Elements and Compounds

Elements and Compounds

Chapter Overview

• Chapter 3 focuses on elements, compounds, and the periodic table, contrasting with the math-heavy Chapter 2.
• Emphasis on definitions related to the periodic table.

Elements

• Elements are known by their appearance (e.g., gold: shiny, lustrous, yellow, dense, valuable, ductile, malleable, conductive).
• Key topics include distribution of elements, names and symbols, the periodic table, natural states, diatomics, compounds, formulas, and composition.
• Definition: An element is a fundamental substance that cannot be broken down into simpler substances by chemical means.
• Elements are the building blocks of matter.
• Elements can occur naturally or be synthesized.

Atoms

• The atom is the basic unit of an element, retaining the element's characteristic properties and chemical behavior.
• Atoms are composed of subatomic particles: protons, neutrons, and electrons.
• Subatomic particles alone do not exhibit the properties of the element; they must be combined in a specific atomic structure.

Elemental Facts

• There are 118 known elements.
• 88 elements occur naturally at room temperature.
• Only 2 elements are liquids at room temperature:
  • Bromine (Br): a nonmetal
  • Mercury (Hg): a metal
• 11 elements are gases at room temperature; all are nonmetals, located on the right side of the periodic table.

Element Distribution

• 10 elements constitute approximately 99% of the Earth's crust, seawater, and atmosphere.
  • Oxygen (O) is the most abundant, present in water, the atmosphere, and combined with minerals in rocks and ores.
  • Silicon (Si) is the next most abundant (e.g., in silicon dioxide SiO_2 - sand).
  • Carbon (C) accounts for only 0.08% in the Earth's crust, seawater, and atmosphere but is significant in living organisms.
• Oxygen accounts for about 20% of the atmosphere and is found in nearly all rock, sand, and soil.
• The human body primarily consists of oxygen, carbon, and hydrogen.

Element Names and Symbols

• Elements are named based on historical, geographical, or scientific significance.
  • Iodine (I) comes from the Greek word meaning violet.
  • Germanium (Ge) is named after Germany.
  • Curium (Cm) commemorates Marie Curie.
• Marie Curie won Nobel Prizes in both physics and chemistry for her work on radioactive elements and died of cancer due to radiation exposure.
• Element symbols can be one or two letters.
  • Examples: Iodine (I), Nitrogen (N), Carbon (C), Hydrogen (H), Oxygen (O).
  • Barium (Ba) is an example of a two-letter symbol derived from its name.
  • Gold (Au) uses a symbol based on its old name (aurum).
• It is important to familiarize yourself with the symbols of common elements; note cards can be helpful.

Writing Element Symbols

• The first letter of an element symbol is always capitalized, and the second letter (if present) is lowercase.
• This convention distinguishes elements like cobalt (Co) from compounds like carbon monoxide (CO).
• Understanding how to write element symbols correctly is essential.

Periodic Table

• The periodic table, developed in the late 1800s, organizes elements based on their properties.
• Elements in the same column (group or family) have similar chemical properties due to their electron configurations.

Groups/Families

• Noble Gases: The far-right column, known for being largely unreactive.
• The first group is the alkali metals.
• The first two groups (including Hydrogen) are the S block, where outermost electrons fill S orbitals.
• The P block consists of the rightmost 6 columns.
• The transition elements are in the D block.

Element Types

• Metals: Found on the left side of the periodic table.
• Metalloids: Located in the middle, along a zigzag line.
• Nonmetals: Found on the right side of the periodic table.

Elemental States

• Bromine (Br) and Mercury (Hg) are liquids at room temperature.
• Gases include noble gases, nitrogen (N), oxygen (O), fluorine (F), and hydrogen (H).

Metals

• Metals are solids at room temperature, except for mercury (Hg).
• They are shiny and good conductors of heat and electricity.
• Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires).
• Most metals have high melting points and densities.
• Common metals include aluminum (Al), gold (Au), platinum (Pt), silver (Ag), tin (Sn), and iron (Fe).
• Metals readily combine with nonmetals to form compounds but rarely combine with other metals.
• Metals tend to give up electrons.

Alloys

• Alloys are homogeneous mixtures of metals, like brass, bronze, and steel.

Nonmetals

• Nonmetals are generally not shiny and have low melting points and densities.
• They are poor conductors of heat and electricity, found on the far right of the periodic table.
• Nonmetals combine with each other to form molecular compounds.
• Common nonmetals include carbon (C), nitrogen (N), oxygen (O), and sulfur (S).
• Sulfur compounds often have strong odors, such as rotten eggs (hydrogen sulfide, H_2S).
• Sulfur compounds are added to natural gas to make leaks detectable.

Metalloids

• Metalloids are located between metals and nonmetals and are semiconductors.
• Examples include boron (B), silicon (Si), and germanium (Ge).
• They are used in electronic devices.

Chemical Reactions

• Elements react to achieve a more stable state.
• Oxygen is commonly found combined with other elements in ores and sand because it helps stabilize them.
• Elements in pure form are rare; examples include gold, silver, and platinum (noble metals) due to their unreactivity.

Air Composition

• Air is a homogeneous mixture primarily composed of nitrogen (N) and oxygen (O).
• Nitrogen exists as a diatomic molecule (N_2).
• Oxygen exists as a diatomic molecule (O_2).

Diatomic Elements

• 7 elements exist as diatomics: hydrogen (H), nitrogen (N), oxygen (O), and the halogens.
• The halogens are one column on the periodic table.

Compounds

• Definition: A compound contains two or more elements chemically combined in a definite proportion by mass.
• Example: Water (H_2O) - two hydrogens and one oxygen.
• Changing the ratio of elements creates a different compound with different properties (e.g., hydrogen peroxide).
• Compounds can be decomposed into simpler substances or elements.

Types of Compounds

• Molecular Compounds: Held together by the sharing of electrons (nonmetal-nonmetal).
• Ionic Compounds: Held together by the attraction of opposite charges (metal-nonmetal).

Molecules and Ions

• Molecules are the smallest unit of a compound formed by two or more atoms.
• Ions are atoms or groups of atoms with a charge.
• Cations: Positive charge (e.g., sodium ion, Na^+, ammonium ion, NH_4^+.
• Anions: Negative charge (e.g., chloride, Cl^-, hydroxide, OH^-, nitrate, NO_3^{2-}.

Compound Properties and Formulas

• Over 50 million compounds are known, each with unique properties.
• The ratio of elements (e.g., hydrogen to oxygen) defines the compound.
• Chemical formulas indicate the types and numbers of atoms present.
• Example: Sulfuric acid (H2SO4) contains 2 hydrogens, 1 sulfur, and 4 oxygens.
• Subscripts indicate the number of each atom.

Chemical Formulas and Subscripts

• Formulas indicate elements present.
• Subscripts indicate quantity.
• Absence implies 1.
• Example: Calcium nitrate (Ca(NO3)2): 1 calcium, 2 nitrogens, 6 oxygens.
• Example: Potassium phosphate (K3PO4): 3 potassiums, 1 phosphorus, 4 oxygens.
• Example: Magnesium hydroxide (Mg(OH)_2): 1 magnesium, 2 oxygens, 2 hydrogens.

Reading Chemical Formulas

• Sodium chloride (NaCl): sodium and chlorine.
• Magnesium oxide (MgO): magnesium and oxygen.
• Barium nitrate (Ba(NO3)2): barium, nitrogen, and oxygen.
• The formula Ba(NO3)2 contains 1 barium, 2 nitrogens, and 6 oxygens.

Laws of Composition

• Law of Definite Composition: A compound always contains the same elements in the same proportions by mass, regardless of the source (e.g., water is always water).
• Law of Multiple Proportions: Atoms of two or more elements may combine in different ratios to produce more than one compound (e.g., carbon and hydrogen).

Scientific Theories

• A law is a summary of observed behavior that is nearly always true under tested conditions.
• A theory or model is an explanation of observed behavior; theories can be modified if they fail.
• A hypothesis is a level below a theory.