have been expanded for clarity and completeness, such as the octet rule, the definition of cations/anions, and the table comparing bond types.