physical and chemical periodicity. copyright © houghton mifflin company. all rights reserved.3 | 2...

Physical and Chemical Periodicity

Copyright © Houghton Mifflin Company. All rights reserved. 3 | 2

The Electron Configuration of All the Elements in the Periodic Table


The Electron Configuration of All the Elements in the Periodic Table

• Note that all the elements in a column of the periodic chart have the same outer electron configuration.• This imparts much of the chemical properties. As

such columns in the periodic chart are called groups.

• Traveling across a row of the periodic chart, elements are generally filling the shells and subshells in the same order. • The results is that we see repetitive patterns. Rows in

the charts are called periods.


Physical Form of the Elements at Room Temperature and One Atmosphere Pressure


Gilbert Lewis and Lewis Dot Structures

• G.N. Lewis recognized the tendency of elements and combinations of elements to form filled s and p shells.• Since there are 2-electrons in an s orbital and 6 in a p-

orbital, the total is 8 and leads to the “octet” rule.• Although this works for simple compounds, we shall

see that when we involve d orbitals, the picture becomes more complicated.

• The formation of an octet between atoms can lead to the prediction of combinations of the elements.


Lewis Dot Structures for the s and p Elements

As can be seen, the dots represent the number of electrons in the outer or “valence” shell of an atom. These can be used to predict combining ratios.


Practical Questions

• You need a material for an aircraft that must withstand high pressure and temperature but not be brittle. It must withstand oxidation from the fuel burning.

• You want a metal for a coin. It cannot corrode or be conductive and must have a high melting point.

• You need a wire for conduction. But because the wire will be strung on poles, it can’t stretch.

• You want to develop a blue laser diode tuned to 450 nm. What materials and doping agents will work and in what concentrations?

• You need an alloy that melts at a specific temperature. What metals will mix with each other to form such an alloy and what concentrations will govern hardness and melting point?


Periodicity• Periodicity refers to recurring trends in the periodic

chart that occur over periods. • The development of the periodic chart suggested the

existence of elements that had not yet been found through periodic behavior.

• Periodicity suggested, not only the shell structure predicted by Bohr, but also the orbital structure that was developed much later with the advent of quantum mechanics.

• Periodicity gives us the ability to identify trends in materials that can lead us to the properties we desire for a given application.


Ionization Energies

• Ionization Energy is the energy required to remove an electron from an atom or an ion.• IE’s are greatly affected by the effective nuclear charge experienced by an electron

which is a combination of electron shielding and electron repulsion.• The ability to remove electrons is critical to the formation of ions, or the conductivity of

the metal among other many physical properties.

Example: Consider the Lithium atom.


Ionization Energies

The Ionization Energy for Li us 5.392 eV and the electron is in the n = 2 shell. Computing Zeff:

So we see that the outer shell electron is shielded by the two n = 1 electrons. Why isn’t the shielding perfect to have the outer electron experience Zeff = 1?

We shall see in a moment. What about removing the next electron? And the last electron? These energies are given next.


The Ionization Energy of the Three Electrons in the Lithium Atom Demonstrate the Shielding of the Outer-

shell Electron by the Two Inner-shell Electrons


Ionization Energies

The 2nd Ionization Energy for Li is about 75.6193 eV and the electron is in the n = 1 shell. Computing Zeff:

So we see that the n = 1 shell electron is shielded by the other n = 1 electron. So shielding within a shell is less effective. Finally, note the 3rd and last electron removed. IE = 122.420 eV


Ionization Energy Jumps After the First Ionization in Group 1A, After the Second Ionization in Group IIA, and

After the Third Ionization in Group IIIA


Ionization Energies


Ionization Energies• Mg has 12 protons vs Be which has 4, thus there is 3 times the

nuclear charge.

• As can be seen, Mg also penetrates into the inner shells and thus experiences a greater nuclear charge so the shielding is less efficient but still quite good. (Shields almost 10 protons!)

• However, this is countered by the fact that the Mg electron is farther away from the nucleus so already experiences an additional E by virtue of being in the n = 3 shell rather than the n = 2 shell so less energy is required to ionize it.

So we see such properties are a complex combination of nuclear charge, location of the electron and the type of orbital in which the electron is located. As further evidence of this, let’s examine the transition metals.


Ionization Energies – Filling within an orbital

IE (eV) Zeff

V 6.750 2.114

Cr 6.766 2.116

Mn 7.434 2.218

Fe 7.902 2.287

Co 7.880 2.284

Consider the filling of the d-orbitals of V, Cr, Me and Fe.

As before, we see that when filling within a shell, the electrons within the shell are inefficient at shielding.


The Highest Ionization Energies are Found in the Upper-right Part of the Periodic Table; The Lowest Appear in the

Lower-left Part of the Periodic Table


IE’s are highest for filled orbitals and for ½ filled orbitals and increase with nuclear charge. Thus the energies are a balance

between charge and shielding.


Electron Affinity

• Electron affinity is the opposite of ionization..it is the energy of adding an electron to an atom to form an ion.• In order to attract an electron, the atoms’ nuclear charge

must be imperfectly shielded.

• We can play the same analysis game as we did for ionization energy. We will use..

• In this instance, however, the ionization energy is now the electron affinity which also depends on Zeff.


Electron Affinity of the Main-group Elements

Note: In this instance, the larger EA means ease of electron addition, thus the addition of an electron to Fluorine, F, gives off 3.4 eV’s of energy.. Note the trend for filled and ½ filled orbitals!


Electron Affinity of the Main-group Elements (cont’d)


Electron Affinity• In order to give off energy by collecting an electron, the

electron must be attracted to the atom by its nuclear charge.

• If the neutral atom were perfectly shielded, no energy would be given off and the atom would tend not to form an ion. Two examples:


Electron Affinity• Note the Zeff of N and of a metal such as Iron, Fe.

• Here, N is virtually 0. The reason is N possesses a ½ filled p orbital. Thus the next electron requires more energy to add.

• Fe has a very low EA. This is due to the free movement of electrons in metals. We will explore this next.


Electronegativity• It should be obvious that ionization energy and electron affinity are

related to similar properties of the atom. Both can be summarized by a parameter called the electronegativity.

• The electronegativity can be described as the relative strength of a particular element to attract electrons to itself.

• When two atoms are sharing electrons as we will see later, the element possessing the higher electronegativity will draw the electrons closer.

• In extreme differences, the type of bonding changes entirely as we will see later.

• Linus Pauling developed a scale of electronegativity based on the relative sharing of electrons. It is called, properly, the “Pauling” scale.


Electronegativity

We’ve seen two types of bonds so far, those formed from ions (ionic bonds) and those that share electrons (covalent bond). The type of bond that forms can be predicted using the Pauling scale.From your periodic chart:


Electronegativity• In many instances, the electronegativity can determine the fate of a

chemical reaction. Two examples:

• Cu+2 and Zn metal. Cu has an electronegativity of 1.90 whereas Zn has a value of 1.65 indicating that Cu has a greater affinity for the electrons. When placed together, the following reaction occurs:

Cu+2 + Zn Cu(s) + Zn+2

This reaction will form the basis for creating batteries later.

• Cl2 is a greenish gas and Br-1 ions are colorless. The electronegativities of these two elements are: Cl(3.16) and Br(2.96). When combined, the greenish gas turns to the red-brown color of Br2.

Cl2(g) + 2 Br-1 2 Cl-1 + Br2(l)


Electronegativity Values of the Main-Group Elements


Electronegativity of Main-group Elements Shown on the Pauling Scale


Elements on the Left-hand Side of the Periodic Table have a Few Loosely Held Valence

Electrons, Those on the Right Side of the Table have Nearly Filled Valence s and p Subshells


Ionization Energy, Electron Affinity (here multiplied by four to show it on the same scale as the ionization

energy), and Electronegativity Values for the Second-Period Elements


Atomic Radius

• The strength of pull of electrons towards the nucleus clearly leads to the chemical effects we’ve discussed. It should be obvious that an increased pull will result in a smaller atom.

• In fact, the atomic radius follows a similar periodic pattern.

• Also note that adding or removing electrons can dramatically alter the atomic radius. We will see later that this alters the distance and, therefore, the size and attraction of elements contained in materials effecting properties such as density and malleability


The Atomic Radius of Elements (in pm) Increases Down a Group and Decreases Across a Period

Note: A graphical error shows He to be larger than it should be. In fact it is the smallest radius element.


The Relative Size of Atoms and their Commonly Associated Ions


Trends in Physical Properties


The Basics

• In general, elements that have strong attraction to each other will be solids.• We have seen that electrons are the principle items

that contribute to bonding.• Elements that have smaller electronegativities will

contribute electrons to the metallic “sea” and are thus metals.

• As elements hold electrons more tightly, less interaction occurs and the materials become non-metals, liquids and gases.

• Other properties follow this scheme including melting points, densities, conductivities, etc.


The Melting Point of the Metallic Elements has a Regular Variation with Position in the Periodic Table


Melting Points are related to the strength of the interaction between a metal nucleus

and the bonding electrons:

•At the beginning of a period, there are few valence electrons to contribute to bonding•At the midpoint, valence d-orbitals are ½ filled maximizing interaction as all electrons are unpaired.•As the shells fill, they begin to take on the spherical character. (Remember the HW problem in which you added p-orbitals together)

Melting Points, Stiffness and Density are Related.


The Flexibility of Five Common Elemental Wires Varies Considerably


The packing in metal atoms relates to the strength of

interaction and affects the physical properties.

We will see in upcoming chapters that the way in which the atoms are packed together affect the strength of attraction and thus the flexibility among other mechanical properties.


Bending a Wire Distorts the Hexagonal Close Pack Layers


There Is a Correlation Between Stiffness and Melting Point in Wires Composed of Five Common Metallic Elements

Both involve breaking of the bonds between the elements.


Comparison of the Density of Metals in Periods 4, 5 and 6 of the Periodic Table Shows that the Density Varies Periodically, with a

Rise and Fall Repeated in Each Period


Summary

The mechanical properties of materials is a complex combination of:

•Atomic size•Orbital interaction and the energy levels of different orbitals•Nuclear charge•Valence electrons

Each of these affect the crystal structure and electron mobility. In the next chapters we will examine the contributions of each of these to the properties.

physical and chemical periodicity. copyright © houghton mifflin company. all rights reserved.3 | 2...

Documents

periodic chart

combinations of elements

tendency of elements

existence of elements

periodic tablenote

periodic behavior

outer electron configuration

s orbital