Periodicity - Ms. Suchy's science site · Periodicity Periodic table Periodic Trends Ms. Thompson - SL Chemistry Wooster High School Wednesday, July 15, 15

PeriodicityPeriodic table

Periodic Trends

Ms. Thompson - SL ChemistryWooster High School

Wednesday, July 15, 15

Topic 3.2Periodic 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.


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.


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


Periodic trends

Atomic radius

• Rely on bonding atomic radius (Rb) of elements• distance, d, between two nuclei of atom X • 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.


Periodic trends

Atomic radius• 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 radiiWednesday, July 15, 15

Periodic trends

Quick questionPredict the bond lengths in:

a) iodine monobromide, IBr

b) trichloromethane (chloroform), CHCl3


Periodic trends

Effective nuclear charge and screening effect

• 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.


Periodic trends


Estimate the effective nuclear charge experienced by the valence electrons in the alkali metal potassium.


Periodic trends



Periodic trends

Periodic trends in atomic radius

• 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


Periodic trends

Periodic trends in atomic radius

atom

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atomic radii decrease across a period


Periodic trends

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

ion charge examplecation + Na+ and Mg2+

anion - Cl- and O2-

Study tip:AOL

AniOn Larger


Periodic trends

Ionization energy

• 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:• X(g) --> X+

(g) + e- (kJ mol-1)• 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)


Periodic trends

Periodic trends in ionization energy

• Ionization energies vary across periodic table• Across periodic table, ionization energies increase

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

• Going down a group from top to bottom, ionization energies decrease1. As atomic radii increase down a group, Zeff is reduced making it easier to

remove a valence electron from an atom2. Shielding effect of the core electrons increases faster than nuclear charge

weakening the attractive force between nucleus and valence electrons


Periodic trends

ioni

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ionization energies increase across a period

Periodic trends in ionization energy

Trends in ionization energies are the opposite of the trends in atomic radius


Periodic trends

Electron affinity

• Electron affinity, Eea, is the energy required to detach an electron from a singly charged negative ion (anion) in the gas phase.• X-

(g) --> X(g) + e- (kJ mol-1)• 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(g) + e- --> X-

(g) (kJ mol-1)• 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


Periodic trends

Electron affinity


Periodic trends

Periodic trends in electron affinity

• 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.


Periodic trends

Electronegativity

• 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 periodWednesday, July 15, 15

Periodic trends

elec

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electronegativies increase across a period

Periodic trends in electronegativity

Trends in electronegativities are the opposite of the trends in atomic radius and the same for ionization energy & Zeff


Periodic trends

Met

allic

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Metallic character decreases down a period

Trends in metallic character are the same for atomic radius

Periodic trends in metallic and non-metallic character

• 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).


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: 1-Chalcogens: 2-


Periodic trends

Trends in properties of metals and non-metals

• 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


Periodic trends

Naming oxoanions and acidsFormula of oxoanion Non-systematic name

CO32- carbonate

C2O42- ethanedioate (oxalate)

NO2- nitrite

NO3- nitrate

SO32- sulfite

SO42- sulfate

PO33- phosphite

PO43- phosphate

ClO- hypochlorite

ClO2- chlorite

ClO3- chlorate

ClO4- perchlorate

OH- hydroxideWednesday, July 15, 15

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 acidClO3- chlorate HClO3 chloric acidClO4- perchlorate HClO4 perchloric acid


Periodic trends

Some interesting oxides• 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!• Al2O3(s) + 2NaOH(aq) --> 2NaAl(OH)4(aq) [Acting as an acid]• Al2O3(s) + 6HCl(aq) --> 2AlCl3(aq) + 3H2O(l) [Acting as a base]

Amphoteric and amphiprotic oxidesThe 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.


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


Periodic trends

Chemical properties within a group: Group 1, the alkali metals

• 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.


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.

Na 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)Both rubidium and cesium react explosively with water

Cs 2Cs(s) + 2H2O(l) --> 2CsOH(aq) + H2(g)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

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)


Periodic trends

Reactions between halogens and alkali metals

• 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• For example:

• Cl2(aq) + 2KBr(aq) --> 2KCl(aq) + Br2(aq)

Displacement reaction!


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.


Periodic trends

Reactions between halogens and halides

X2(aq) Cl-(aq) Br-(aq) I-(aq)

Cl2(aq) no reactionCl2(aq) + 2Br-(aq) --> 2Cl-(aq) + Br2(aq)

observation: yellow/orange solution due to formation of Br2(aq)

Cl2(aq) + 2I-(aq) --> 2Cl-(aq) + I2(aq)observation: dark red/brown solution due to formation of

I2(aq)

Br2(aq) no reaction no reactionBr2(aq) + 2I-(aq) --> 2Br-(aq) + I2(aq)

observation: yellow/orange solution due to formation of Br2(aq)

I2(aq) no reaction no reaction no reaction


Study tip!

1. The order of oxidizing ability for group 17 elements follows the order of electronegativity:

F > Cl > Br > I4.0 > 3.2 > 3.0 > 2.7F2 > 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.

χp:oxidizing ability:


Practice Problem... I Do ...

Construct a balanced equation, including state symbols, to explain the pH changes for the reaction of nitrogen dioxide with water.

2NO2 + H2O --> HNO2 + HNO3

• Finally, we include state symbols:

• 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(g) + H2O(l) --> HNO2(aq) + HNO3(aq)

• Because a mixture of acids is formed the pH of the solution will be less than 7


Practice Problem... We Do ...

1. How many of the following oxides are amphoteric?

Na2O, MgO, Al2O3, SiO2a) noneb) 1c) 2d) 4


Practice Problem... 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.

20 mins


Topic 3.2Periodic 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.


Documents

Periodicity - Ms. Suchy's science site · Periodicity Periodic table Periodic Trends Ms. Thompson - SL Chemistry Wooster High School Wednesday, July 15, 15