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 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 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 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-}. 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 (H2SO 4) contains 2 hydrogens, 1 sulfur, and 4 oxygens. Subscripts indicate the number of each atom. 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 (K3PO 4): 3 potassiums, 1 phosphorus, 4 oxygens. Example: Magnesium hydroxide (Mg(OH)_2): 1 magnesium, 2 oxygens, 2 hydrogens. 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. Knowt Play Call Kai