The Periodic Table

Objectives

• Relate the organization of periodic table to the arrangement of electrons within an atom

• Explain why some atoms gain or lose electrons to form ions

• Determine how many protons, neutrons, and electrons an atom has, given its symbol, atomic number, & mass number

• You will be able to describe how the abundance of isotopes affects an element’s average atomic mass

History of the Periodic Table• Russian chemist Dmitri Mendeleev is

generally credited as being the “father” of the periodic table

• His work was based on earlier versions of periodic tables by a number of scientists, including:

A. E. Béguyer de Chancourtois

John A. R. Newlands.

Lothar Meyer

History of the Periodic Table cont.• The modern periodic table is based on one

presented by Mendeleev in 1869

• He realized that certain properties repeat periodically when the elements are arranged horizontally in order of atomic weight

• He also placed chemically similar elements in vertical columns, leaving empty spaces as needed

• From the empty spaces, Mendeleev deduced the existence of unknown elements and predicted some of their properties

• When Mendeleev published his first table, scientists did not know about subatomic particles

The Modern Periodic Table• Today, elements are arranged by atomic number instead of by atomic weight

• The modern periodic table has 118 elements

• Only 114 have official names

• Many more than the 63 elements known to scientists in Mendeleev’s time

Organization of the Periodic Table

• It groups similar elements together

• The organization makes it easier to predict the properties of elements

• Elements are represented by their symbols

• Position in the table helps to determine properties of elements

• The order of arrangement is based on the number of protons an atom of that element has in the nucleus

Periodic Law

• States that the repeating chemical and physical properties of elements change periodically with the atomic numbers of the elements

Transuranium Elements

• Are elements past uranium on the periodic table

• Have been difficult to study because they do not exist in nature.

• They must be created in a laboratory, and many exist for a very short period of time.

• One particularly troublesome element is element 104• American scientists Albert Ghiorso and James Harris created this elusive element at the Lawrence Radiation Laboratory at the University of California at Berkeley in 1969

Determining Electron Arrangement

• You can determine how an atom’s electrons are arranged if you know where that element is located in the table

• A group is a column of elements in the periodic table

• Elements in the same group have the same number of valence electrons!

• Elements in the same group have similar properties

• Periods are the horizontal rows in the periodic table

• The number of protons & the number of electrons increases as you move across a period left to right

Electron Arrangement Examples

•Hydrogen & Helium are in the same period

Energy Level Orbital # of Electrons

1 s 1

1 s 2

• Lithium & Carbon are in the same period

Energy Level Orbital Number of Electrons

1 s 2

2 s 1

Energy Level Orbital Number of Electrons

1 s 2

2 s 2

2 p 2

Electron Arrangement Examples

Electron Arrangement Table

You Try!---What about oxygen?Energy Level Orbital # of Electrons

Atoms Form Ions

• Atoms that do not have filled outer orbitals can experience ionization

• Ionization is the ability to gain or lose valence electrons, resulting in charged atoms

• Ions are charged atoms and can be positive or negative

• Cations are positive ions

• Anions are negative ions

Specific Examples

• Lithium loses 1 electron to form a 1+ charged ion

• Lithium has 3 electrons• 1 in the outer most orbital

• Fluorine gains 1 electron to form a 1- charged ion

• It has 7 electrons in its outer orbital

Specific Examples

How Do the Structures of Atoms Differ?

• Atomic number (Z) equals the number of protons as well as the number of electrons• Atoms are always neutral!

• Mass number (A) equals the total number of subatomic particles in the nucleus • The number of protons & neutrons• It is also the “average atomic weight” of an element

Isotopes

• Are atoms of the same element that differ by the number of neutrons in the nucleus

• They DO NOT differ in atomic number

• They ONLY differ in mass number & their physical properties

Calculating the # of Neutrons

Let’s Practice!

• K

• Au

• B

• Si

• U-235

K = 20

Au = 118

B = 6

Si = 14

U-235 = 143

The Mass of an Atom• Is very small

• We usually work with atomic mass units

• Atomic mass unit (amu) is equal to one-twelfth of the mass of carbon-12 atom

• Average atomic mass is a weighted average of the masses of all naturally occurring isotopes of an element

• The more common the isotope the greater the effect on the average

Mass of an Atoms

More Practice!• Calculate the # of protons, electrons,

& neutrons in the following isotopes

1. Carbon-14

2. Nitrogen-15

3. Sulfur-35

4. Calcium-45

5. Iodine-131