A.P. Chemistry: Notes on Chapter 2
“The properties of matter that we can see are explained by the properties and behavior of
atoms that we cannot see.”
Atomic Structure:
Subatomic particles: electron (negative charge -1, mass = 9.109 x 10-28g, about 1/1800 of mass
of a protom), proton (positive charge +1, mass = 1.673 x 10-24g ), neutron (neutral, mass 1.675
x 10-24g).
Nucleus: contains protons and neutrons, very small, almost all the mass of the atom
Electrons: surround nucleus, much smaller mass, most of the volume of the atom, largely
determine the chemical properties of the atom
Atom: electrically neutral – number of protons and electrons is equal
Atomic Number, Z = number of protons in the nucleus
The mass of an atom is standardized to the mass of a carbon atom which has 6 p and 6 n and
therefore is defined to have a mass of 12 atomic mass units (amu). One amu=1/12 themass of a
C atom and has a mass of 1.66 x 10-24 g. 1n and 1p each weigh 1 amu.
Mass Number , A = the sum of the number of p and n in an atom
𝑚𝑎𝑠𝑠 𝑛𝑢𝑚𝑏𝑒𝑟
23
Notation: 𝑎𝑡𝑜𝑚𝑖𝑐
𝑛𝑢𝑚𝑏𝑒𝑟𝐸𝑙𝑒𝑚𝑒𝑛𝑡 𝑠𝑦𝑚𝑏𝑜𝑙 Type equation here.e.g. 11𝑁𝑎
Isotopes are elements having the same Z but different A (because they have different numbers of
neutrons). All atoms of the same element have the same number of protons.
Hydrogen isotopes: protium 11𝐻 , deuterium 21𝐻 , tritium 31𝐻
Percent Abundance: the number of atoms of a particular isotope found in a natural sample of
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
that element. Calculated: 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 𝑎𝑙𝑙 𝑖𝑠𝑜𝑡𝑜𝑝𝑒𝑠 𝑥 100%
Atomic mass = the weighted average mass of all isotopes of an element
Weighted Average Atomic Mass =
% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝑜𝑓 𝑖𝑠𝑜𝑡𝑜𝑝𝑒 1
% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝑜𝑓 𝑖𝑠𝑜𝑡𝑜𝑝𝑒 2
(𝑚𝑎𝑠𝑠 𝑜𝑓 𝑖𝑠𝑜𝑡𝑜𝑝𝑒 1) +
(mass of isotope 2)
100
100
Go through Example problem 2.3, p. 57
Periodic Table: elements are arranged in ascending order by Z.
Be able to locate: metals, nonmetals, metalloids, periods 1-7, families or groups, main group
elements, transition metals
Characteristics of metals: soilids (except Hg), conduct electricity, ductile, malleable, can form
alloys, have metallic luster
Characteristics of nonmetals: solid, liquid, or gas at room T, do not conduct electricity
Metalloids: B, Si, Ge, As, Sb, Te, some act as semiconductors
History of organization of the periodic table:
Dmitri Mendeleev (1869) realized that if elements were arranged in order by atomic mass, there
was a periodicity in the properties of the elements. He arranged the elements leaving space to
retain his periodicity and deduced that these spaces belonged to elements which had not yet been
discovered.
Law of Periodicity: properties of elements are periodic functions of the atomic number
The most important inference form the periodic table is that elements in a group react w tih other
elements to form similar compounds.
Allotropes: nonmetal element which can exist in several different, distinct forms (carbon –
graphite, diamond, buckyball
Molecule – the smallest unit which a molecular compound can be divided and still retain the
properties of the compound
Ionic compounds – consist of positively charged cation (formed when an atom or group of
atoms loses electrons) and negatively charged anion (formed when an atom or group of atoms
gains electrons). The ratio of ions results in a compound which is electrically neutral.
Metals form cations, nonmetals form anions. In the formula, the cation is written first, followed
by the anion.
Monoatomic ions – for main group elements, charges are determined by location on the periodic
table (see figure 2.18). Transition metal ions are exceptions and may have more than one
charge; if in doubt, the charge of most transition metals is +2.
H ion may have a +1 or a -1 charge since it can gain or lose its one electron
When forming ions, atoms gain or lose electrons to achieve the same number of electrons as the
nearest noble gas.
Polyatomic ions: two or more atoms which bond covalently and have a collective charge.
You should know all of the polyatomic ions in Table 2.4.
You should be able to write formulas for ionic compounds by balancing charges of the ions.
Cation names are generally the name of the metal unless they have more than one charge.
In this case the metal name is followed by the charge in Roman number, e.g. Iron (III). In the
old Latin naming system the lower charged ion ends in “ous” and the higher charged ion ends in
“ic”, e.g. ferrous v. ferric.
Anion names for monoatomic ions contain the stem of the atom name, but the ending is changed
to “ide,” e.g. fluorine  fluoride
Polyatomic oxoanions are named by the number of oxygens. The “ite” ending has one less
oxygen atom than the “ate” ending. (See p. 77 for more detail.)
Coulombs Law refers to the force of attraction between ions in a compound. It states that the
greater the magnitude of the charges of the ions, the greater the forces of attraction and the
smaller the ions, the greater the forces of attraction (because they are closer together).
Ionic compounds form solid crystal lattices. It takes a great deal of energy to break this lattice
and therefore the melting points of ionic compounds are very high. Ionic compounds break along
a sharp boundary when struck with a hammer.
Molecular compounds: may be solid, liquid or gas and generally consist of a combination of
nonmetals. Because the ratio of atoms in the compound can change, prefixes are used to indicate
the numbers of atoms present (e.g. carbon monoxide, carbon dioxide)
Mono: 1
Di: 2
Tri: 3
Tetra: 4
Penta: 5
Hexa: 6
Hepta: 7
Octa: 8
Nona: 9
Deca: 10
The Mole: term was used by Wilhelm Otwald and is Latin for heap or pile
Mole = the amount of substance that contains as many particles as there are in exactly 12 g of
C-12 = 6.022 x 1023 particles. Amedeo Avodagro conceived of the concept of 1 mole but did not
actually determine the number.
Type equation here.
1 mol of unpopped popcorn would cover the U.S. nine miles deep. If a computer counted 10
million atoms / second, it would take 2 billion ears to count all the atoms in a mol.
Molar mass, M = the mass in grams of 1 mol of any element
The molar mass is equivalent to the atomic mass, but the unit is g/mol instead of amu..
To convert:
𝑔𝑟𝑎𝑚
Mols to mass: 𝑚𝑜𝑙 (1 𝑚𝑜𝑙 ) = 𝑔𝑟𝑎𝑚
1 𝑚𝑜𝑙
Mass to mol: 𝑔𝑟𝑎𝑚 (𝑔𝑟𝑎𝑚𝑠) = 𝑚𝑜𝑙
6.022 𝑥 1023
Mol to particles: 𝑚𝑜𝑙 (
1 𝑚𝑜𝑙
) = # 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠
When using molar mass in calculations, use one more significant figure in the molar mass than in
any of the data given in the problem.
𝒅𝒆𝒔𝒊𝒓𝒆𝒅 𝒖𝒏𝒕
For any conversion: 𝑮𝒊𝒗𝒆𝒏 𝒖𝒏𝒊𝒕 ( 𝒈𝒊𝒗𝒆𝒏 𝒖𝒏𝒊𝒕 ) = 𝒂𝒏𝒔𝒘𝒆𝒓 𝒊𝒏 𝒅𝒆𝒔𝒊𝒓𝒆𝒅 𝒖𝒏𝒊𝒕
The conversion factor used must be equal to one.
Percent composition, empirical and molecular formulas – we will go over this in class.