CHAPTER 5

History of The Periodic Table

Organization of the Elements

• 1869: Dmitri Mendeleev -–Published an organizational scheme for the elements.

–He called it the Periodic Table .

• Organized his elements by atomic mass in two rows of 7 elements and two rows of 17 elements.

• Noticed that the properties of the elements repeated so he put elements that behaved the same in the same vertical column.

• Left blank spots on his periodic table and predicted the discovery and properties of several undiscovered elements.

_____________________________• Problems were found with the

table.• Ar -- K Te -- I Co -- Ni

• If Mendeleev ’s Table was to be correct, these elements should have been reversed.

• Elements could not be reversed because of their properties.

• This lead to a new version of the Periodic Table.

The Modern Periodic Law

• Henry Mosely – Early 1900s

•The properties of the elements are periodic when they are arranged in increasing order of their Atomic Number or Protons.

Bombarded the atom with X-rays to determine the number of protons in atoms of known elements.

“Patterns” on the Periodic Table.

• Columns:Group or family - elements with similar properties.

• Several Groups have descriptive names.

Alkali Metals - Group IA

• React with water to form an alkaline or basic solution.

Base

Na(S) + H2O(L) --> NaOH(aq) + H2(g)

s11 valence electron

Soft silvery active metals

Alkaline Earth Metals - Group IIA

• Elements also react with water to form a base.

s2

2 valence electrons

Halogens - Group VIIA

• “Salt Formers”

“Salt”

Na(S) + Cl2(g) --> NaCl(S)

s2p5 7 valence electrons

Most reactive non-metals

Noble Gases - Group VIIIA

• Gases that are highly unreactivebecause their electron configuration is very stable.

s2p68 valence electrons

Transition Metals

• Elements in the center section of the periodic table.

d sublevels

Inner Transition Metals

• Lanthanide Series–Elements Ce- Lu

• Actinide Series–Elements Th- Lr

4f

5f

RADIOACTIVE

18

Metalloids• Elements that border the zig-zag

line (steps) separating the metals

from the non-metals are

metalloids.

Period

• Row on the periodic table.• There are 7 periods on the

periodic table.–One period for each “layer”or energy level of electrons.

Trends in Some Important

Periodic Atomic Properties

All physical and chemical behavior of the elements is based ultimately on the electron configurations of their atoms.Trends in Atomic Size

Trends in Ionization EnergyTrends in Electron Affinity

These trends are periodic.

Trends in Electronegativity

Trends in Atomic Size

Definition is based on how closely one atom lies next to another identical atom.

Generally, the distance between the atoms is measured and then divided by two to give the atomic radius.

Trends in the Main Group Elements

1. As n increases, the outer electrons spend more time farther from the nucleus. Hence, the atoms are larger.

2. As the effective nuclear charge (Zeff)- the positive charge “felt” by an electron increases, outer electrons are pulled closer to the nucleus. Hence the atoms are smaller.

The net effect:

1. Down a group, n dominates.

Inner electrons shield outer electrons very effectively. Hence, atomic radius generally increasesdown a group.

2. Across a period, Zeff dominates.

As electrons are added to the same outer level, the shielding of the inner electrons does not change. Zeff rises and the outer electrons are pulled closer. Hence, atomic radius generally decreases across a period (L to R).

i

(a) Ca, Mg, Sr

Sr > Ca > Mg

These elements are in Group 2A(2).

Ranking Elements by Atomic Size (Radii)

Using only the periodic table rank each set of main group elements in order of decreasing atomic size:

(b) K, Ga, Ca

(c) Br, Rb, Kr

K > Ca > Ga

These elements are in Period 4.

Rb > Br > Kr

Rb has a higher energy level and is far to the left. Br is to the left of Kr.

(d) Sr, Ca, Rb

Rb > Sr > Ca

Ca is one energy level smaller than Rb and Sr. Rbis to the left of Sr.

Ranking Elements by Size

Problem: Rank the following elements in each group according to decreasing size ( largest first!):

a) Na, K, Rb b) Sr, In, Rb c) Cl, Ar, K

a)Rb > K > Na These elements are all alkali metals and the elements increase in size as you go down the group.

b) Rb > Sr > In These elements are in Period 5 and the size decreases as you go across the period.

c) K > Cl > Ar These elements border a noble gas, and the noble is the smallest diameter.

Trends in Ionization Energy

Ionization Energy (IE) is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms or ions.

Removing an electron always requires energy to break the attraction of the electron towards the nucleus.

IE is always positive since energy flows into the system like an endothermic reaction.

IE1 removes an outermost electron (highest energy sublevel).

Atom(g) ion+(g) + e-

∆E = IE1 > 0Cations: + ions

Atom(g) + energy ion+(g) + e-

Variations in IE 1

Generally, as the size of the atom increases, it takes less energy to remove it.

1. Down a group: As the distance between the nucleus and the outer electron increases, IE decreases.

2. Across a period: Zeff increases so atomic size decreases. This causes the attraction between the nucleus and the outer electron to increase.

Hence, IE generally increases across a period.

1. Between Group 2A - Group 3A

Full s sublevel to a p1 configuration.

2. Between Group 5A - Group 6A

p3 configuration – p4 configuration.

Exception: Stability of full and half-full sublevels.

p sublevels are at a higher energy state than s sublevels and are hence removed easier.

Periodicity of First IonizationEnergy (IE1)

IE 2 removes a second electron.

Ion+1(g) ion2+

(g) + e-

∆E = IE 2 (always> IE1)

Atoms with a low IE1 tend to form positive ions (cations) during reactions while those with a high IE1(except the Noble Gases) often form negative ions (anions)

IE 1 of the main-group

elements

Ranking Elements by First Ionization Energy

Using the periodic table only, rank the elements in each of the following sets in order of decreasing IE 1:

IE decreases as you proceed down in a group; IE increases as you go across a period.

He > Ar > Kr

(a) Kr, He, Ar

Group 8A- IE decreases down a group.

Te > Sb> Sn

Period 5 elements - IE increases across a period.

(b) Sb, Te, Sn

(c) K, Ca, Rb

Ca is to the right of K; Rb is below K. Ca > K > Rb

Xe > I > Cs

I is to the left of Xe; Cs is further to the left and down one period.

(d) I, Xe, Cs

Trends in Electron AffinityElectron Affinity (EA) is the energy change accompanying the ADDITION of 1 mole of electrons to 1 mole of gaseous atoms or ions.

Atom(g) + e- ion-(g)

Anions: - ions

In most cases, energy is released when the first electron is added because of the attraction to the nucleus.

EA1 is usually negative just like ∆H is negative for exothermic reactions.

Atom(g) + e- ion-(g) + energy

Electron Affinities of the Main-Group Elements

EA2 is always positive (energy absorbed) because electrons are being added to something that is already negatively charged.

There is no clear cut trend for EA.

Reactive nonmetals: Have high IEsand very negative (exothermic) EAs– high liking for electrons.

These elements gain electrons with ease and lose them with difficulty.

Reactive metals: Have low IEsand slightly negative or slightly positive (endothermic) EAs– low liking for electrons.These elements lose electrons with ease and gain them with relative difficulty.

Noble Gases: Have IEsand slightly positive (endothermic) EAs.

These elements tend to neither lose or gain electrons.

Trends in Three Atomic Properties

Trends in Metallic Behavior

Metals tend to lose electrons during chemical reactions because of their low IEs as compared to nonmetals.

Hence, the elements with the most “metallic ” behaviors are those on the left and towards the bottom of the periodic table.

Trends in Metallic Behavior

Electronegativity and Polarity

Linus Pauling developed an EN scale for the elements used to determine the type of bond between elements.

•Electronegativity (EN) – the ability of a bonded atom to attract a shared pair of electrons.

Electronegativity (EN) increases across a period.The EN is inversely proportional to the atomic size.

Electronegativity Trends

The Periodic Table of the Elements2.1

0.9 1.5

0.9 1.2

0.8 1.0 1.3

0.8

0.7

0.7

1.0

0.9

1.5 1.6 1.61.5 1.8

1.2

1.1

1.8 1.8 1.9 1.6

1.4 1.6

1.5

1.8

1.7

1.9

1.9

2.2 2.2

2.2

2.2

2.2

1.9

2.4

1.7

1.9

2.0 2.5 3.0 3.54.0

He

Ne

Ar1.5 1.8 2.1 2.5 3.0

1.6 1.8 2.0 2.4 2.8 Kr

Xe

Rn

2.52.1

2.2

1.9

2.01.9

1.81.7

1.81.8

1.1 1.1 1.1 1.1

1.3

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.21.3

1.5 1.7 1.3 1.3 1.3 1.3 1.3 1.3 1.31.3 1.5

0.9

1.3 2.2

Electronegativity

1.1

1.3

Electronegativity and Atomic Size.

Bonds

• Ionic Compounds – Electrons are transferred from one atom to another to form ionic compounds. (Ionic Bonds)

•Chemical Bonds –The electrostatic forces that hold the atoms of elements together in the compound.

Mono-atomic ions form binary ionic compounds.

Covalent Compounds -Electrons are shared between atoms of different elements to form covalent compounds.

Each nucleus attracts the other atom's electrons.Sharing electrons is the way that most atoms interact chemically.

Shared electron pairs = covalent bond

∆EN

3.0

2.0

0.0

Boundary ranges for classifying ionic character of chemical bonds.

What kind of a bond exists between … ?

Cl2 Cl-Cl = 3.0 – 3.0= 0.0

= Nonpolar

Nonpolar covalent bond– bond in which the bonding electrons are shared equally by the bonding atoms –resulting in a balanced distribution of electric charge.

X

What kind of a bond exists between … ?

H-O 3.5 – 2.1 = 1.4

= Polar

Polar covalent bond– bond in which the bonding electrons are shared unequally by the bonding atoms –resulting in a uneven distribution of electric charge.

What kind of a bond exists between … ?

Na-Cl 3.0 – 0.9 = 2.1

= Ionic

Ionic bond – bond in which the bonding electrons are transferred from one bonding atom to another – resulting in oppositely charged ions.

• Another way to look at electronegativitydifference …

• If difference is 0.0 then the compound has 0 % ionic character …

• If the difference is 1.7 it has 50% ionic character …

• If the difference is 3.3 it has 100% ionic character.

x

The End