Chapter 5

Periodic Law

SECTION 1 History of the Periodic Table

Mendeleev and Chemical Periodicity

When the Russian chemist Dmitri Mendeleev heard about the new atomic masses he decided to include the new values in a chemistry textbook that he was writing

Mendeleev hoped to organize the elements according to their properties

He placed the name of each known element on a card, together with the atomic mass of the element and a list of its observed physical and chemical properties

He then arranged the cards according to various properties and looked for trends or patterns

Mendeleev noticed that when the elements were arranged in order of increasing atomic mass, certain similarities in their chemical properties appeared at regular intervals

Such a repeating pattern is referred to as periodic

Mendeleev’s procedure left several empty spaces in his periodic table

In 1871, he predicted the existence and properties of the elements that would fill three of the spaces

By 1886, all three elements had been discovered

scandium, Sc, gallium, Ga, and germanium, Ge Their properties are very similar to those

predicted by Mendeleev

Success of Mendeleev’s predictions persuaded most chemists to accept his periodic table and earned him credit as the discoverer of the periodic law

Two questions remained (1) Why could most of the elements be arranged

in the order of increasing atomic mass but a few could not?

(2) What was the reason for chemical periodicity?

Moseley and the Periodic Law

In 1911 English scientist Henry Moseley examined the spectra of 38 different metals

Moseley discovered a previously unknown pattern

The elements in the periodic table fit into patterns better when they were arranged in increasing order according to nuclear charge, or the number of protons in the nucleus

Moseley’s work led to both the modern definition of atomic number and the recognition that atomic number, not atomic mass, is the basis for the organization of the periodic table

SECTION 2

Electron Configuration and the Periodic Table

The Modern Periodic Table

“Periodic” - Repeating patterns

Listed in order of increasing number of protons (atomic #)

Properties of elements repeat Periodic Law-Periodic Law- “the physical and chemical properties of

the elements are periodic functions of their atomic numbers.”

Periods and Blocks of the Periodic Table

Elements are arranged vertically in the periodic table in groups that share similar chemical properties

They are also organized horizontally in rows, or periods

The length of each period is determined by the number of electrons that can occupy the sublevels being filled in that period

main group elements

Metals Most solids (Hg is liquid) Luster – shiny. Ductile – drawn into thin wires. Malleable – hammered into

sheets. Conductors of heat and

electricity. Include transition metals –

“bridge” between elements on left & right of table

Non-Metals

Properties are generally opposite of metals

Poor conductors of heat and electricity

Low boiling points Many are gases at room

temperature Solid, non-metals are brittle (break

easily) Chemical properties vary

Metalloids

stair-step pattern Have properties similar to

metals and non-metals Ability to conduct heat

and electricity varies with temp Better than non-metals but

not metals semiconductors

Group 1 – The Alkali Metals

The elements of Group 1 of the periodic table (lithium, sodium, potassium, rubidium, cesium, and francium) are known as the alkali metals

In their pure state, all of the alkali metals have a silvery appearance and are soft enough to cut with a knife

Because they are so reactive, alkali metals are not found in nature as free elements

They combine strongly with most nonmetals

And they react strongly with water to produce hydrogen gas and aqueous solutions of substances known as alkalis

Because of their extreme reactivity with air or moisture, alkali metals are usually stored in kerosene or oil

Group 2 – The Alkaline-Earth Metals The elements of

Group 2 of the periodic table are called the alkaline-earth metals

Atoms of alkaline-earth metals contain a pair of electrons in their outermost s sublevel

Group 2 metals are harder, denser, and stronger than the alkali metals

They also have higher melting points Less reactive than the alkali metals, but also

too reactive to be found in nature as free elements

Transition Elements

Good conductors of electricity and have a high luster

They are typically less reactive than the alkali metals and the alkaline-earth metals

Some are so unreactive that they do not easily form compounds, existing in nature as free elements

TungstenMercury

Vanadium

Rare Earth ElementsLanthanide series (period 6)Actinide Series (period 7)

Some radioactive Separated from table to make easy to

read/print silver, silvery-white, or gray metals. Conduct electricity

uranium

Halogen Family (“salt-former”)

-7 Valence Electrons-most active nonmetals-never found pure in nature-react with alkali metals

easily (forms salts)-F most active halogen

Halogens cont…

F compounds in toothpasteCl kills bacteriaI keeps thyroid gland

working properly

bromine

The Noble Gases (Inert Gases)

- non-reactive- outermost e-

shell is full (8 VE)

- In “neon” lights-in earth’s

atmosphere (less than 1%)

Neon

Section 5.3Electron Configuration and

Periodic Properties

Periodic Trends

In periodic table, there is a DECREASE in atomic radii across the periods from left to right

Caused by increasing positive charge of nucleus (more protons = more positive charge)

Group Trends

Radii of elements decrease as you go UP a group

Electrons occupy sublevels in consecutively higher main energy levels (located further away from nucleus)

In general, the atomic radii of the main-groups elements decrease from the bottom to the top of a group

2. Ionization Energy

Electrons can be removed from an atom if enough energy is supplied

Using A as a symbol for an atom of ANY element, the process can be expressed as follows:

A + energy A+ + e-

A+ represents an ion of element A with single positive charge (a 1+ ion)

Ion an atom or group of bonded atoms that have a positive or negative charge

Ionization any process that results in the formation of an ion

Period Trends

In general, ionization energies of the main-group elements INCREASE across each period

Caused by increasing nuclear charge

Higher charge more strongly attracts electrons in same energy level

Group Trends

Ionization energies generally INCREASE going UP a group

Electrons going down in group are in higher energy levels, so further away from the nucleus

Removed more easily Also more electrons between outermost

electrons and the nucleus (shields them from attraction to positive nucleus)

What are Valence electrons?

outermost e-’s Responsible for chem props Elements in same group… same # of VE ALL atoms want full outer energy level (usually 8 VE) To get full outer energy level, some elements:

lose e- (metals) gain e- (non-metals) share electrons (some non-metals & metalloids)

Main-group elements – valence e- are in outermost s and p sublevels

Inner e- held too tightly by nucleus to be involved in compound formation

6. Electronegativity

Valence e- hold atoms together in compounds

In many compounds, negative charge centered around one atom more than another

Uneven distribution of charge has effect on compound’s properties

Useful to have measurement of how strongly one atom attracts e- of another atom

Electronegativity measure of the ability of an atom in a chemical compound to attract electrons

Most e-neg element (fluorine) – randomly assigned value of 4

Other values calculated in relation to F

Period Trends

e-negs tend to INCREASE across each period

There are exceptions (of course)

Alkali and alkaline-earth metals are least e-neg

In compounds, their atoms have low attraction for e-

Group Trends

Electronegativities tend to INCREASE going UP a group or stay the same

At higher energy levels electrons being added are further away from the nucleus

Therefore, less attraction to the nucleus Also more electrons between outermost

electrons and the nucleus (shields them from attraction to positive nucleus)