Download Periodic table Periodic Trends

Periodicity
Periodic table
Periodic Trends
Ms. Thompson - SL Chemistry
Wooster High School
Wednesday, July 15, 15
Topic 3.2
Periodic trends
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•
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Vertical and horizontal trends in the periodic table exist for
atomic radius, ionic radius, ionization energy, electronic affinity,
and electronegativity.
Trends in metallic and non-metallic behavior are due to the
trends above.
Oxides change from basic through amphoteric to acidic across
a period.
Wednesday, July 15, 15
Periodic trends
Nature of science
•
Looking for patterns - the position of an element in the periodic
table allows scientists to make accurate predictions of its
physical and chemical properties. This gives scientists the ability
to synthesize new substances based on the expected reactivity
of elements.
Wednesday, July 15, 15
Periodic trends
Atomic radius
•
Based on Bohr’s model of the atom, we would be able to easily
measure the radius of an element.
• However, Bohr’s model of an atom was too simplistic and
electrons are not fixed in orbit around nucleus.
• They are found in orbitals (regions of space) where there is a
high probability of finding an electron.
• Means we cannot find the radius of an element the same way
we measure the radius of a circle
• Must resort to other methods
Wednesday, July 15, 15
Periodic trends
Atomic radius
Rely on bonding atomic radius (Rb) of elements
• distance, d, between two nuclei of atom X
•
•
•
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i.e. diatomic atom X2 --> (H2)
Rb = 1/2d
{
•
d = 2Rb
Bonding atomic radius is sometimes termed covalent radius
For metals, the bonding atomic radius is 1/2d’ where d’ now
represents distance between two atoms adjacent to each other
in the crystal lattice of the metal.
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Periodic trends
Atomic radius
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Another term is non-bonding atomic
radius, Rnb.
Also known as van der Waals’ radius
Atoms are touching but not chemically
bonded
Rb < Rnb -- always!
Approximate bond length between two
elements can also be estimated from their
atomic radii.
• i.e. BrF
• Atomic radius of Br = 117pm
• Atomic radius of F = 60pm
• Bond length of BrF = 177pm
(experimental data* shows 176pm in gas phase)
* Section 9 of data booklet provides data for the covalent atomic radii
Wednesday, July 15, 15
Periodic trends
Quick question
Predict the bond lengths in:
a) iodine monobromide, IBr
b) trichloromethane (chloroform), CHCl3
Wednesday, July 15, 15
Periodic trends
Effective nuclear charge and screening effect
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•
•
In an atom the negatively charged electrons are attracted to the
positively charged nucleus (opposites attract) but at the same
the outer most electrons are repelled by other electrons.
The inner electrons reduce the positive attraction of the
nucleus felt by the outer electrons - shielding or screening
The net charge experienced by an electron is known as
effective nuclear charge, Zeff.
• Zeff = Z - S, where Z is the atomic number and S is the number of electrons shielding the
other electrons.
Wednesday, July 15, 15
Periodic trends
Effective nuclear charge and screening effect
Estimate the effective
nuclear charge experienced
by the valence electrons in
the alkali metal potassium.
Wednesday, July 15, 15
Periodic trends
Effective nuclear charge and screening effect
Wednesday, July 15, 15
Periodic trends
Periodic trends in atomic radius
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•
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Across periodic table left to right, atomic radii decrease
• Due to increasing effective nuclear charge, Zeff, going L to R
• Pulls valence electrons closer to nucleus
Down a group from top to bottom, atomic radii increase
• In each new period, atoms have an additional energy level
• This has a greater affect on Zeff as the electrons get further
and further from the nucleus
Transition metals do not change greatly across a period
• Due to outermost electrons are almost constant
• As electrons are added, the enter the n-l rather than the nth
energy level
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Periodic trends
atomic radii increase down a group
Periodic trends in atomic radius
atomic radii decrease across a period
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Periodic trends
ion
charge
example
cation
+
Na+ and Mg2+
anion
-
Cl- and O2-
Periodic trends in ionic radius
•
The radii of cations and anions vary from the parent atoms in
the following way:
• radii of cations are smaller - more protons than electrons so
valence electrons are strongly attracted to the nucleus, high Zeff
• radii of anions are larger - more electrons than protons so
greater repulsion between valence electrons, low Zeff
Study tip:
AOL
AniOn Larger
Wednesday, July 15, 15
Periodic trends
Ionization energy
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The ionization energy, IE, is the minimum energy required
to remove an electron from a neutral gaseous atom in its
ground state.
The first ionization energy, IE1, of a gaseous atom relates
to the process:
+ + e- (kJ mol-1)
X
-X
• (g) > (g)
• The second ionization energy relates to the removal of a further electron from the
ion X+(g) and the third ionization energy is associated with the removal of another
electron from ion X2+(g).
A positive number indicates that energy is absorbed during
this process (endothermic)
Wednesday, July 15, 15
Periodic trends
Periodic trends in ionization energy
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•
Ionization energies vary across periodic table
Across periodic table, ionization energies increase
•
Going down a group from top to bottom, ionization energies
decrease
1. As the Zeff increases L to R the valence electrons are pulled closer to
nucleus and thus becomes harder to remove an electron from the atom
2. As atomic radii decreases across a period the distance between the valence
electrons and nucleus is also decreased thus increasing the Zeff - making it
harder to remove an electron from the atom
1. As atomic radii increase down a group, Zeff is reduced making it easier to
remove a valence electron from an atom
2. Shielding effect of the core electrons increases faster than nuclear charge
weakening the attractive force between nucleus and valence electrons
Wednesday, July 15, 15
Periodic trends
ionization energies decrease down a group
Periodic trends in ionization energy
ionization energies increase across a period
Trends in ionization energies are the opposite of the trends in atomic radius
Wednesday, July 15, 15
Periodic trends
Electron affinity
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Electron affinity, Eea, is the energy required to detach an
electron from a singly charged negative ion (anion) in the gas phase.
- -- X + e- (kJ mol-1)
X
• (g) > (g)
Electron affinity is the energy released (Einitial - Efinal) when 1 mol of
electrons is attached to 1 mol of neutral atoms or molecules in the
gas phase
- -- X- (kJ mol-1)
X
+
e
(g)
> (g)
•
A negative number indicates that energy is released during this
process (exothermic)
The more negative the Eea value, the greater the is the attraction of
the ion for the electron
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Periodic trends
Electron affinity
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Periodic trends
Periodic trends in electron affinity
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Not as apparent as atomic radii and ionization trends
In general, across a period electron affinity values become more
negative (with some exceptions)
• i.e. N Eea is positive
Group 17 (halogens) have the most negative Eea values
Expected because gaining an electrons means element attains
noble gas configuration (stable!)
Electron affinity does not have an apparent trend down groups
• Group 1 (alkali metals) generally, Eea, becomes less negative.
Wednesday, July 15, 15
Periodic trends
Electronegativity
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Electronegativity (χ) is defined as the relative attraction
that an atom has for the shared pair of electrons in a covalent
bond.
Linus Pauling proposed concept of electronegativity and defined
it as “the power of an atom in a molecule to attract electrons to
itself.”
The Pauling Scale (χp) is found in section 8 of data booklet
• Fluorine is most electronegative element and has a value of
4.0
Across a period L to R, electronegativities increase because both
Zeff and atomic radii increase across a period
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Periodic trends
electronegativities decrease down a group
Periodic trends in electronegativity
electronegativies increase across a period
Trends in electronegativities are the opposite of the trends in atomic radius
and the same for ionization energy & Zeff
Wednesday, July 15, 15
Periodic trends
Periodic trends in metallic and non-metallic
character
Metallic character increases down a group
•
•
•
Metallic character decreases across a period and
increases down a group
Metals have low ionization energies - lose
electrons during chemical reactions (oxidized).
Non-metals show highly negative electron
affinities - gain electrons during chemical
reactions (reduced).
Metallic character decreases down a period
Trends in metallic character are the same for atomic radius
Wednesday, July 15, 15
Periodic trends
Periodic trends in metallic and non-metallic
character
Some common ions of metals and non-metals.
Alkali metals: 1+
Alkaline earth metals: 2+
Halogens: 1Chalcogens: 2-
Wednesday, July 15, 15
Periodic trends
Trends in properties of metals and non-metals
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An oxide is formed from the combination of an element with oxygen
Use the charge of a metal cation to deduce the chemical formula of a metal oxide:
• Na+ combines with O2- to form Na2O
• Ca2+ combines with O2- to form CaO
• Al3+ combines with O2- to form Al2O3
Oxides of metals are basic: they react with water to form metal hydroxides:
• CaO(s) + H2O(l) --> Ca(OH)2(aq)
• Na2O(s) + H2O(l) --> 2Na(OH)(aq)
Oxides of non-metals are acidic: they react with water to form acidic solutions:
• CO2(g) + H2O(l) --> H2CO3(aq) carbonic acid
• SO3(l) + H2O(l) --> H2SO4(aq) sulfuric acid
• SO2(g) + H2O(l) --> H2SO3(aq) sulfurous acid
• P4O10(g) + 6H2O(l) --> 4H3PO4(aq) phosphoric acid
Wednesday, July 15, 15
Periodic trends
Naming oxoanions and acids
Formula of oxoanion
CO32C2O42NO2NO3SO32SO42PO33PO43ClOClO2ClO3ClO4OHWednesday, July 15, 15
Non-systematic name
carbonate
ethanedioate (oxalate)
nitrite
nitrate
sulfite
sulfate
phosphite
phosphate
hypochlorite
chlorite
chlorate
perchlorate
hydroxide
Periodic trends
Naming oxoanions and acids
Formula of
oxoanion
Non-systematic
name
Formula of
oxoacid
Non-systematic
name
ClO-
hypochlorite
HClO
hypochlorous
acid
ClO2-
chlorite
HClO2
chlorous acid
ClO3-
chlorate
HClO3
chloric acid
ClO4-
perchlorate
HClO4
perchloric acid
Wednesday, July 15, 15
Periodic trends
Some interesting oxides
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Silicon dioxide, SiO2, does not dissolve in water. Classified as an acidic axide
because it reacts with sodium hydroxide NaOH to form sodium silicate,
Na2SiO3(aq).
• SiO2(s) + 2NaOH(aq) --> Na2SiO3(aq) + H2O(l)
Aluminum oxide, Al2O3, is classified as an amphoteric oxide. This means is can
react both as an acid and a base!
[Acting as an acid]
• Al2O3(s) + 2NaOH(aq) --> 2NaAl(OH)4(aq)
• Al2O3(s) + 6HCl(aq) --> 2AlCl3(aq) + 3H2O(l) [Acting as a base]
Amphoteric and amphiprotic oxides
The terms amphoteric and amphiprotic are often mixed up.
Amphoteric - a species that behaves as both an acid and a base i.e. Aluminum oxide
Amphiprotic - a specific type of amphoteric species. These species are either proton (H+) donors
or proton acceptors. i.e. self-ionizing solvents such as water, methanol, amino acids, and proteins.
Wednesday, July 15, 15
Periodic trends
Some interesting oxides
Formula
of oxide
Na2O(s)
MgO(s)
Al2O3(s)
SiO2(s)
P4O10(s)
SO3(l) and
SO2(g)
Nature of
oxide
basic
basic
amphoteric
acidic
acidic
acidic
Wednesday, July 15, 15
Periodic trends
Chemical properties within a group: Group 1,
the alkali metals
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Lithium Li, sodium Na, potassium K, rubidium Rb, cesium Cs,
and francium, Fr.
One valence electron
Form 1+ cations by losing one electron in chemical reactions
Going down group, atomic radius increases and ionization
energy decreases - reactions with water become more vigorous
down the group to form metal hydroxides.
Less energy is required to remove an electron from potassium,
K than from sodium, Na and so on.
Wednesday, July 15, 15
Periodic trends
Chemical properties within a group: Group 1,
alkali metals
Group 1
metal
Reaction with water
Description
Li
2Li(s) + 2H2O(l) --> 2LiOH(aq) + H2(g)
Lithium reacts slowly and floats on the water (due to it
low density). Bubbling is observed.
2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g)
Sodium reacts vigorously. Heat is evolved and the sodium
melts to form a ball of molten metal which whizzes around
on the surface of the water
K
2K(s) + 2H2O(l) --> 2KOH(aq) + H2(g)
Potassium reacts more vigorously than sodium: the
reaction is violent. It evolves enough heat to ignite the
hydrogen, so bursts into flames instantly. A characteristic
lilac-colored flame is observed.
Rb
2Rb(s) + 2H2O(l) --> 2RbOH(aq) + H2(g)
Cs
2Cs(s) + 2H2O(l) --> 2CsOH(aq) + H2(g)
Na
Wednesday, July 15, 15
Both rubidium and cesium react explosively with water
Periodic trends
Chemical properties within a group: Group 17, the
halogens
• Non-metals: fluorine, F, chlorine, Cl, bromine, Br, iodine, I, and asatine, At.
• Seven valence electrons (one electron away from a full valence)
• Gain an electron (reduced) in chemical reactions to obtain noble gas
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configuration.
Diatomic molecules, X2
F and Cl are gases, Br is liquid, and I and At are solids at room temp and
pressure
Form ionic compounds with metals (1- anions)
Form covalent compounds with non-metals
Highly reactive but decreases down the group. As atomic radius
increases it becomes harder to gain an electron (further from nucleus)
Wednesday, July 15, 15
Periodic trends
Reactions between halogens and alkali metals
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Halogens, X2, react with alkali metals, M(s), to form ionic alkali metal halide salts,
MX(s) of the form:
• 2M(s) + X2(g) --> 2MX(s)
For example:
• 2Na(s) + Cl2(g) --> 2NaCl(s)
Reactions between halogens and halides
A solution of a more reactive halogen, X2(aq), reacts with a solution of halide ions,
X-(aq), formed by a less reactive halogen
Represented by ionic equations
Displ
acem
For example:
e
nt
r
Cl
+
2KBr
-2KCl
+
Br
e
(aq) >
(aq)
2(aq)
• 2(aq)
acti
on!
Wednesday, July 15, 15
Study tip!
You can think of this displacement reaction as being a
competition between the chlorine in the bromine for an extra
electron. Remember that the atomic radius increases down a
group. The atomic radius of chlorine (100pm) is smaller than that
of bromine (117pm) so chlorine has a stronger attraction for a
valence electron than does bromine. Therefore chlorine forms
the chloride anion, Cl- more readily than bromine forms the
bromide anion, Br -. Going down group 17 the oxidizing
ability, that is, the ability to gain an electron, decreases.
Wednesday, July 15, 15
Periodic trends
Reactions between halogens and halides
X2(aq)
Cl2(aq)
Cl-(aq)
no reaction
Br-(aq)
I-(aq)
Cl2(aq) + 2Br-(aq) --> 2Cl-(aq) + Br2(aq)
Cl2(aq) + 2I-(aq) --> 2Cl-(aq) + I2(aq)
observation: yellow/orange solution due to formation of
Br2(aq)
observation: dark red/brown solution due to formation of
I2(aq)
Br2(aq)
no reaction
no reaction
I2(aq)
no reaction
no reaction
Wednesday, July 15, 15
Br2(aq) + 2I-(aq) --> 2Br-(aq) + I2(aq)
observation: yellow/orange solution due to formation of
Br2(aq)
no reaction
Study tip!
1. The order of oxidizing ability for group 17 elements follows
the order of electronegativity:
F > Cl > Br > I
χp: 4.0 > 3.2 > 3.0 > 2.7
oxidizing ability: F2 > Cl2 > Br2 > I2
2. Be careful with the term observation when describing a
chemical reaction . An observation is something you can
directly witness., such as bubbles of gas, the color of a solution,
or a precipitate forming. The formation of a gas is not in itselfan
observation.
Wednesday, July 15, 15
Practice Problem
... I Do ...
Construct a balanced equation, including state symbols, to explain
the pH changes for the reaction of nitrogen dioxide with water.
• Nitrogen is a non-metal and therefore may form an acidic oxide. Reacts with
water to form a 1:1 mixture of nitrous acid, HNO2(aq) and nitric acid, HNO3(aq).
• We write the balanced chemical equation:
2NO2 + H2O --> HNO2 + HNO3
• Finally, we include state symbols:
2NO2(g) + H2O(l) --> HNO2(aq) + HNO3(aq)
• Because a mixture of acids is formed the pH of the solution will be less than 7
Wednesday, July 15, 15
Practice Problem
... We Do ...
1. How many of the following oxides are
amphoteric?
Na2O, MgO, Al2O3, SiO2
a) none
b) 1
c) 2
d) 4
Wednesday, July 15, 15
Practice Problem
in
20 m
s
... You Do ...
1. The periodic table shows the relationship between electron
arrangement and the properties of elements and is a valuable
tool for making predictions in chemistry.
a) Identify the property used to arrange the elements in the
periodic table.
b) Outline two reasons why electronegativity increases
across period 3 in the periodic table and one reason why
noble gases are not assigned electronegativity values.
Wednesday, July 15, 15
Topic 3.2
Periodic trends
➡Vertical and horizontal trends in the periodic table exist for
atomic radius, ionic radius, ionization energy, electronic affinity,
and electronegativity.
➡Trends in metallic and non-metallic behavior are due to the
trends above.
➡Oxides change from basic through amphoteric to acidic across
a period.
Wednesday, July 15, 15