A German scientist named Johann Dobereiner put forward his law of triads in 1817. Each of Dobereiner's triads was a group of three elements. The appearance and reactions of the elements in a triad were similar to each other. At this time, scientists had begun to find out the relative atomic masses of the elements. Dobereiner discovered that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements. This suggested that atomic mass might be important to arranging the elements.

With this idea in mind nearly 50 years later, in 1864, an Englishman, John Newlands, arranged the known elements into seven rows.Newlands' Octave Arrangement

from Chemical News [1866]

No. No. No. No. No. No. No. No.

H 1Li 2G 3Bo 4C 5N 6O 7

F 8Na 9Mg 10Al 11Si 12P 13S 14

Cl 15K 16Ca 17Cr 19Ti 18Mn 20Fe 21

Co & Ni 22Cu 23Zn 25Y 24In 26As 27Se 28

Br 29Rb 30Sr 31Ce & La 33Zr 32Di & Mo 34Ro & Ru 35

Pd 36Ag 37Cd 38U 40Sn 39Sb 41Te 43

I 42Cs 44Ba & V 45Ta 46W 47Nb 48Au 49

Pt & Ir 50Os 51Hg 52Tl 53Pb 54Bi 55Th 56

The elements were placed in order of increasing atomic mass and arranged so elements with similar chemical properties are in the same group.

Since the properties were repeated every eighth element, Newlands referred to his arrangement as theLaw of Octaves. Unfortunately for Newland’s, there were a number of problems with his arrangement and it really only worked up through calcium.About five years later, Dmitri Mendeleev a Russian chemist who was unaware of the work of Newlands,designed his own table.After about 18 months of gathering information and arranging element cards, it was finished.Like Newlands, Mendeleev arranged the 63 known elements in order of increasing atomic mass, having elements with similar chemical properties in the same group.

Unlike Newlands his table had groups of varying lengths.

He also left gaps in the table for elements he believed had not yet been discovered. He even made predictions about the properties of some of these elements.

Although his successful predictions allowed many to accept his periodic idea, there were still anomalies that Mendeleev could not explain.

One of these is the order of iodine and tellurium.The problem was finally solved in 1913 by a 25-year-old English physicist named Henry Moseley.Moseley showed that the ordering of X-ray spectral lines was dependent upon the ordering of nuclear charge,that is, in order of the atomic number.When the elements were placed in order of increasing atomic number,the anomalies in Mendeleev’s table were eliminated.

Moseley’s work gave rise to the modern periodic law:

The properties of the elements are a periodic function of their increasing atomic numbers.Tragically for the development of science, Moseley was killed in battle during World War I, only two years later.

Moseley’s revised periodic table looked something like this

The Modern Periodic TableThe Modern Periodic Table

• seven horizontal rows called seven horizontal rows called periodsperiods

• 18 vertical columns called 18 vertical columns called groups groups or familiesor families

• groups 1 and 2 and groups 13-18 groups 1 and 2 and groups 13-18 are called are called representative elementsrepresentative elements

• groups 3-12 are the groups 3-12 are the transition transition metalsmetals

Modern Periodic Table Modern Periodic Table cont.cont.• elements in any group have similar elements in any group have similar

physical and chemical propertiesphysical and chemical properties

• properties of elements in periods properties of elements in periods change from group to groupchange from group to group

• symbol placed in a squaresymbol placed in a square

• atomic number above the symbolatomic number above the symbol

• atomic mass below the symbolatomic mass below the symbol

Metallic CharacterMetallic Character

• MetalsMetals– malleable & malleable &

ductileductile– shiny, lustrousshiny, lustrous– conduct heat conduct heat

and electricityand electricity– lose electrons in lose electrons in

reactions reactions

• Nonmetalsbrittle in solid statedullelectrical and

thermal insulatorsgain electrons in

reactions

• MetalloidsAlso known as

semi-metalsShow some

metal and some nonmetal properties

Metallic CharacterMetallic Character• Metals are found on the left of the table, Metals are found on the left of the table,

nonmetals on the right, and metalloids nonmetals on the right, and metalloids in betweenin between

• Most metallic element always to the left Most metallic element always to the left of the Period, least metallic to the right, of the Period, least metallic to the right, and 1 or 2 metalloids are in the middleand 1 or 2 metalloids are in the middle

• Most metallic element always at the Most metallic element always at the bottom of a column, least metallic on bottom of a column, least metallic on the top, and 1 or 2 metalloids are in the the top, and 1 or 2 metalloids are in the middle of columns 4A, 5A, and 6Amiddle of columns 4A, 5A, and 6A

Other Important Groups to Other Important Groups to KnowKnow

• Group IA Group IA alkali metals alkali metals

• Group IIA Group IIA alkaline earth metals alkaline earth metals

• Group VIIIA Group VIIIA noble gases noble gases

• Group VIIA Group VIIA halogens – “salt halogens – “salt formers”formers”

• Group VIA Group VIA chalcogens chalcogens

• Group VA Group VA Nitrogen group Nitrogen group

• Group IVA Group IVA IVA group IVA group

• Group IIIA Group IIIA IIIA group IIIA group

Other GroupsOther Groups

• s & p block filling s & p block filling representative representative elementselements

• d block filling d block filling transition metals transition metals• f block filling f block filling inner transition metals inner transition metals

– 4f 4f lanthanides lanthanides– 5f 5f actinides actinides

• f elements that are naturally occurring f elements that are naturally occurring rare earth elements rare earth elements

What are Periodic TrendsWhat are Periodic Trends

• trends are general patterns or trends are general patterns or tendenciestendencies– they are general not definite – there are they are general not definite – there are

exceptionsexceptions

• when looking at trends we look for when looking at trends we look for increases & decreasesincreases & decreases– across across periodic periodic– down down group group

Effects on the TrendsEffects on the Trends

1.1. Nuclear ChargeNuclear Charge- the “pull” of the nucleusthe “pull” of the nucleus- proportional to the number of protons proportional to the number of protons

in an atomin an atom- the greater the number of protons, the the greater the number of protons, the

stronger the nuclear charge (“pull”)stronger the nuclear charge (“pull”)- this has its greatest effect across a this has its greatest effect across a

periodperiod

Effects on Trends Cont.Effects on Trends Cont.

2.2. ShieldingShielding- - the electron protection from the the electron protection from the

nuclear “pull”nuclear “pull”- - shield = an energy level of shield = an energy level of electronselectrons- - we are not concerned with single we are not concerned with single

electrons, only energy levels of electrons, only energy levels of electronselectrons

- - these electrons reduce the nuclear these electrons reduce the nuclear pullpull- - affects group trendsaffects group trends

Effects on Trends Cont.Effects on Trends Cont.

3.3. StabilityStability- - where electron arrangement is where electron arrangement is compared to stable octet (or other compared to stable octet (or other special stabilities)special stabilities)- - determines if atom gains or determines if atom gains or loses electronsloses electrons-- can be used to explain can be used to explain anomalies in trendsanomalies in trends

Trend in Atomic Size

• Decreases across period Decreases across period – left to right because of the nuclear “pull”left to right because of the nuclear “pull”– adding electrons to same valence shelladding electrons to same valence shell– valence shell held closer because more valence shell held closer because more

protons in nucleusprotons in nucleus

• Increases down columnIncreases down column– valence shell farther from nucleus valence shell farther from nucleus

because of increased shieldingbecause of increased shielding

• IllustrationIllustration

Trend in Ionization EnergyTrend in Ionization Energy• Minimum energy needed to remove a valence Minimum energy needed to remove a valence

electron from an atomelectron from an atom– 1 mole of electrons in the gaseous state (kJ/mol)1 mole of electrons in the gaseous state (kJ/mol)

• The lower the ionization energy, the easier it is to The lower the ionization energy, the easier it is to remove the electronremove the electron– metals have low ionization energiesmetals have low ionization energies

• Ionization Energy decreases down the groupIonization Energy decreases down the group– valence electron farther from nucleusvalence electron farther from nucleus

• Ionization Energy increases across the periodIonization Energy increases across the period– left to rightleft to right– harder to remove an electron from the atom harder to remove an electron from the atom

because of the increased nuclear “pull”because of the increased nuclear “pull”

• Exceptions: Group 3 less than Group 2, Group 6 Exceptions: Group 3 less than Group 2, Group 6 (chalcogens) less than Group 5(chalcogens) less than Group 5

Ionization Energy Cont.Ionization Energy Cont.• Li + energy Li + energy Li Li++ + e + e--

– 11stst ionization = 520 kJ/mol ionization = 520 kJ/mol

• LiLi+ + + energy + energy Li Li+2+2 + e + e--

– 22ndnd ionization = 7297 kJ/mol ionization = 7297 kJ/mol

• LiLi+2+2 + energy + energy Li Li+3+3 + e + e--

– 33rdrd ionization = 11,810 kJ/mol ionization = 11,810 kJ/mol

• Notice, each successive ionization energy is Notice, each successive ionization energy is greater than the preceding one – there is a greater than the preceding one – there is a greater “pull” between the nucleus and the greater “pull” between the nucleus and the electron and thus more energy is needed to break electron and thus more energy is needed to break the attraction.the attraction.

• Examining ionization energies can help you Examining ionization energies can help you predict what ions the element will form.predict what ions the element will form.– easy to remove an electron from Group IA, but difficult easy to remove an electron from Group IA, but difficult

to remove a second electron. So group IA metals form to remove a second electron. So group IA metals form ions with a 1+ charge.ions with a 1+ charge.

Ionization Energies for aluminum (kj/mol):

1st - 578

2nd - 1817

3rd - 2745

4th - 11580

5th - 14840

Electron AffinityElectron Affinity

• atoms tendency to attract (gain) an electronatoms tendency to attract (gain) an electron

• it is the energy change that accompanies the it is the energy change that accompanies the

addition of an electron to a gaseous atomaddition of an electron to a gaseous atom

• Basically the “opposite” of ionization energyBasically the “opposite” of ionization energy

• Across a PeriodAcross a Period– electron affinity increases because of increased electron affinity increases because of increased

“pull”“pull”

• Down a GroupDown a Group– electron affinity decreases because the electrons electron affinity decreases because the electrons

are shielded from the pull of the nucleusare shielded from the pull of the nucleus

Exceptions: 2A; Nitrogen Group & Noble GasesExceptions: 2A; Nitrogen Group & Noble Gases

Ionic SizeIonic Size• cations – lose electrons (positively charged)cations – lose electrons (positively charged)

• anions – gain electrons (negatively charged)anions – gain electrons (negatively charged)

• elements gain or lose eelements gain or lose e- - to become stable – to become stable – being like noble gases (filled outer sublevel)being like noble gases (filled outer sublevel)

• IA - +1IA - +1 VA - -3VA - -3

• IIA - +2IIA - +2 VIA - -2VIA - -2

• IIIA - +3IIIA - +3 VIIA - -1VIIA - -1

• IVA – shareIVA – share VIIIA – 0, stableVIIIA – 0, stable– IllustrationIllustration

Ionic Size Cont.Ionic Size Cont.• Across a PeriodAcross a Period

– cations decrease (I-III) because of greater cations decrease (I-III) because of greater pull on electrons (protons pull on fewer pull on electrons (protons pull on fewer electrons = more +)electrons = more +)

– anions decrease (V-VII) because of anions decrease (V-VII) because of decrease in electron repulsion (protons pull decrease in electron repulsion (protons pull on fewer electrons i. e. less negative)on fewer electrons i. e. less negative)

• Down a GroupDown a Group– both cations and anions increase sizeboth cations and anions increase size

• GOOD RULE OF THUMBGOOD RULE OF THUMB– anions are always larger than their neutral anions are always larger than their neutral

atomatom– cations are always smaller than neutral cations are always smaller than neutral

atomatom

ElectronegativityElectronegativity• the ability of an atom to attract the ability of an atom to attract

electrons when the atom is in a electrons when the atom is in a compoundcompound

• electron “tug-of-war”electron “tug-of-war”

• similar to electron affinity, but not the similar to electron affinity, but not the same same

• Across a PeriodAcross a Period– increases because of increased pullincreases because of increased pull

• Down a GroupDown a Group– decreases because of shieldingdecreases because of shielding

• Fluorine – most electronegative elementFluorine – most electronegative element

ReactivityReactivity

• Reactivity of metals increases to Reactivity of metals increases to the left on the Period and down in the left on the Period and down in the columnthe column– follows ease of losing an electronfollows ease of losing an electron

• Reactivity of nonmetals (excluding Reactivity of nonmetals (excluding the noble gases) increases to the the noble gases) increases to the right on the Period and up in the right on the Period and up in the columncolumn

• Reactivity VideoReactivity Video

PracticePractice

• Which element has a greater Which element has a greater ionization energy – Mg or Baionization energy – Mg or Ba

• Which element has a greater atomic Which element has a greater atomic radius – N or Fradius – N or F

• Which element has a greater Which element has a greater electron affinity – S or Pbelectron affinity – S or Pb