IB Chem: Topic 3

Modern Periodic Law

When elements are arranged in order of increasing atomic number, their properties shoow a periodic recurrence and gradual change

Periodic Recurrence (Periodicity)

Elements in the same groups have similar properties

Gradual change

The properties of elements gradually change as you move down a group or across a period (trends)

Physical Properties

Description of a substance that does not involve changing its chemical composition

Chemical Properties

Description of how a substance behaves as its chemical composition is changed

Nuclear Charge

total positive charge in the nucleus (number of protons)

Shielding Effect, ShE

The interference between the attraction of the nucleus and valence electrons. It is provided by core electrons and the distance between the nucleus and Ve- (# E-levels)

Effective Nuclear Charge, Zeff

The net force of attraction experienced by Ve- of the nucleus (Z-Ce-)

Zeff _ across a period because…

increases, the number of shielding electrons remain the same while the magnitude of positive charge in the nucleus increases

Zeff _ down a group because…

remains constant, the magnitude of positive charge in the nucleus and the number of shielding electrons increase proportionally

ShE _ across a period because…

remains relatively constant, the number of shielding electrons and energy levels do not change

ShE down a group because…

increases, there is an increase in the # of shielding electrons

Atomic radius across a period because…

decreases, Zeff increases due to additional p+, but the distance over which it has to act and the # of Ce- remains relatively constant. Therefore an increase in attraction b/n the nucleus and Ve- leads to a decrease in atomic radius.

Atomic radius _ down a group because…

increases, # of E levels increase down a group resulting in an increase in the distance b/n nucleus and Ve-. Zeff remains constant, while ShE (Ce- + E levels) increases.

Cations have a _ radius than their parent atom because…

smaller, the loss of an energy level decreases the number of shielding electrons therefore the positive charge of the nucleus has to act over a short distance to attract Ve-

Anions have a _ radius than their parent atom because…

larger, the extra Ve- contribute to e-e repulsion, therefore taking up more space.

Atomic radius

half the distance between neighbouring atoms’ nuclei.

First Ionization Energy

the energy required to remove one mole of electrons from one mole of gaseous atoms

Electronegativity

The measure of the attraction of an atom for a pair of bonding electrons.

Electron Affinity

The energy change (+ or -) when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous ions.

Ionization Energy _ down a group because…

Decreases, Ve- are less attracted to the nucleus due to an increase in ShE; meaning there is a greater distance b/n the nucleus and the Ve’ while Zeff remains constant.

Ionization Energy _ across a period because…

increases, the attraction to the nucleus increases across a period because zeff increases while ShE remains constant'; means there is a greater magnitude of positive charge in the nucleus acting over the same distance, making the attraction stronger therefore requiring more energy to remove Ve-

Ionization energy exceptions - Group 2 → Group 13

Removing the Ve- from group 13 elements requires less energy because it is in the p subshell, whereas group 2 element Ve- are in the s subshell (the p subshell is further from the nucleus).

Ionization energy exceptions - Group 15 → Group 16

It requires less E to remove a Ve- from a paired e- than a singly occupied orbital because of added e-e repulsion. Therefore it is easier to remove the first Ve- from group 16 than 15.

General EN trends on PT

Follows same trends as IE: decreases down a group due to increased ShE and constant Zeff, increases across a period due to increased Zeff and constant ShE

Ionization Energy vs Electronegativity

• IE can be measured directly and is a property of only gaseous atoms.
• EN is a property of an atom in a molecule or FU and its values are assigned/derived indirectly from experimental bond energy data.

General trends in mp across period 3

Metallic bonds increase in strength (increasing mp) → network covalent solids (highest mp) → molecular compounds decrease in size LDFs decrease in strength (decreasing mp)

A negative e-A value means…

the change in energy is exothermic and the atom has a HIGH e-A

A positive e-A value means…

the change in energy is endothermic and the atom has a LOW e-A

Units of e-A

kJmol^-1

e-A _ across a period because…

increases, higher Zeff and a constant ShE result in a greater attraction for an additional e-

e-A down a group because…

decreases, Zeff remains constant while ShE increases resulting in a decreased attraction for Ve-. Therefore energy is required (endothermic) to add additional e-.

e-A trend exceptions: Group 15

The group 15 elements have a half-full p-subshell, which has more stability than the e- configurations of group 14 and 16 elements. Therefore its e-A is decreased

e-A trend exceptions: Group 2

The group 2 elements have a full s subshell, which is more energetically favourable than adding an electron to the unoccupied p subshell.

The strength of metallic bonds is influenced by

the magnitude of positive charge in the central ions (Zeff vs ShE) and the number of delocalized e- surrounding them. Also the ionic radius of the positive ions (kernels)

Theory of reactivity

Atoms will lose, gain or share valence e- to become isoelectronic with the nearest noble gas (ie. obtain a stable octet/full s and p subshells)

Monatomic molecules

molecules formed form one atom, formed by unreactive elements

Reactivity of Alkali Metals down a group because…

increases, the attraction of group 1 atoms to their Ve- decreases dramatically down a group due to unchanging Zeff and increasing ShE

Reactivity of Halogens down a group because…

decreases, halogens form anions in ionic bonds, so as the hold on e- decreases down a group due to increased ShE and constant Zeff, the reactivity decreases as well

Alkali metal + H2O reaction

2M(s) + 2H2O(l) → H2(g) + 2MOH(aq)

alkali metal + water → hydrogen gas + alkali metal hydroxide

Silver Halide Reaction

2Ag(s) + X2 (g/l/s) → 2AgX(s)

silver + halogen → silver halide precipitate

Metal Halide Reaction

2M(s) + X2(s/l/g) → 2MX

alkali metal + halogen → alkali metal halide

Halogen displacement reaction

a MORE REACTIVE halogen (X) will displace a LESS REACTIVE halide ion (x) in a single displacement reaction

X2(s/l/g) + 2Mx(aq) → 2MX(aq) + x2(s/l/g)

Colour change when Br2 is replaced by Cl2 in a halide displacement reaction

CLEAR → ORANGE

Colour change when I2 is replaced by Br2 in a halide displacement reaction

COLOUR DARKENS, Turns purple when mixed with a hydrocarbon

Metallic character _ across a period because…

decreases, metallic properties can be explained by loosely held Ve- and the underlying d-subshell e-

Metallic character _ down a group because…

increases, metallic properties can be explained by loosely held Ve- and the underlying d-subshell e-

Period 3 Oxides trend in pH

basic (metals) → amphoteric (Al2O3) → acidic (non-metals)

Period 3 Oxides trend in ionic character

decreases across, difference in EN decreases

Example of a measure of ionic character

electrical conductivity

Amphoteric

can behave as an acid or base depending on what it is reacting with

Rxn of Basic Oxides and Water

Na2O(s) + H2O(l) → 2NaOH (aq)

basic oxide + water → metal hydroxide

Rxn of Basic Oxides and an Acid

Na2O(s) + 2HCl(aq) → H20(l) + 2NaCl(aq)

basic oxide + acid → water + salt

double displacement neutralization

Amphoteric Oxides Rxn /w H2O

DOES NOT REACT

Rxn of Amphoteric Oxides and an Acid

Al2O3(s) + 6HCl(aq) → 3H2O(l) + 2AlCl3(aq)

amphoteric oxide + acid → water + salt

Rxn of Amphoteric Oxides and a Base

Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)

amphoteric oxide + base + water → sodium tetrahydroxoaluminate

Rxn of an Acidic Oxide with H2O

P4O10(g) + 6H20(l) → 4H3PO4

acidic oxide + water → acidic solution

Rxn of an Acidic Oxide with a Base

P4H10(g) + 12NaOH(aq) → 6H2O(l) + 4Na3PO4(aq)

acidic oxide + base → water + salt

Double displacement neutralization