Download Atoms, molecules and ions

Atoms, molecules
and ions
Dalton’s Atomic Theory (1808)
• The elements are formed of
extremely small particles,
called atoms. All the atoms of
a given element are identical.
The atoms of an elements are
different than those of all
other elements.
• Compounds are formed of
more than one element. In all
compounds, the ratio between
the number of atoms of two
elements is either an integer
or a simple fraction .
Dalton’s Atomic Theory (1808)
• A chemical reaction is the
separation, combination, or
rearrangement of atoms; it
causes neither the destruction
nor the creation of atoms.
• N.B. Dalton did not know the
structure of the atom (i.e.,
electron, proton, neutron,
nucleus, etc.)
- Dalton imagined an
atom was small and
indivisible
Dalton’s Atomic Theory (1808)
• Dalton’s work explained some
empirical observations
– The law of definite proportions
– The law of multiple proportions
– The law of conservation of mass
The Structure of the Atom: the Electron
• With a cathode ray tube, the
existence of a subatomic
particle with a negative charge
was discovered by
Thomson…the electron.
• Thomson established the
charge/mass ratio of the
electron.
The Structure of the Atom: the Electron
• Millikan established the
charge of an electron in the
early 1900’s (-1.60 x 10-19 C)
• With the help of Thomson’s
work, Millikan deduced the
mass of an electron
mass 
charge
charge
mass
 1.60 10 19 C
 28


9.09

10
g
8 C
 1.76 10
g
Radioactivity
• Radioactivity: the spontaneous
emission of particles and/or radiation
• Three types of particle/rays are
produced by the decay of radioactive
substances :
– α particles (helium nuclei)
– β particles (electrons)
– γ rays (rays of high energy, and
no charge)
The Structure of the Atom: the Proton and the Nucleus
• Before 1910, Thompson’s “plum
pudding” model of the atom was
the accepted model.
• In this model, the electrons are
dispersed within a much bigger
sphere of uniform and positive
matter.
– This is “logical" considering
the small mass of the electron
compared to the overall mass
of the atom
The Structure of the Atom: the Proton and the
Nucleus
• Rutherford bombarded thin
sheets of gold with α particles
• According to the “plum
pudding” model, α particles
should pass through without
their trajectories being greatly
affected
• However, the trajectories of
some particles are greatly
perturbed
The Structure of the Atom: the Proton and the Nucleus
• In Rutherford’s experiment, the deviations are the result of
large repulsions
• Because α particles are positively charged, the positive
charge in the atom must be concentrated in a very small and
solid nucleus in the center of the atom (about 10-13 of the
volume of the atom, but almost 100% of the mass)
• The positive charges that are located in the nucleus are the
protons
• The charge of a proton is the same as the electron, but with
the opposite sign (i.e., positive)
• The mass of the proton is 1.67252 x 10-24 g (about 1840
times that of the electron)
The Structure of the Atom: the Neutron
• Before 1932, a great mystery was the fact that H had one
proton and He had two, but He was four times more massive
than H!
• Many people predicted the existence of the neutron, but it
was Chadwick that discovered it in 1932
• The neutron has no charge and is slightly larger than the
proton (1.67493 x 10-24 g for the neutron vs 1.67252 x 10-24 g
for the proton)
Atomic number, Atomic Mass, and Isotopes
• Atomic number: The number of protons in the nucleus
of each atom of a given element
• Atomic mass: The total number of neutrons and
protons contained in the nucleus of an atom
• All atoms of an element have the same number of protons,
but not necessarily the same number of neutrons
• Atoms that have the same atomic number but different
atomic masses are called isotopes
Atomic number, Atomic Mass, and Isotopes
• To identify an isotope, we use the notation AZ X , where A is the
atomic mass, Z is the atomic number and X is the element in
question
• Isotopes of an element have essentially the same chemistry since
the number of neutrons has little influence on the chemistry of an
element
• We use the atomic mass to identify isotopes (e.g..; 235
is
92 U
uranium 235 and 238
is uranium 238)
92 U
• Hydrogen has three isotopes (only hydrogen has special names for
its isotopes)
hydrogen:
1
1
H
deuterium:
2
1
H
tritium:
3
1
H
The Periodic Table
• Table in which elements are
grouped based on similar physical
and chemical properties.
• The horizontal rows are called
periods
• The vertical columns are called
groups (or families)
• Three categories of elements:
– metal (good conductor)
– non-metal (poor conductor)
– metalloid (between metal and
non-metal)
Molecules and Ions
• A molecule is an assembly of at least two atoms held
together in a fixed arrangement, by chemical forces
• An ion is an atom or molecule that has gained or lost
electrons
– Cation: ion with a positive charge
– Anion: ion with a negative charge
• An ionic compound is a composed of cations and anions
– e.g..; NaOH(s) is a group of Na+ and OH- ions and
there are no distinct molecules of NaOH
Chemical Formulas
• Molecular formula: indicates the exact number of atoms of each
element contained in the smallest unit of a substance
• Empirical Formula: indicates the lowest whole number ratio of
atoms present in a substance
• e.g.; hydrogen peroxide is a molecule that contains two atoms
of oxygen and two atoms of hydrogen.
• The molecular formula is H2O2
• The empirical formula is HO
• For many molecules, the molecular and empirical formulas are
identical (e.g.; H2O)
Chemical Formulas
• For an ionic compound, there are
no distinct molecules, so we use
their empirical formula to identify
them.
• In the figure, each Na+ is
associated with six Cl-, each Cl- is
associated with six Na+, and the
whole solid is electrically neutral.
• Other ionic compounds may have
different structures and different
charges (as long as the crystal
remains electrically neutral)
– ex.; Al3+ and O2- forms Al2O3
Nomenclature of Inorganic Compounds:
Ionic Compounds
• For ionic compounds, we name the cation followed by the anion
• The anion or the cation is sometimes a polyatomic ion
– ex.; NH4+ : ammonium
CO32- : carbonate
OH- : hydroxide
PO43- : phosphate
NO3- : nitrate
SO42- : sulfate
NO2- : nitrite
SO32- : sulfite
• ex.;
KBr : potassium bromide
Al2O3 : aluminium oxide
KCN : potassium cyanide
ZnI2 : zinc iodide
NaOH : sodium hydroxide
NH4Cl : ammonium chloride
Nomenclature of Inorganic Compounds:
Covalent Compounds
• Covalent compounds consist of distinct molecules
• For a binary covalent compound, we name the first element in
the formula first
– ex.; HCl : hydrogen chloride
NO : nitric oxide
SiC : silicon carbide
Nomenclature of Inorganic Compounds:
Covalent Compounds
• When two elements form several compounds, Greek prefixes
are used to specify the number of atoms of each element
– e.g.; CO : carbon monoxide
CO2 : carbon dioxide
NO2 : nitrogen dioxide
N2O4 : dinitrogen tetroxide
• Many covalent compounds containing hydrogen are referred to
by their common name
– e.g.; B2H6 : diborane
CH4 : methane
NH3 : ammonia
H2O : water
SiH4 : silane
PH3 : phosphine
Nomenclature of Inorganic Compounds:
Acids and Bases
• A definition of an acid is: a substance that releases an H+ when
dissolved in water.
• If the acid is not an oxyacid, i.e., does not contain oxygen, we use
the prefix “hydro” and the suffix “ic”
– ex.; HF : hydrofluoric acid
HI : hydroiodic acid
H2S : hydrosulfuric acid
• N.B. HCl(g) is hydrogen chloride, but once it is dissolved in
water, it become hydrochloric acid and releases H+
Nomenclature of Inorganic Compounds:
Acids and Bases
• For an oxyacid (general formula HmXOn) it often happens that there
are multiple possible values of n for each element X, and as such,
within this series of compounds,
– There is always an acid in the series that ends with “ic”
• Adding another oxygen to the “ic” acid produces the “per….ic”
acid
• Removing an oxygen atom to the “ic” acid produces the “ous”
acid
• Removing two oxygen atoms to the “ic” acid produces the
“hypo…ous” acid
Nomenclature of Inorganic Compounds:
Acids and Bases
• ex.;
H2CO3 : carbonic acid
HNO3 : nitric acid
HNO2 : nitrous acid
H2SO4 : sulfuric acid
H2SO3 : sulfurous acid
HClO4
HClO3
HClO2
HOCl
:
:
:
:
perchloric acid
chloric acid
chlorous acid
hypochlorous acid
Nomenclature of Inorganic Compounds:
Acids and Bases
• When an oxyacid loses its H+ to produce an oxyanion
– The “ic” acids produce the “ate” anions
e.g.; ClO4- : perchlorate
ClO3- : chlorate
– The “ous” acids produce the “ite” anions
e.g.; ClO2- : chlorite
OCl- : hypochlorite
• When not all of the possible H+ ions are lost, we have to specify
the number of H’s present
e.g.; H3PO4 : phosphoric acid
H2PO4- : dihydrogenphosphate
HPO42- : hydrogenphosphate
PO43- : phosphate