Periodic Trends (Cambridge (CIE) O Level Chemistry): Revision Note

The Metallic Character of Elements

• The metallic character of the elements decreases as you move across a Period on the Periodic Table, from left to right, and it increases as you move down a Group

• Across a Period, atoms gain electrons more readily than they lose them, so the elements become less metallic

• Down a Group, the outer electrons are further from the nucleus and are lost more easily, so the elements become more metallic

• Metals occur on the left-hand side of the Periodic Table and non-metals on the right-hand side

• Between the metals and the non-metals lie the elements which display some properties of both

• These elements are referred to as metalloids or semi-metals

Properties of metals and non-metals

A zig-zag line in this diagram separates the metals on the left, from the non-metals on the right

Periodic Trends & Electronic Configuration

• The electronic configuration is the arrangement of electrons into shells for an atom (e.g: the electronic configuration of carbon is 2,4)

• There is a link between the electronic configuration of the elements and their position on the Periodic Table

• The number of notations in the electronic configuration will show the number of occupied shells of electrons the atom has, showing the period

• The last notation shows the number of outer electrons the atom has, showing the group number

Example:  Electronic configuration of chlorine:

The electronic configuration of chlorine as it should be written

Period: The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 shells of electrons.

Group: The final notation, which is 7 in the example, shows that a chlorine atom has 7 outer electrons and is in Group VII

The position of chlorine on the Periodic Table

• Elements in the same group in the Periodic Table have similar chemical properties

• When atoms collide and react, it is the outermost electrons that interact

• The similarity in their chemical properties stems from having the same number of electrons in their outer shell

• For example, both lithium and sodium are in Group I and can react with elements in Group VII to form an ionic compound (charges of Group I ions are 1+, charges of Group VII ions are 1-) by reacting in a similar manner and each donating one electron to the Group VII element

• As you look down a group, a full shell of electrons is added to each subsequent element

  • Lithium's electronic configuration: 2,1

  • Sodium's electronic configuration: 2,8,1

  • Potassium's electronic configuration: 2,8,8,1

Predicting Properties

• Because there are patterns in the way the elements are arranged on the Periodic Table, there are also patterns and trends in the chemical behaviour of the elements and their physical properties

• These trends in properties occur down groups and across the periods of the Periodic Table

• As a result, we can use the Periodic Table to predict properties such as:

  • boiling point

  • melting point

  • density 

  • reactivity 

• Some common properties / trends in properties include:

  • Group I elements react very quickly with water

  • Noble gases are unreactive

  • Transition elements are denser than Group I elements 

  • Reactivity decreases going down Group VII 

  • Melting point decreases going down Group I

• In this way the Periodic Table can be used to predict how a particular element will behave

Identifying Trends

• Using given information about elements, we can identify trends in properties 

• An example of when this might be used is to determine the trend in reactivity of Group I metals 

• The table below shows the reactions of the first three elements in Group I with water 

Observations of lithium, sodium and potassium with water

Lithium

• Reaction equation:

2Li (s) + 2H₂O (l) → 2LiOH (aq) + H₂ (g)

• Slower reaction than sodium

• Bubbles of hydrogen gas

• Lithium moves around the surface of the water

  • Floats on water because it is less dense

Sodium

• Reaction equation:

2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

• Bubbles of hydrogen gas

• Sodium melts into a shiny ball that moves around the surface of the water

  • Floats on water because it is less dense

• Melts because sodium has a low melting point and a lot of heat is released in the reaction

• Hydrogen is evolved, which causes the ball of sodium to move around the surface of the water

• A white trail of sodium hydroxide is produced, which dissolves in the water, producing an alkaline solution

Potassium

• Reaction equation:

2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

• Reacts more violently than sodium

• Bubbles of hydrogen gas

• Melts into a shiny ball that dashes around the surface

• Burns with a lilac flame

• A white trail of potassium hydroxide is produced, which dissolves in the water, producing an alkaline solution

• The observations show that reactivity of the Group I metals increases as you go down the group

• Using this information we can predict the trend going further down Group I for the elements rubidium, caesium and francium

• As the reactivity of alkali metals increases down the group, rubidium, caesium and francium will react more vigorously with air and water than lithium, sodium and potassium

• Lithium will be the least reactive metal in the group at the top, and francium will be the most reactive at the bottom

• Francium is rare and radioactive so is difficult to confirm predictions

Table to Show the Predicted Reaction of other Group I Elements with Water