Chapter 2 - Atoms, Molecules, and Ions

2.1 - Early Ideas in Atomic Theory

  • Dalton’s Atomic Theory:

    • 1. Matter is composed of exceedingly small particles called atoms, an atom is the smallest unit of an element that can participate in a chemical change

    • 2. An element consists of only one type of atom, which has a mass that is a characteristic of the element and is the same for all atoms of that element

    • 3. Atoms of one element differ in properties from atoms of all other elements

    • 4. A compound consists of atoms of two or more elements combined in a small, whole number ratio, in a given compound the numbers of atoms of each are always present in the same ratio

    • 5. Atoms are neither created nor destroyed during a chemical change, but are instead rearranged to yield substances that differ from those present before the change

  • Law of Definite Proportions or Law of Constant Composition:

    • numbers of atoms of the elements in a given compound always exist in the same ratio is consistent with these observations

      • although all samples of a particular compound have the same mass ratio, the converse is not true in general —» samples that have the same ratio are not necessarily the same substance

  • Law of Multiple Proportions:

    • when two elements react to form more than one compound a fixed mass of one element will react with masses of the other element in a ratio of small whole numbers

2.2 - Evolution of Atomic Theory

  • Electron: a negatively charged subatomic particle with a mass more than one thousand times less that of an atom

  • Alpha Particles:

    • produced by the radioactive decay of radium, a particles consist of two protons and two neutrons

    • Positively charged so deflections arose when they encountered another positive charge

    • 1. The volume occupied by an atom must consist of a large amount of empty space

    • 2. A small relatively heavy positively charged body, the nucleus must be at the center of each atom

  • Proton: a positively charged subatomic particle found in the nucleus

  • Isotopes: atoms of the same element that differ in mass

  • Neutrons: uncharged subatomic particles with a mass approximately the same as that of protons

    • explains isotopes since they differ in mass because they have different numbers of neutrons but they are chemically identical because they have have the same number of protons

2.3 - Atomic Structure and Symbolism

  • Nucleus contains the majority of an atom’s mass because protons and neutrons are much heavier than electrons whereas electrons occupy almost all of an atom’s volume

  • Atomic Mass Unit (AMU) and Fundamental Unit of Charge (e):

    • used when describing the properties of tiny objects such as atoms

  • Dalton (Da) and unified atomic mass unit (u) are alternative units that are equivalent to to amu

2.4 - Chemical Formulas

  • Molecular Formula: a representation of a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the number of atoms of each type in the molecule; used as abbreviations

    • subscript is used only when more than one atom of a given type is present

    • ex: CH4

  • Structural Formula: for a compound gives the same information as its molecular formula (types and numbers of atoms in the molecule) but also shows how the atoms are connected in the molecule

    • lines are an indication of how atoms are connected in a molecule

  • Many elements consist of discrete individual atoms

    • some exist as molecules made up of two or more atoms in the element chemically bonded together

  • A subscript following a symbol and a number in front of a symbol do not represent the same thing

    • ex: H2 and 2H —» molecular formula vs two separate hydrogen atoms that are not combined as a unit

  • Compounds: formed when two or more elements chemically combine resulting in the formation of bonds

    • Empirical Formula: indicates the types of atoms present and the simplest whole number ratio of the number of atoms/ions in the compound

      • sometimes describe composition of compounds with this

  • In many cases the molecular formula of a substance is derived from experimental determination of both its empirical formula and its molecular mass (sum of atomic masses for all atoms composing molecule)

    • ex: molecular formula for acetic acid C2H4O2 —» molecule of acetic acid and ratio is 2:4:2 —» dividing lowest common denominator (2) gives the simplest whole number ratio of atoms 1:2:1 —» empirical formula is CH2O

      • molecular formula is a whole number multiple of an empirical formula

  • Compounds for the same atom may be arranged in different aways

    • compounds with the same molecular formula can have different atom-to-atom bonding and different structures

  • Isomers: compounds with the same chemical formula but different molecular structures

    • has a major effect on chemical properties

  • Spatial Isomers: relative orientations of the atoms in space can be different

2.5 - Periodic Table

  • Periodic Law: the properties of the elements are periodic functions of their atomic numbers

  • Period Table: arranges the elements in increasing order of their atomic numbers and groups atoms with similar properties in the same vertical column; each box represents an element and contains its atomic number, symbol, average atomic mass, and sometimes name

  • Periods/Series: elements are arranged in seven horizontal rows

  • Groups: elements are arrange in 18 vertical columns; labeled at the top of each column

  • Many elements differ dramatically in their chemical and physical properties, but some elements are similar in their behaviors

  • Can sort elements into large classes with common properties

    • Metals: elements are shiny, malleable, good conductors of heat and electricity

    • Nonmetals: elements that appear dull, poor conductors of heat and electricity

    • Metalloids: elements that conduct heat and electricity moderately well and possess some properties of metals and some properties of nonmetals

  • Main-Group Elements: can also be classified in here; in columns labeled 1,2 and 13-18

  • Transition Metals: in columns 3-12

  • Inner Transition Metals: two rows at the bottom of the table (top-row elements are called lanthanides and the bottom row elements are actinides)

  • Alkali Metals: elements in group 1 (first column) and they all have similar chemical properties

  • Alkaline Earth Metals: elements in group 2 (second column) form compounds consisting of one atom of the element and two atoms of hydrogen

  • Pnictogens: group 15

  • Chalcogens: group 16

  • Halogens: group 17

  • Noble Gases: group 18, aka inert gases

2.6 - Ionic and Molecular Compounds

  • In normal chemical reactions the nucleus of each atom remains unchanged

    • electrons can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms