5.6 Periodic Trends

PERIODIC TRENDS GROUP PROPERTIES TRANSITION ELEMENTS

By the end of the topic students should be able to: □ Describe how the elements are arranged in the Periodic Table: - in order of proton number; - so that elements with the same number of outermost shell electrons are put together. □Describe the change from metal to non-metal across the Periods from left to right. □ Describe the relationship between Group number, number of valency electrons and metallic/non-metallic nature. □ Describe the main physical properties of the Alkali metals, lithium to potassium. □ Describe the reactions of Group I elements, lithium to potassium with cold water. □ Describe the trends in properties of the Group I elements going down the Group. □ Predict the properties of other Group I elements from their positions in the Group. □ Describe the main physical properties of the Group VII elements, chlorine to iodine. □ Describe the displacement reactions of the Group VII elements, chlorine to iodine. □ Predict the properties of the other VII elements from their positions in the Group. □ Describe the main properties of the Transition Elements. □ Predict properties of other elements from their positions in the Periodic Table. Please tick in the box if you can do the above. FEATURES OF THE PERIODIC TABLE

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 The Periodic Table is a list of elements arranged in order of their increasing proton (atomic) numbers.  The Periodic Table divides the elements into periods and groups.

Figure 1 showing a simplified Periodic Table.

a) Periods  The Periodic Table consists of 7 periods of elements, numbered 1 to 7.  The periods run horizontally from left to right. ⇒ On the simplified block of the periodic table below, sketch the positions of Period 1 and Period 3.

b) Groups  The Periodic Table has 8 groups of elements, numbered from 1 to 0.  The groups run vertically from top to bottom.  Group 0 is sometimes called Group VIII. ⇒ On the simplified block of the periodic table below, sketch the positions of Group I and Group 0.

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c) Transition Elements  The block of metals between II and III is known as the transition elements.  They are so called because they represent the transition from metals to non-metals as we move from left to right across a period. ⇒ On the simplified block of the periodic table below, sketch the positions of the transition elements.

CHANGES IN ELEMENTS ACROSS A PERIOD (PERIODIC TRENDS)

Figure 2 showing the positions of metals and non-metals in the Periodic Table.

 The Periodic Table divides the elements into metals and nonmetals. The bold line in Figure 2 divides the metals from the nonmetals.  Several elements, for example silicon (Si) and germanium (Ge) are located close to bold line. Because of their positions, these 3 Prepared by Kartini Ishak

elements have the properties of both a metal and non-metal. Silicon and germanium are known as metalloids.  As the elements across a Period changes from metals to nonmetals, the electrical conductivity changes from good to bad conductor of electricity.  The oxides of metals are usually basic and the oxides of nonmetals are usually acidic.  The oxides of metals close to the dividing line have basic and acidic properties, i.e. they are amphoteric. Examples of these oxides are Beryllium oxide and Aluminium oxide.  A summary of what happened across the period is shown below:

 An example of how the properties and the nature of oxide changes across Period 3.

 What do group and period numbers tell us about the electronic structures of elements? ⇒ Refer to the table below and what do you notice about the relationship between the electronic structures with group and period numbers.

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 Electronic structure and group number: The number of valence electrons (outer shell electrons) is the same as the group number of the elements.  Electronic structure and period number: The number of electron shells is the same as the period number of the elements. CHANGES IN ELEMENTS GOING DOWN A GROUP (GROUP PROPERTIES)  In any group of the Periodic Table, the following trends can be seen as you go down the group: - the proton number becomes bigger. - the atoms become bigger. - the properties of the elements become more metallic. This is because on going down the group, elements lose electrons more easily.  The changes from non-metallic to metallic character when going down a group are less obvious for elements in Group I, II, VII and 0. However, for elements in the Periodic Table (e.g. Groups IV, V and VI), the changes are significant. For example, in Group IV, the element at the top (Carbon) is a non-metal but the elements at the bottom (tin and lead) are metals.  How are the types of ion formed by elements related to group number? ⇒ Refer to the table below and what do you notice about the relationship between group numbers and the ion formed by an element.

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 The elements in Groups I, II and II are metals. -their atoms lose electrons to form positive ions, e.g. Na+ - The charge of the ion is the same as the group number of the elements.  The elements in Groups IV and V of the Periodic Table are less likely to form ions. They - share electrons to form covalent bonds. - have a maximum oxidation state that is the same as the group number of the element as shown on the table below.

 The elements in Groups VI and VII - are non-metals - tend to gain electrons and form negative ions, e.g. Cl The elements in Group 0 of the Periodic Table - have stable electronic configurations - do not form compounds.

ELEMENTS YOU NEED TO BE FAMILIAR WITH FOR O’LEVEL

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GROUP I ELEMENTS – ALKALI METALS

 Group I elements are called alkali metals because they react with water to form alkalis. PHYSICAL PROPERTIES OF THE ALKALI METALS  All atoms have 1 electron in their outermost shell.  Their atoms lose the outermost shell electrons to form ions with charge +1.  The elements are soft silvery solids/  The elements are metals and so conduct electricity.  The elements have low melting and boiling points. 7 Prepared by Kartini Ishak

 The elements have low densities. Lithium, sodium and potassium float on water.  How do the physical properties change on going down Group I?

⇒ From the table above, what do you notice about the melting point, boiling point and density as you go down Group I? - Melting and boiling point: Decreases down the group - Density: Increases down the group. CHEMICAL PROPERTIES OF THE ALKALI METALS  The alkali metals are reactive metals. They are stored in oil to prevent them from reacting with air and water.  Due to their similar electronic structures, all alkali metals have similar chemical properties.  All the alkali metals react with cold water to form hydrogen and an alkali. ⇒ If a drop of the solution is placed on a piece of red litmus paper after the reaction, what will be colour of the litmus paper? Blue  The table on the next page summarizes the reactions of alkali metals with water.

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 How do the chemical properties change on going down Group I? - As we go down Group I, the size of the atom increases. - It is easier to lose the valence electron from bigger atoms. - Hence reactivity increases on going down Group I. - This is illustrated by the reactions of the alkali metals with water on the table above.  Compounds of the alkali metals are ionic, soluble in water and have similar chemical formulas.

GROUP VII ELEMENTS – HALOGENS 9 Prepared by Kartini Ishak

 The elements in Group VII of the Periodic Table are called halogens.  The elements in the group are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). PHYSICAL PROPERTIES OF THE HALOGENS    

The halogens are non-metals. They exist as diatomic covalent molecules: F2, Cl2, Br2, I2. The halogens have low melting and boiling points. They are also coloured.

 How do the physical properties of the halogens change on going down the group?

⇒ From the table above, what do you notice about the melting point, boiling point and appearance as you go down the Group? - Melting and boiling point: Increases down the group. Chlorine is a gas, bromine is a liquid while iodine is a solid at room temperature. - Appearance: the colours of the halogens become darker. CHEMICAL PROPERTIES OF THE HALOGENS  Halogens are reactive non-metals. Why are they so reactive?

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- The valence shell of each halogen contains seven valence electrons. This means that only one more electron is needed to achieve a stable noble gas structure.  Halogens react with most metals to form salts called halides. Examples of ions include fluoride ions, chloride ions, bromide ions and iodide ions. Displacement Reactions  What are the displacement reactions of the halogens? A displacement reaction is a reaction in which one element takes the place of another element in a compound. The main rule: A more reactive halogen will displace a less reactive halogen from its halide solution.

 The reactivity of halogens decreases down the group.  For example, when aqueous chlorine is added to aqueous sodium bromide, a reddish brown solution is obtained. Chlorine, being more reactive than bromide, displaces bromine from the bromide solution.

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⇒ Write down the chemical reaction of chlorine displacing iodine from an iodide solution of potassium iodide and take note of the colour changes.

⇒ Write down the chemical reaction of bromine displacing iodine from an iodide solution of potassium iodide solution and take note of the colour changes.

⇒ When bromine water was added to potassium chloride solution, no reaction occurs. Explain why? _______________________________________________________________ _______________________________________________________________ GROUP O ELEMENTS- NOBLE GASES

 Group 0 elements are the least reactive elements in the Periodic Table. Explain why. They have a stable electronic configuration because the last shell is full.  The Group 0 elements are also referred to as inert gases (because they are unreactive) or rare gases (because less than 1% of the air is made up of these gases). PROPERTIES OF NOBLE GASES  The noble gases 12 Prepared by Kartini Ishak

- are monatomic elements - are all colourless gases at room temperature. - have low melting and boiling points that increase on going down the group. - are insoluble in water. - are unreactive. MAIN USES OF NOBLE GASES  Neon: Used in making lights.  Helium: Used for filling weather or advertisement balloons and airships.  Argon: Used to fill electric bulbs because it provides an inert atmosphere which prevents the filament from oxidation in air.  Argon is also used for certain processes such as welding of stainless steel.  Divers working underwater breathe a mixture of 80% helium and 20% oxygen instead of air. Helium is used in preference to nitrogen because it is less soluble in blood than nitrogen. THE TRANSITION METALS

 The transition elements are a block of metals in the middle of the Periodic Table. They have similar properties. PROPERTIES OF TRANSITION METALS  Transition metals have: 1) high density, melting and boiling point. - most other metals have lower densities and melting points. The comparison is shown in the Table below.

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2) coloured compounds - solutions of their ions in water are always coloured. E.g. a blue solution suggest that copper ions, Cu2+ , are present. 3) Variable Oxidation State - An example is iron, which commonly forms compounds containing Fe2+ ions or Fe3+ ions, with oxidation state of +2 and +3 respectively. 4) been known as good catalyst Transition metals are often used to spread up chemical reactions. E.g. Iron is used in the Haber Process and Vanadium (V) oxide is used in the Contact Process.

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WORKSHEET 1

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WORKSHEET 3

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WORKSHEET 3

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