Download Atoms, Molecules and Ions

Atoms, Molecules and Ions
Chapter 2 - A.P. Chemistry
Laws Governing Atoms
• law of definite proportions
• law of multiple proportions
• law of conservation of mass
Laws Governing Atoms
• law of definite proportions
• in a compound the proportion of
mass of each element is consistent
• law states that the elemental
composition of a pure substance is
always the same
Laws Governing Atoms
• law of multiple proportions
• when elements combine to form different
compounds the ratio of elements is
always a small whole number
• law states that when two elements form a
series of compounds the ratios of the
masses to second element to 1g of the first
element can always be reduced to small
whole numbers
Laws Governing Atoms
• law of conservation of mass
• matter cannot be created or destroyed
• since all matter has mass, this must be
conserved as matter is conserved
• mass can be neither created or
destroyed by a chemical reaction
Atomic Theory
• Dalton’s Atomic Theory
•
each element is composed of extremely small particles called
atoms
•
all atoms of a given element are identical, the atoms of one
element are different than atoms of another element
•
compounds are formed when atoms of more than one
element combine, a given compound always has the same
number of elements and atoms
•
atoms of one element cannot be changed into atoms of
another element by chemical means, a chemical reaction
involves only separation, combination or rearrangement of
atoms, they cannot be created or destroyed
Atomic Theory
• Democritus
• Bohr
• Dalton
• Thomson
• Proust
• Schrodinger
• Rutherford
• Lavoisier
• Boyle
• History’s Key
Players
Atoms
• the smallest particle of an element that
retains the characteristic chemical
properties of that element
Atoms
• electrons - negatively charged particles
of insignificant mass orbiting around
the nucleus
• nucleus - small dense center of an atom
• protons - positively charged particles
with a mass of 1 amu
• neutrons - neutral particles with a
mass slightly higher than that of a
Atoms
• Radioactivity
• spontaneous breakdown of an atom
by emission of particles and/or
radiation
• Alpha, Beta or gamma rays
• end result of the breakdown is a
stable element
Atoms
• Radioactivity
• Alpha rays - positively charged
particles called alpha particles
• Beta rays - electrons (negative)
particles called beta particles
• Gamma rays - no charge, very high
energy waves
Atoms
• Numbers
• atomic number
• atomic mass
• mass number
Atoms
• atomic number - number of protons in
the nucleus of each atom of an element
Atoms
• mass number - the total number of
neutrons and protons present in the
nucleus of an element
• all nuclei have protons and neutrons
• exception is hydrogen
Atoms
• mass number = # protons + # neutrons
• mass number = atomic number +
number of neutrons
Atoms
• Isotopes - atoms that have the same
atomic number but different mass
numbers
• same number of protons, different
number of neutrons
Atoms
11
6C
12
6C
13
6C
14
6C
• How many protons, neutrons, and electrons are in (a) an atom of 197Au; (b) an atom of stron:um-­‐90?
Atomic Mass
• Average mass is calculated from the
isotopes of an element weighted by
their relative abundances.
• Because in the real world we use large
amounts of atoms and molecules, we
use average masses in calculations.
Atomic Mass
• Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine.
Atoms
• Give the chemical symbol, including mass number, for each of the following ions:
(a) The ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and 18 electrons.
Atoms
• The diameter of a US penny is 19 mm. The diameter of a silver atom, by comparison, is only 2.88 Å. How many silver atoms could be arranged side by side in a straight line across the diameter of a penny?
Periodic Table
• It is a
systematic
catalog of
elements
• Elements are
arranged in
order of
atomic
number
Periodic Table
• patterns of chemical and physical
properties can be seen in the order of
the elements.
Periodic Table
• the rows on the
periodic chart are periods
• columns are groups or families
• elements in the same group have
similar chemical properties
Periodic Table
• these five groups are known by their
names
Periodic Table
• nonmetals are
on the right side
of the table
(exception is H)
Periodic Table
• metalloids
border the stair
step line (except
Al, Po, At)
Periodic Table
• metals are on the
left side of the
table
Molecules and Ions
• only the noble gases (group 8A) of the
periodic table exist in nature as single
atoms
• monatomic (single or one atom)
Molecules and Ions
• molecule - a grouping of at least two
atoms in a definite arrangement held
together by chemical forces (chemical
bonds)
Chemical Formulas
• molecular
compounds are
compounds
composed of
molecules and
almost always
contain only
nonmetals
Diatomic Molecules
• these seven elements occur naturally as
molecules with two atoms
Types of Formulas
• Empirical formulas give the lowest
whole-number ratio of atoms of each
element in a compound.
• Molecular formulas give the exact
number of atoms of each element in a
compound.
• Example: B2H6 vs. BH3
Types of Formulas
• Structural formulas show the order in
which atoms are bonded.
• Perspective drawings also show the
three-dimensional array of atoms in a
compound.
Formulas
• various structures can be
used to model the same
formula
• When atoms lose or gain electrons, they
become ions.
• Cations are positive and are formed by
elements on the left side of the periodic chart.
• Anions are negative and are formed by
elements on the right side of the periodic
chart.
Ionic Compound
• Ionic compounds (such as NaCl) are
generally formed between metals and
nonmetals, cations and anions
• Because compounds are electrically neutral, one
can determine the formula of a compound this
way:
• The charge on the cation becomes the subscript
on the anion.
• The charge on the anion becomes the subscript
on the cation.
• If these subscripts are not in the lowest wholenumber ratio, divide them by the greatest
common factor.
Common Cations
Common Anions
Practice Problem
• What are the empirical formulas of the
compounds formed by
(a) Al3+ and Cl– ions,
(b) Al3+ and O2– ions,
(c) Mg2+ and NO3– ions?