Photo by Magnus Manske

Historical Development of the Periodic Table

Dmitri Mendeleev

Since scientists have been able to isolate and study single elements, they have known that some elements have very

similar physical and chemical properties.

Lithium Sodium

Photo by Magnus Manske

Potassium

Lithium, Sodium, and Potassium are all silvery-grey metals that are soft enough to cut with a knife. All 3 of them violently react with water.

One of the easiest things to determine about each element was its atomic mass (also called its atomic

weight). Early chemists noticed that the atomic weights of elements with similar physical and chemical

properties seemed to show that these elements were related on a deeper level. Of the 60 or so elements

known by chemists at the time, these similar elements seemed to come in sets of 3 which they called triads.

Photo by Magnus Manske

Lithium6.9 grams

Sodium23.0 grams

Potassium39.1 grams

When early chemists looked at a group of 3 similar elements, it seemed like the average mass of the

elements was always very close to the middle element.

(6.9 + 23.0 + 39.1) / 3 = 23.0

This became known as the Law of Triads, and scientists concluded that this meant these elements were related

at the atomic level.

Lithium6.9 grams

Sodium23.0 grams

Potassium39.1 grams

Photo by Magnus Manske

Calcium40.0 grams

Strontium87.6 grams

Barium137.3 grams

Scientists found many of these triads. The idea was so popular some scientists

tried to say things were triads when they were not.

While lists of elements had existed for a while, the idea that there might be a natural order to the elements started people on

the quest to find the pattern within the elements.

John Newlands and The Law of Octaves

John Newlands was one of the first to present his idea about how the elements were related. He noticed that if your place them in

a table together according to increasing atomic masses, the pattern of similarities repeated after every 8th element.

He called this the Law of Octaves because it reminded him of the musical scale.

John Newlands and The Law of Octaves

Newlands’ idea was NOT taken very seriously at the time because of the problems that were apparent in his table.

Cobalt and nickel appear in the same column as chlorine. Iron appears in the same column as oxygen and sulfur. Copper is in lithium, sodium and potassium’s column.

John Newlands and The Law of Octaves

It turns out that Newlands’ idea works perfectly, but only for elements 3-18! Since some of these elements were still undiscovered and Newlands

used all known elements, his table was obviously not correct.

Dmitri MendeleevDmitri Mendeleev and the 1st Periodic Table

A Russian chemist named Dmitri Mendeleev created the first widely accepted periodic table. He still relied on this idea of 8 groups, but he understood that many elements

were still missing. He placed gaps in his periodic table where he thought these elements belonged, and this kept

his groups more properly aligned.

Dmitri MendeleevDmitri Mendeleev and the 1st Periodic Table

He predicted the properties of these undiscovered elements and when

one of them (Germanium) was discovered shortly after, it had

properties almost identical to what Mendeleev had predicted. Photo by Jurii

Henry Moseley and Atomic Numbers

For decades scientists used atomic masses to place elements in order, but this seemed to throw some of the groups out of

alignment. Henry Moseley, an English physicist, understood the work being done on the atom by Rutherford and Bohr and realized that if you order them by atomic number instead of atomic mass,

then the groups fall into their expected order.

Cobalt has a higher atomic mass than nickel, but it has a smaller atomic number.

The Modern Periodic

Table

The modern periodic table that we use in class is based on our current knowledge of the atom. We now understand that the electron energy levels and energy sublevels of atoms are the reason for the repeating, periodic arrangement of elements.

In science, PERIODIC means repeated, regular intervals or patterns.

There are several things you will need to memorize about the modern periodic table.

Find the dark jagged line on the Periodic Table

On the left side of this line are all of the metals

On the right side of this line are

all of the nonmetals

The elements that lay on either side of the dark, jagged have rather unique properties. Sometimes they act like metals, and sometimes they act like nonmetals.

Because of this, they are called Metalloids

The metals in the middle of the periodic table are known as the transition metals.

Transition Metals

Elements in the same group/family have similar

physical and chemical properties because they all have the same number of

valence electrons!

All of the elements in group 17 have 7 valence electrons

Columns on the periodic table are known as groups

or families.

Valence electrons are the electrons in the outer energy level.

How many valence electrons do each of the following atoms have?

1 valence electron

2 valence electrons

3 valence electrons

How many valence electrons do the elements of group 16 have?

6A = 6

How many valence electrons does phosphorous have?

5A = 5

Remember: We can determine the number of valence electrons in the atom of an element by using the number attached to the As and Bs at the top of the group.

Use your periodic table to determine how many valence electrons are in each

of the elements on your notes.

67

4

81

2

Use your periodic table to determine the number of valence electrons for elements below. Decide which Lewis dot diagram is an accurate representation.

Which Lewis Dot Diagram below accurately represents lithium?

Li Li Li

Which Lewis Dot Diagram below accurately represents boron?

B B BWhich Lewis Dot Diagram below accurately

represents phosphorous?

P P P

For each of the examples on your notes, circle the Lewis Dot Diagram that shows the correct number of

valence electrons for each element.

There are several groups/families on the periodic table whose name you should have memorized.

Alka

li M

etal

sAl

kalin

e E

arth

Met

als

Halo

gens

Nob

le G

ases

Group 1 = Alkali MetalsGroup 2 = Alkaline Earth MetalsGroup 17 = HalogensGroup 18 = Noble Gases

Decide which group number goes with the group names found on

your notes.

1614

1315

Rows on the periodic table are known as periods.

All of the elements in a period have the same number of energy levels.

s p

The elements whose valence electrons are in the s and p sublevels are known as the representative

elements. The number of valence electrons of these elements is followed by an A.

d

f

The elements whose valence electrons are in the d and f sublevels are known as the transition

elements. The number of valence electrons of these elements is followed by a B.

You also need to know that Iron, Cobalt, and Nickel have special properties such as the ability to be made into magnets. Together, these three elements are known as the .Iron Triad

Iron Triad

Which group on the periodic table undergoes the fewest chemical reactions?

Group 18The Noble Gases

They are inert gases. This is because they have 8 valence electrons and their s and p sublevels are full.

Inert means they resist chemical reactions!