Chapter 2 Atoms, Molecules and Ions

Atomic Structures and Nomenclature

Dalton's Atomic Theory

Element — fundamental substance that can not be broken down chemically into simpler units.

Atomic Symbol — representation of elements in one or two-letter symbol

All are listed in the Periodic Table — Remember period and group from Chapter 1.

Dalton proposed:

  • Element of chemistry = atoms
  • Atoms in elements have the same mass and the same property
  • Compounds (molecules) are made of different elements of simple numerical ratio
  • In chemical rxn, no atom is created nor destroyed

    Conservation of Mass

    Matter can neither be created nor destroyed during a chem rxn. It also means that the mass of all reactants add up to the mass of all products.

  • Priestly (1774) Isolation of O2 by Heating HgO

    2HgO → 2Hg + O2
  • Lavoisier (1774) Oxygen is involved in combustion.

    Law of Definite Proportions

    Compounds contain — same proportions of elements by mass

    Law of Multiple Proportions

    Mass of compounds == Integral multiples of two or more elements

  • Elements A and B can form compounds in: AB, AB2, A2B, AB3, etc.

    Example: If there is 17g of Ammonia, then there must be 14g of Nitrogen and 3g of Hydrogen. In ammonia, the ratio of H:N is always 1:4.632. Always.


    Example: In a closed vessel, 0.455g of Mg is allowed to burn in O2 with its mass 2.315g. The unreacted O2 was found t be 2.015g. What is the mass of MgO2 produced?

    Using Conservation of Mass, we know that:

    Before Rxn: Mg(0.455g) + O2 → Total mass = 2.770g

    After Rxn: O2(2.015g) + MgO( ? g) → Total mass = 2.770g

    Therefore, m(MgO) = 2.770g - 2.015g = 0.775g MgO.

    The Structure of the Atom


  • First component of atom discovered -- Electron
      In cathode ray tube (electron beam)
      Thomson (1898): found negatively charged partilces in all atoms
      Subsequent experiments determined me- ≈ mp+/2000
      Devised an atomic structure to be "plum pudding"

    The yellow sponge cake portion is the smeared positive charges

    The electrons are embedded as chunks of plums, which oscillate back and forth.



      ... but this is a wrong picutre!
















  • X ray → Pentrating radiation from Radioactive substance
  • α particle
  • β particle
  • γ ray

    Radioactivity = spontaneous emission of particles or radiation

    Proton and Nucleus

  • Rutherford, Geiger, Marsden (1909) -- nuclear atom

    Applied α particles (from radioactive decay) to tink gold foil




      From this and other experiments, they concluded:

  • Atom -- mostly empty space
  • Magnitude of positive charge -- different for different atoms, approximately 1/2 of atomic mass
  • The charge of nucleus = # of electrons -- and that atom is electrically neutral.

    They proposed a "planetary model of the atom".

    .... but this is also wrong!




    H structure was solved by Rutherford. But problem remained.
    Helium mass was 4 times as much as Hydrogen, for where Rutherford knew there is one proton. Helium is known for having 2 protons, then why 4 time as much mass?

    Chadwick (1932, long after Rutherford's experiment of proton) found Neutron


    Today's View

    This is how we view today. more details in chapters 5 and 6.

      Nuclear Components:

    How atom should look



  • Proton discovered by Rutherford in 1919
  • Neutron discovered by Chudwick in 1932
    Each of them is made up of 3 quarks.



    It's just a fuzzy ball.


    Today, this is how we view atom. A fuzzy ball. The blue halo is fast-moving electron cloud, and lighter the blue more electrons are found in that region. The halo effect comes from the fact that we can not pin point where the electrons are since they are in constant motion.


    Pictorial representation of atom

    This picture is much more familiar, but perhaps not very accurate. In the center, there is a nucleus, containing number of protons and nuetrons and surrounding the nucleus we have electrons moving.

    Atomic Number, Mass Number, and Isotopes

    In an atom

    Define Mass # → A
    Define # of protons → Z
    Define # of neutrons → N


    Z → differentiate atoms

    If atoms share the same Z, but different NIsotopes

    A = Z + N

    # of electrons = Z → therefore atom is electrically neutral

    If # of electron in excess or deficient → Ion


    If # of electrons > ZAnion

    If # of electrons < ZCation


    For smaller atoms, the number of protons is the same as the number of neutrons. However, for larger atoms, the number of neutrons exceeds that of the protons.


    Size Comparison: Baseball proton and baseball Sun Let's compare the size of Sun-Earth system with that of a simplest atom hydrogen, consisting only one proton and one electron. We're going to visualize them by making a baseball to either Sun or proton. The electron and Earth are represented by a smallest bead I could possibly find in my wife's hobby box! The following table gives the pertinent distances.

    DiameterDistance Diameter
    Sun1.39 x 106 km1.5 x 108 km Earth1.28 x 104 km
    Proton 1 fm0.53 ÅElectrona point
    Baseball7.5 cm Small bead 0.8 mm

    The ratio between diameters of Earth to Sun and small bead to a baseball is about the same. In both cases, the smaller one is about 1 % of the larger. Since electron is a point particle (to a good approximation), so there is no size. But, to visualize the system, we need to make electron a particle with finite size. The distance between the two parts of the systems are also listed. From these we can calculate the distance from Sun to Earth if the size of the Sun is shrunk to that of a baseball. ? m baseball =1.5× 10 8 km( 7.5c m baseball 1.39× 10 6 km )( 1m 100cm )=8.1m=8m

    If we use size of Earth as our reference, then

    ? m bead =1.5× 10 8 km( 0.8m m bead 1.28× 10 4 km )( 1m 10 3 mm )=9.4m=9m

    1 m or so difference. Not too bad. Two calculations are just consistent. Now, calculate for baseball hydrogen.

    ? m baseball =0.53Å( 10 10 m 1Å )( 7.5c m baseball 10 15 m )( 1m 100cm )=4000m=4km Wow!! So, if the baseball was our Sun, Earth would be orbiting only 8~9m away, but if the baseball was a proton in hydrogen atom, it would be zipping around near South Ferry in Manhattan!! (For those of you who are outside of LIU, we are located near Manhattan Bridge in Brooklyn, New York.)

    Atomic Symbol

    There are about 100 naturally occuring elements, listed on the Periodic Table.

  • Each element, represented by an atomic symbol (one- or two-letter symbol)
  • Nuclear charge (aka Atomic Number), Z = # of protons in nucleus
  • Average mass in amu (atomic mass unit)

    Representation on the left is from the Periodic Table and on the right is the isotopic notation.

    In the above example, Xenon atom, with atomic symbol Xe, is given here. The information we can obtain from these are,

    It has 54 protons — from both
    Its A = 131 — from isotopic notation
    The average mass <m> = 131.30 — from Perodic Table notation

    From these information we can calculate:

    N = A - Z = 131 - 54 = 77 neutrons
    # of electron = 54 = Z

    Example: From 2.100. How many protons, neutrons, and electrons are in each of the following: a. N 7 15     b. C 27 60 o     c. I 53 131     d. C 58 148 e

    a. In this notation, the subscripted number, that is Z, is 7. It means that we have 7 protons. The superscripted number, A, is 15. It means that we have combined nuclear particles (protons and neutrons) counts 15. Then, the number of neutrons is N = A - Z and there are 8 neutrons. The number of electrons are the same as the number of protons in an atom, because atom is electrically neutral.

    b. Similarly, Z = 27, N = 60 - 27 = 33, e- = 27

    c. Z = 53, N = 131 - 53 = 78 , e- = 53

    d. Z = 58, N = 148 - 58 = 90 , e- = 58

    Atomic Mass

    Atomic mass is represented by atomic mass unit (amu), and is defined by 1amu= m( C- 12 ) 12 =1.660539× 10 24 g

    where m(C-12) is the mass of carbon-12 isotope.

    Using the above Xe example, hence, the average mass of Xe is 131.30 amu.

    Example: How many atoms are there in 10.0 g of Na?

    As we've seen in Chapter 1, we can use dimensional analysis to solve this problem. As you read the question, you can write the equation down as,

    ?Na = 10.0gNa

    should suffice. The conversion of 1 amu is 1.660539 x 10-24 g.

    ? Na =10.0 g Na ( 1amu 1.660539× 10 24 g )( 1 Na 22.99amu )=0.2619× 10 24 Na

    That's a huge number!

    The Periodic Table

    Over 90 different naturally occuring elements listed in the Periodic Table of Elements

    Group = elements in the same column
    Period = elements in the same row

    Each group has similar chemical properties

    For example, Carbon belongs to group 14 so as silicon. Both can form up to 4 bonds.

    Majority of elements are metals, i.e. conducts electricity and has shiny colors

    Group 1Alkali metals
    Group 2Alkali Earth metals
    Groups 3 - 12Transition metals
    Groups 13 - 18Main group elements
    Elements 57 - 71Lanthanides
    Elements 89 - 103Actinides
    Group 15Pnictogens
    Group 16Chalcogens
    Group 17Halogens
    Group 18Noble gas

    Molecules and Ions


  • Molecule — one unit of chemical compound
  • Consisting of two or more elements in a definite arrangement
  • Atoms in molecule are held by chemical bond

  • Molecules are represented by chemical formula. Water's chemical formula is H2O, representing 2 hydrogens andn 1 oxygen.


    Ion — Charged chemical compound.
      If positively charged — Cation
      If negatively charged — Anion

    Since atoms are electrically neutral, in comparison with the parent atom

  • cation — missing electrons
  • anion — extra electrons

    Example: How many electrons are in the following atoms and ions? H, H+, Cl, Cl-, Mg2+, O2-

    Remember that atoms are electrically neutral. It means that the number of protons, Z, is the same as the number of electrons. So, for H, the number of electron is 1. For Cl, it is 17. For cations, the number of electron is less than the number of protons. For H+, an electron with -1 charge is missiing from H, terefore, there is no electron, zero. For Cl-, it is 18, because when you add one -1 charge to 17 electron Cl atom, there will be an -1 charge overall, so there are 18 electrons in Cl-. Similar line of arguments will give you the number of electrons in Mg2+ to be 10, and O2- to be 10.

      Between cation and anion — there exits ionic force
        + charge and - charge attract each other
      If the ion is consist of more than one atom — polyatomic ion

    Table salt, sodium chloride (NaCl), is an ionic compound. It consists of Na+ cation and Cl- anion.

    Vinegar is a solution of acetic acid and water. Acetic acid in water is partially ionic, consisting H+ ion and acetate ion (anion), as shown below.

    Phase of the compounds. You'll see often notation of chemical formula with its phase. NaCl(s) is solid phase of sodium chloride.

    Gas(g)gas phase
    Solid(s)sold phase
    Solution(aq)aqueous solution

    Chemical Formulas

    Chemical formulas — representing the composition of molecules and ions

    Atoms in a molecule are held by chemical bond.
    Several different chemical formulas

  • Molecular formula — shows exact number of elements
      Structural formula — shows how atoms are connected to each other
  • Empirical formula — simplest representation of molecule, derived from experiments
      For example, chemical formula for benzene is C6H6 and the ratio between C and H is 1:1. So, empirical formula of benzene is CH.

    Below is an example of formulas for water.

    Chemical formulas and structural formula of water

    The representation on the right is structural formula for describing the chemical bonding in the molecule. In the figure above, there are lines connected the oxygen atom to two hydrogen atoms. These lines represent chemical bonds.
    There are two types of chemical bonds:
      Covalent bond—sharing two electrons between two or more nuclei
      Ionic bond—force associated with two or more opposite charged ions bond

    Here represented in four ways, vanillin molecule. This molecule is responsible for the very pleasant aroma of vanilla.

    Chemical Formula Structural FormulaOrganic Chemists 3-D Ball-stick3d Space Filling

    Two models on the left will be used in CHM3, but other noteworthy ones are also listed. The representation used by organic chemists (labeled as Organic Chemists) the one you learn in Organic Chemistry next year. The 3-D representation can be done in a number of ways. Here we list two, ball and stick model and space filling model.

    Formula of Ionic Compounds

    As shown above, table salt, NaCl, has ionic character. It can be separated into Na+ and Cl- ions. Again, between Na+ and Cl- ions, there exists the ionic force that holds these ions. Interaction between ions are:
      Positive and Positive — Repulsive (repel one another)
      Negative and Negative — Repulsive (repel one another)
      Positive and Negative — Attractive (come together)
    So, when ionic compound is made, the ions of opposite charges are combined, and the resulting compound is electrically neutral.

    For example, magnesium bromide, where magnesium ion is Mg2+ and bromine ion (called bromide ion) is Br-, must be neutral, then there must be two Br- so that +2 charge on magnesium ion is cancelled. Thus, magnesium bromide is written MgBr2.

    Nomenclature of Binary Ionic Compounds

    Nomenclature — systematic naming scheme

    Binary compound — made up of metal cation and main group anion

      Use atom name for cation
        For iron, Fe2+ = ferrous, Fe3+ = ferric
      Use —ide ending for anion

    So sodium chloride is: NaCl. First one is cation Na+ and the latter is anion Cl-.

    Example: Name the following compounds. LiCl, MgCl2, BeO, AlCl3, Al2O3

      LiCl = lithium chloride
      MgCl2 = magnesium chloride
      BeO = Berrium oxide
      AlCl3 = aluminum chloride
      Al2O3 = aluminum oxide

    Example: Obtain the chemical formula for the following. Ferric chloride, Potassium telluride, Strontium bromide, Rubidium sulfide.

      Ferric chloride = FeCl3
      Potasium telluride = K2Te
      Strontium bromide = SrBr2
      Rubidium sulfide = Rb2S

    When the compound contains some number of atoms, we need to know how many. We use Greek-derived prefix for the purpose. For example, TiO2
    is commonly known as Titanium dioxide. Di- represents two. The following table lists the prefixes.

    numberprefix numberprefix
    1mono 11hendeca
    2di 12dodeca
    3tri 13triskadeca
    4tetra 14tetrakaideca
    5penta 15pentakaideca
    6hexa 16hexakaideca
    7hepta 17heptakaideca
    8octa 18octakaideca
    9nona 19nonakaideca
    10deca 20icosa

    Example: Give names of the following compounds. CO, CO2, N2O3, BrF3, S2F2

    CO — Carbon monoxide
    CO2 — Carbon dioxide
    N2O3 — Dinitrogen trioxide
    BrF3 — Bromine trifluoride
    S2F2 — Disulfur difluoride

    Naming with Polyatomic Ions

    Somewhat preserved systematic naming in compounds with polyatomic ions. However, many of them use common names. Important ones that you encounter in this course are listed in the following table. Most of them are anions.

    We have already talked about acetic acid above. Acetate ion is written as CH3CO2-. If acetate ion is combined with sodium ion, it would be called sodium acetate. Naming scheme is still the same.

    Nomenclature of Acid As described above, acid produces H+ in aqueous solution. For example, hydrochloric acid is HCl in water, therefore, it is written as HCl(aq). This notation of adding (aq) and having dissociable H+ tells that the species is an acid.

    Some of the common acids and anion produced from the acids are listed in the following table.

    Example: Name the following compounds. HCl(g), HCl(aq), NaOH(s), KMnO4(s), HNO3(aq), Fe(ClO4)3(s), H2SO4(aq)

      HCl(g) — hydrogen chloride
      HCl(aq) — hydrochloric acid
      NaOH(s) — sodium hydroxide
      KMnO4(s) — Potassium permanganate
      HNO3(aq) — nitric acid
      Fe(ClO4)3(s) — ferric perchlorate
      H2SO4(aq) — sulfuric acid

    Introduction to Organic Compounds

    The names of hydrocarbon species one carbon to 10 carbons. You can rotate the molecules around.