Chapters 4–5: Atoms, Elements, Molecules & Compounds — Study Materials Flashcards

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Law of Composition

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The Law of Constant Composition states that all pure substances have a constant composition; every sample contains the same elements in the same percentage by mass. For example, water is always $H_2O$ (2:1 H:O) and carbon dioxide is always $CO_2$ (1:2 C:O).

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Molecule

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A molecule is a particle composed of two or more nonmetal atoms bonded together. A chemical formula represents the number and types of atoms in a molecule, e.g., sulfuric acid is $H_2SO_4$.

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Chemical Formula

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A chemical formula expresses the types and numbers of atoms in a compound using element symbols and subscripts. Subscripts indicate atom counts; a subscript of 1 is not written, so niacin with 6 C, 6 H, 2 N, and 1 O is $C_6H_6N_2O$.

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Parentheses Rule

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Parentheses in chemical formulas group polyatomic units and indicate multiplication by a subscript outside the parentheses. For example, antifreeze $C_2H_4(OH)_2$ contains 2 OH groups, giving a total of 6 H and 2 O atoms.

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Order of Elements

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In formulas, metals (cations) are written first followed by nonmetals; among nonmetals a conventional order (C P N H S I Br Cl O F) is often used. Historical exceptions exist, such as $H_2O$ and $NaOH$.

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Compound Types

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Compounds are classified by composition and state: ionic (metal + nonmetal), molecular (nonmetals only), aqueous (dissolved in water), binary (two elements), ternary (three or more elements), and acids (contain hydrogen and nonmetals). Oxyacids are acids containing hydrogen and a polyatomic oxyanion, e.g., $H_2SO_4$.

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Cation

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A cation is a positively charged ion formed when an atom (usually a metal) loses one or more electrons. Example cations include $Na^+$, $Mg^{2+}$, and $Al^{3+}$.

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Anion

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An anion is a negatively charged ion formed when an atom (usually a nonmetal) gains electrons. Common anions include $Cl^-$, $O^{2-}$, and $N^{3-}$ and their names typically end in -ide (oxide, nitride, fluoride).

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Monoatomic Ion

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A monoatomic ion is a charged species formed from a single atom by loss or gain of electrons. Group IA metals form $+1$ ions, Group IIA form $+2$, Group IIIA often $+3$, while many transition metals can form multiple charges.

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Stock System

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The Stock system names cations of elements that form multiple charges by using Roman numerals to indicate the ion charge. For example, $Fe^{2+}$ is iron(II) and $Fe^{3+}$ is iron(III).

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Polyatomic Ion

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A polyatomic ion is a charged species composed of two or more atoms covalently bonded that act as a single ion, e.g., $NH_4^+$, $SO_4^{2-}$, and $NO_3^-$. These ions often appear as intact units in ionic formulas and names.

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Oxyanions

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Oxyanions are polyatomic ions containing oxygen bonded to another element; common endings are -ate for the more-oxygen form and -ite for one fewer oxygen. Examples include $SO_4^{2-}$ sulfate and $SO_3^{2-}$ sulfite.

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Ammonium

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Ammonium ($NH_4^+$) is the most common polyatomic cation and combines with many anions to form ionic compounds even in the absence of metals. For example, ammonium sulfate is $(NH_4)_2SO_4$.

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Crossover Rule

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The crossover rule creates ionic formulas by using the magnitude of each ion's charge as the subscript for the other ion, omitting signs. For instance, $Mg^{2+}$ and $Cl^-$ become $MgCl_2$, and polyatomic ions require parentheses when more than one is present, e.g., $Sr^{2+}$ and $NO_3^-$ become $Sr(NO_3)_2$.

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Lowest Ratio

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Ionic formulas must be reduced to the lowest whole-number ratio of ions; do not leave formulas that imply a common factor. For example, $Pb^{4+}$ and $O^{2-}$ yield $PbO_2$, not $Pb_2O_4$.

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Ionic Naming

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For ionic compounds with metals that have only one charge, name the cation (metal) followed by the anion name (anion stem + -ide or polyatomic name). For metals with multiple charges, include the cation charge in Roman numerals, e.g., $CoCl_3$ is cobalt(III) chloride.

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Binary Molecular

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Binary molecular compounds contain two nonmetals and are named using Greek prefixes to indicate the number of atoms of each element, with the second element's name ending in -ide. The prefix mono- is often omitted for the first element (e.g., $CO$ is carbon monoxide, $PCl_3$ is phosphorus trichloride).

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Binary Acid

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Binary acids are aqueous solutions of hydrogen plus a nonmetal and are named with the prefix hydro- and the nonmetal stem + -ic acid, e.g., $HCl(aq)$ is hydrobromic acid. The formula contains hydrogen sufficient to neutralize the anion charge.

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Oxyacid Naming

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Ternary (oxy)acids derived from oxyanions use the anion stem with -ic for -ate ions and -ous for -ite ions; for example, $NO_3^-$ gives nitric acid $HNO_3$ and $NO_2^-$ gives nitrous acid $HNO_2$. The name indicates the corresponding oxyanion and oxygen content.

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Atom

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An atom is the smallest identifiable unit of an element that retains the element's chemical properties. Elements are composed of atoms, and there are roughly 91 natural elements with about 20 synthetic ones.

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Dalton Model

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Dalton's atomic theory proposed that all matter is made of indivisible atoms, that atoms of a given element are identical in mass and properties, and that atoms combine in simple whole-number ratios to form compounds. He used laws like conservation of mass and definite composition to support the model.

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Thomson Model

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J.J. Thomson discovered the electron and proposed the plum pudding model, in which negatively charged electrons were embedded in a diffuse positive sphere to account for atomic neutrality. This model explained the presence of electrons but was later superseded by nuclear models.

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Electron Mass

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The electron has a very small mass of about $9.11 \times 10^{-28}\,$g, while the proton has mass about $1.67 \times 10^{-24}\,$g. Millikan determined the electron charge, enabling calculation of its mass from measured mass-to-charge ratios.

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Rutherford Experiment

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Rutherford's gold foil experiment fired alpha particles at thin gold foil and found most passed through while some were deflected, implying a small dense positively charged nucleus. This led to a nuclear model where electrons orbit a tiny nucleus in mostly empty space.

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Nucleus

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The nucleus is the tiny, dense central region of an atom containing protons and neutrons; it accounts for nearly all the atom's mass. If an atom were the size of a stadium, the nucleus would be comparable to a marble in its center.

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Neutron

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A neutron ($n^0$) is a neutral subatomic particle found in the nucleus with a mass similar to a proton but no charge. Neutrons contribute to atomic mass and cause isotopic variation.

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Atomic Notation

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Atomic notation uses $A$ (mass number) over $Z$ (atomic number) next to an element symbol to show nucleus composition; $Z$ equals the number of protons and $A$ equals protons plus neutrons. The number of neutrons is $A-Z$, and in neutral atoms electrons equal protons.

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Isotopes

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Isotopes are atoms of the same element (same $Z$) that differ in neutron number and thus mass number $A$. They have nearly identical chemical behavior but different masses and some isotopes may be unstable (radioactive).

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Hydrogen Isotopes

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Hydrogen has three common isotopes: protium with 1 proton and 0 neutrons, deuterium with 1 proton and 1 neutron, and tritium with 1 proton and 2 neutrons. Tritium is radioactive while protium and deuterium are stable.

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Atomic Mass

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Atomic mass is the weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu). It equals the sum of each isotope's mass times its natural abundance fraction, e.g., chlorine atomic mass ≈ $35.45\,$amu.

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Mendeleev

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Dmitri Mendeleev arranged elements by increasing atomic mass and grouped elements with similar properties, leaving blank spaces to predict undiscovered elements. His periodic organization anticipated the modern periodic table and predicted properties of elements like germanium.

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Periodic Law

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The Periodic Law states that when elements are arranged by increasing atomic number $Z$, their properties repeat periodically. Modern tables reflect electron sublevels ($s,p,d,f$) and account for periodic trends and some exceptions to mass orderings.

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Groups Periods

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Groups are vertical columns (18 in the modern table) whose elements share similar chemical properties; periods are horizontal rows (7) that correspond to occupied principal energy levels. Elements in the same group typically have the same valence electron configuration.

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Metals

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Metals are typically shiny, good conductors of heat and electricity, malleable and ductile, and usually form cations by losing electrons. They occupy the lower-left region of the periodic table and make up about 75% of elements.

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Nonmetals

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Nonmetals are found in all three states at room temperature, are poor conductors of heat and electricity, and tend to be brittle in the solid state. They gain electrons to form anions and are located on the upper-right of the periodic table (except hydrogen).

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Metalloids

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Metalloids (semimetals) have intermediate properties between metals and nonmetals and often act as semiconductors. Silicon is a typical metalloid: shiny, brittle, conducts electricity but not heat well.

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Ionic Charge

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Ionic charge is determined by the tendency of atoms to gain or lose electrons to achieve a noble gas configuration; Group IA metals form $+1$, Group IIA form $+2$, Group VIIA nonmetals form $-1$, and so on. Charges are often written as $Mg^{2+}$ or $Cl^-$ and determine ionic formulas.