Chapter 2 Atoms Molecules and Ions
Atomic Theory of Matter
Democritus – Proposed the idea that matter is made up of small, indivisible
particles (atoms from the Greek atomos indivisible). This idea not accepted by
his contemporaries (e.g., Plato, Aristotle).
Evidence mounted for the existence of atoms Note => 1808; John Dalton
formulated his ideas (atomic theory); basically 3 postulates.
DALTON'S ATOMIC THEORY
1. Elements made up of ATOMS. All atoms of same element are alike;
atoms of different elements are different.
2. Chemical Reactions: Separation and joining of atoms. No atom is
created or destroyed, no conversion of an atom from one element to
– explains LAW OF CONSERVATION OF MASS
3. A chemical COMPOUND is the result of the combination of 2 or more
elements in a simple numerical ratio.
– explains LAW OF DEFINITE PROPORTIONS
Notes on Dalton’s Atomic Theory
1. Properties of 1 atom of Na different from 1 atom of Cl but the properties of 1
atom of Na are no different than 1 gram of Na (we just have more of the
2. We can't have compounds like CO½; CH3/5; OH1/6; CO1/3.
3. Alternate statement for the law of conservation of mass
Based on Dalton, we can define an atom as the basis unit of an element that
still retains the properties characteristic of the element (can enter into
Atoms are not the smallest structures; they possess an internal structure.
We now know that atoms can be split and created in nuclear reactions.
Atoms consist of protons, neutrons, and electrons; these are the "sub–
J.J. Thomson's cathode ray tube experiment determined the mass/charge
ratio of the electron.
Later experiments by R.A. Milliken estimated the electronic charge as 1.60 x
Thomson's number i.e., – 1 g / 1.76 * 108C * 1.60 x *19 C
= 9.09 * 10–28g
X–Rays and Radioactivity
Radioactivity – Spontaneous emission of particles and/or radiation.
Radioactive substances (radionuclides) break down or decay.
Roentgen – Cathode rays striking glass and metals resulted in new and
unusual rays (x–rays).
Becquerel – Certain uranium compounds darkened photographic plates.
The radiation resembled X–rays, but it didn't consist of particles [–rays
(gamma)] (Marie Curie suggested the term "radioactivity” for this
Note – units of radioactivity are the Becquerel (Bq) and the Curie (Ci)
named in honour of these pioneers in radioactive chemistry.
– particle – 24He (Mostly from Rutherford’s work)
– particle – e–
– ray – energy released when the metastable radionuclide returns to
the ground state
Proton and Nucleus
Thomson's Model – Uniform, positive sphere in which electrons are
Rutherford's Model – the result of the Rutherford, Marsden, and Geiger
The – Scattering Experiment
Since most of the – particles passed through the gold fail undeflected, he
concluded that the nucleus was only a small fraction of the total volume of
the atom, but it did contain most of the mass. The electrons occupied the
large volume outside the nucleus.
Mass of proton ~2000 times mass of electron.
FACT: He: H problem Mass 4:1 (1840) charge 2:1
Chadwick – 1932; solved the problem of the "missing nuclear mass” by
proposing the existence of neutrons in the nucleus.
FACT: Mass of neutrons = mass of protons, however, the neutrons possess
no charge, while the proton possesses a charge of +1.
Protons and electrons have the same magnitude of charge, but opposite sign
Mass Relationships of Atoms
A = mass number = number of protons + neutrons
Z = atomic number = number of protons
number of neutrons = A – Z
A bromine atom with 35 protons and (79–35) = 44 neutrons
Technetium–99 (a synthetic element) – used heavily in nuclear
medicine; 43 protons and 56 neutrons
Technetium – 97 – technetium atom with 43 protons 54 neutrons
Isotopes – atoms of a given element that differ in the number of neutrons,
and hence, the mass number A.
An atom of a specific isotope is called a nuclide
e.g. three isotopes of hydrogen
1 H Protium – name is rarely used for atomic hydrogen.
1 H Deuterium – the D in D2O or heavy water.
1 H Tritium – radioactive isotope of hydrogen.
Molecules and Chemical Formulas
Molecules – aggregates of atoms joined together in a definite arrangement
by chemical forces
Chemical Formula – combination of symbols representing the atoms in a
molecule of a compound. Describes not only the type of atoms but also the
number of atoms, e.g., H2O2.
Molecular Formula – the exact # of atoms of each element in a molecule;
CO2; N2O; H2O; O2; N2; H2; HBr.
diatomic molecules – two – atoms; e.g., O2; N2; H2
polyatomic – molecules with more than two atoms; e.g., MnO2; MnCl2; O3;
Allotropes – different forms of the same element
O2; O3 (diamond, graphite, "buckyballs")
Molecule vs. compound
Molecule – unit of substance composed of two or more atoms of the same
element or of different elements.
Compound – a substance composed of atoms of two or more elements. 64
Cl2; H2 – they are molecules but not compounds.
NH3 – molecule and compound.
Empirical Formula – Expresses the simplest whole number ratio of the
atoms in a substance.
H2O2 – ratio of H:O = 2:2 or 1:1; empirical formula = HO
N2H4 – ratio of H:N = 4:2 or 2:1; empirical formula = NH2
Many molecules have the same molecular (or true) formula and the empirical
formula, e.g., H2O.
Often we write the molecular formula showing how its atoms are joined
together – structural formula
Ions and Ionic Compounds
Ions – atoms or groups of atoms that possess a charge. Ions are formed
when electrons are add or removed form a neutral atom (or molecule).
Example Li atom, Z = 3, 3 protons 3 electrons
3 protons 2 electrons; cation – positively charged ion.
Ca2+ (20 protons 18 electrons)
Br(atom) 35 protons; 35 electrons
Br 35 protons; 36 electrons; anion – a negatively charged ion.
monatomic ions – ions that contain only one type of atom
polyatomic ions – ions containing two or more types of atoms, e.g., SO42–
(sulfate ion), CO32– (carbonate), NH4+ (ammonium ion) SO32– (sulphite), OH–
(hydroxide ion). (note: Table 2.4 in text, very important).
Compounds containing cations and anions are ionic compounds e.g. solid
sodium chloride (NaCl) contains an equal # of anions (Cl–) and cations (Na+),
i.e., the solid compound is electrically neutral.
In most cases, ionic compounds contain an metallic ion as the cation and a
non–metallic ion as the anion. exception NH4+ (IMPORTANT EXCEPTION)
Na Na+ +e– Na+ + Cl– NaCl (ionic compound) note that discrete units of
NaCl don't really exist.
We generally write NaCl for the ionic compound formed between Na and Cl;
the formula unit = NaCl
NOTE: when we write the empirical formula for an ionic compound, we
must know the changes of the ions comprising the compound. All we
have to do is keep the total # of negative charges equal to the total # of
NaCl 1 Na+ 1 Cl–
BaCl2 1 Ba2+: need 2 Cl– ions to to preserve electrical neutrality
CaCl2 Ca Ca2+ + 2 e–; Cl + e– Cl–