## The Periodic Table of Elements

### History

• • In 1869, Dmitri Mendeleev created the first periodic table.

He organized elements into rows and groups based on their properties.

• • The picture to the left shows his early table.

How is this table different from today ’ s?

• • •

### GROUPS

Vertical columns Same number of valence electrons (electrons in outermost energy level) Share similar properties

Memory trick: Group has the word

up

in it, and groups go up and down!

### GROUPS

• Groups are also called families since they share similar properties.

• Look at the Bohr Models of elements to the right… can you figure out what they have in common?

### GROUPS

• All the elements have the same number of electrons in their outer shells: 1.

• Electrons in the outer shell are called valence electrons. • The number of valence electrons determine the chemical reactivity of the element.

### Group 1: Alkali Metals

• 1 valence electron (1 electron in outer shell) • Have shared properties:  React with water and oxygen  Good conductors  Ductile, malleable and soft  Silvery luster  Low density  Low melting point

• •

### Periods

• Look at the Bohr Models of elements below… can you figure out the pattern?

### Periods

• As you go down the period to the right, the number of Protons increases • The number of protons is called the Atomic Number • This means the periods are arranged by increasing Atomic Number.

• •

### Periods

You may think that the atoms get bigger as they go across the period, but they don ’ t!

The atoms get smaller as they go across. Why do you think this is?

• HINT: As you go along the row there are more protons and electrons. Think about the charges of these particles.

### Periods

• As you move to the right along each period, there is a stronger positive and negative charge, since there are more protons and electrons. • This pulls the shell closer together, causing it to be smaller.

### Practice:

Which two elements are in the same group?

Which two elements are in the same period?

Which two elements have the most similar chemical properties?

Find another element that has the same properties as C.

A B C D

### Central Questions:

Consider these questions about the periodic table: 1. What is the significance of patterns among the elements in the periodic table?

2. Can the theory of the atom explain the organization of the periodic table?

3. What does the periodic table allow us to predict about the elements?

### Conclusion:

1. The patterns of the elements in the periodic table allow scientists to better understand the properties of each element. 2. The structure of the atom (atomic theory) plays a key role in how the periodic table is structure, since groups are arranged by outer valence electrons and periods by increasing atomic number 3. The periodic table allows us to predict the properties, outer valence shell, atomic number and size of unknown elements.

### Patterns Across and Down the Table

• • The physical and chemical properties of the elements change in a predictable way.

Many nonmetals are gases and most metals and metalloids are solids.

### Patterns Across and Down the Table

• • • Density of the elements increases as you move down a group.

The melting points of metals decrease as you move down a group.

The melting points of nonmetals increases as you move down a group.

### Patterns Across and Down the Table

• • The ease at which a metal reacts with other substances decreases from the left to right across a period.

The elements in Group 1 are highly reactive, but the elements in Group 11 are slow to react.

### Patterns Across and Down the Table

• • Nonmetals in Group 18 almost never react.

Nonmetals in Groups 13-17 are more reactive from left to right.

### Why the Periodic Table Works

• • • The periodic table works because it is based on the structures of atoms, especially the valence electrons.

Each element in a period has one more electron than the element to its left.

Elements in a column or group have the same number of valence electrons, which is why these elements react in similar ways.