Chapter 6: The Periodic Table

Section 6.1: Organizing the elements

How do we know that we have found all of the elements? ● Some elements have been known to

exist for thousands of years● By 1700 only 13 elements had been

discovered● When scientists starting using the

scientific method to analyze elements the rate of discovery significantly increased

How can you group elements together? ● In the 1820s, J.W. Dobereiner began

grouping elements together into Triads

● A Triad is a set of three elements with similar properties - One example is Chlorine, Bromine and Iodine

● Grouped together because they all react easily with metals

● Did not work for all elements

Who is Dmitri Mendeleev? How did he change our understanding of chemistry?

● Dmitri Mendeleev was a Russian Chemist working in the mid 19th Century

● He was writing a textbook, and investigated how elements were related

● He wrote properties on cards, and arranged them until he found an organization which worked

● His table was based upon the atomic mass of an element

● He left gaps were he thought elements should go that had not been discovered, and was ultimately proved correct

How is Mendeleev’s periodic table different to todays? ● For same elements, Mendeleev had to ignore atomic mass in order to group

elements with similar properties together ● Instead, we no know that elements should be arranged in order of increasing

atomic number rather than mass

The modern periodic table

● Period: Row● Group: Column● Periodic law:

When elements are arranged in order of increasing atomic number; there is a periodic repitition of their properties

There are other ways to classify elements rather than simply groups

There are three classes of elements; metals, nonmetals and metalloids

What are the key characteristics of metals? ● Metals make up 80% of the

periodic table● They are good conductors of heat

and electric current● They often have a high luster, and

are shiny (reflect a lot of light)● With the excpetion of mercury, all

are solid at room temperature ● Many are ductile, meaning they

can be drawn into wires● Many are malleable, meaning they

can be hammered into thin sheets

What are the key characteristics of nonmetals? ● Most are gasses at room

temperature, some are solid and only one is liquid

● There is much more variation among nonmetals than metals

● Hard to give general properties, although most tend to be the opposite of metals

● There are exceptions - eg. carbon will conduct electricity

What about metalloids? ● A metalloid has some properties that are similar to

metals and some that are similar to nonmetals ● At times it behave like a metal, at times it may

behave like a non-metal● Scientists can sometimes change the behavior of a

metalloid● For example - silicon is a poor conductor of

electricity, but add a small amount of boron and it will become a great conductor of electric current

● This mixture is used to make computer chips

Classifying the elementsSection 6.2

Why is the periodic table so useful?

It contains so much information!

What information exactly? ● Individual elemental squares can contain: ● Atomic number and atomic mass, element symbol and name, AND number of

electrons in each energy level● The color of the Chemical symbol can be used to distinguish state of matter,

or whether it is found in nature● Background color of square links elements that have similar properties

○ Example - Group 1A and Group 2A elements are orange, but different shades ○ Same idea with halogens, group 7A

What is the connection between an element’s electron configuration and many elemental properties?

Elements can be sorted into four classes on the basis of the their electron configurations

What is special about the nobel gasses? ● These are the elements found in group 8A● Often referred to as inert gasses, as they rarely take part in reactions; largely

due to their electron configuration● They are the only elements whose highest energy levels are completely filled.

What is a representative element? ● An element in group 1A through 7A● There are a wide range of physical and chemical properties ● Characteristic is that the s and p sublevels of the highest occupied energy

levels are not filled ● For any representative element, the group number is equal to the number of

electrons in the highest occupied energy level

What are the transition elements? ● These are the B group elements found in the middle of the periodic table● Transition metals: Group B elements in the main body of the table. They

contain electrons in d orbitals● Inner transition metals: They do not appear in the main periodic table● Electron contained in a f orbital

Block view of periodic table

● Why didn’t I just teach you electron configuration this way? ● What is the electron configuration of Nickel?

Periodic trends Section 6.3

What is atomic size, why is it important? ● Atoms have different sizes● These can be calculated from looking at molecules, and measuring the

distance between nuclei (half this distance is the atomic radius)○ This distance is very small○ It is often expressed in picometers (pm). There are one trillion (1012) picometers in a meter.

Group trend in Atomic radius

Group trends in atomic radius

What changes across the periodic table?

As atomic number increases the nucleus has a greater positive charges, drawing the electrons closer to the nucleus

Why do atoms get bigger then as you move down a group?

● Electron shielding● The electrons in the lower energy levels acts as a shield between the charge

of the nucleus and the electrons in the highest energy levels

Why does size decrease along a period?

Period trends explained

● The shielding effect remains the same as you move across the period, but the number of protons remains the same

● The increased positive charge in the nucleus draws the outer electrons in closer, decreasing atomic radii.

What is an ion? ● An ion is an atom or a group of

atoms that has a positive or a negative charge. They form when electrons are transferred between atoms

● Ions with positive charge (more protons than electrons) are called cations

● Ions with a negative charge (more electrons than protons) are called anions

What is Ionization energy?

● Ionization energy is the energy required to remove an electron from an atom● The first ionization energy is the energy required to remove the first electron

from an atom

Trends in Ionization energy● As you move down a group less energy is required to remove an electron

from that level, so the ionization energy is smaller ● As move from left to right along a period, nuclear charge increase yet

shielding remains the same, so more energy is required to remove an electron from an atom

Second and third ionization energies ● Sometimes more than one electron can be removed from an atom ● The energy needed to remove a second electron is the second ionization

energy● The energy needed to remove the third is the third ionization energy ● The energy level needed increases as you move to subsequent ionization

energies● Ionization energies allow you to predict the possible ions that an element can

form

Are there any other trend you need to know about!

● Yes! ● Metals tend to lose electrons

and form cations● Nonmetals tend to gain

electrons and form anions● Cations are always smaller

than the atom from which they formed

● Anions are always larger than the atom from which they formed

What is electronegativity?● Electronegativity is the

ability of an atom to attract electrons when the atom is in a compound

● Fluorine is the most electronegative element (value of 4)

● Caesium is the least electronegative element (value of 0.7)

Periodic trend summary