Starter 1 If M can be any element

Starter. . . . 1. If M can be any element what group do you think MO is the general formula for? Why? 2. Write equations for M reacting with water and oxygen. 3. Write any other general formulas for M.

Groups 1 and 2 elements 20 February 2021

Groups 1 and 2 elements Lesson Objective: To know the chemical and physical properties of the Group 2 elements and to know the flame tests for Groups 1 and 2 Success criteria: • understand reasons for the trend in ionisation energy down Group 2 • understand reasons for the trend in reactivity of the Group 2 elements down the group • know the reactions of the elements Mg to Ba in Group 2 with oxygen, chlorine and water • understand the formation of characteristic flame colours by Group 1 and 2 compounds in terms of electron transitions • know the flame colours for Groups 1 and 2 compounds • understand experimental procedures to show flame colours in compounds of Group 1 and 2 elements

Group 2 Elements (The Alkaline Earth Metals): Physical Properties • High melting point • Light metals with low densities • They form colourless compounds

Group 2 Elements: Melting Points

Trend in first ionisation energy and atomic radius down a group • 1 st IE’s decrease • The number of shells increases, so the distance between the nucleus and the outer electrons – weaker attractive force. • More inner shells, so the shielding increases – less attraction.

• Ionic radius increases • More shells of electrons • Increased electron shielding • Also less attraction means electrons are pulled in less by the nucleus For each element the +2 ion is smaller than the atom because of the loss of the outer electrons.

Group 2 Elements: Electron Configuration Element Electron Configuration 1 st I. E. /k. J mol-1 Be [He]2 s 2 900 Mg [Ne]3 s 2 736 Ca [Ar]4 s 2 590 Sr [Kr]5 s 2 548 Ba [Xe]6 s 2 502 Ra [Rn]7 s 2 509 • Highest energy electrons in an s sub-shell • Each element has two more electrons than the preceding noble gas • two electrons in the outer shell. (form 2+ ions) • What will their oxidation states be?

Group 2 Elements: Reactivity • Reactive metals that are strong reducing agents. • They are oxidised in reactions – lose 2 electrons M M 2+ + 2 e- (+2 oxidation state) • Reactivity increases down the group • Reactivity increases because of the same factors that cause 1 st I. E. to decrease. Reducing agents reduce (add electrons to) another species Oxidation is the loss of electrons

Explaining the trend in reactivity The reactivity of the elements down group 2 from beryllium to barium increases. Mg This is because it is successively easier to remove electrons to form the 2+ ion. Ca Although increased shielding cancels the increased nuclear charge down the group, the increase in atomic radius results in a decrease in the attractive force between the outer electrons and the nucleus. Sr Ba

Group 2 Elements: Reaction with Oxygen • Group 2 metals react vigorously with oxygen (apart from Be) • It is a redox reaction • Produces an ionic oxide with general formula MO What are the oxidation numbers? e. g. 2 Ca(s) + O 2(g) 2 Ca. O(s) 0 0 +2 Oxidation Numbers – Ca – Oxidised O – Reduced -2 An oxidation number is a measure of the number of electrons that an atom uses to bond with atoms of another element.

Group 2 Elements: Reaction with Water • Group 2 metals (Mg – Ba) react with water to form hydroxides – M(OH)2 • Hydrogen gas is also formed e. g. Ca(s) + 2 H 2 O(l) Ca(OH)2(aq) + H 2(g) What are the oxidation numbers? 0 +1 -2 +2 -2 +1 0 Oxidation Numbers – Ca – Oxidised H – Reduced Only one H atom in each water has been reduced – The other doesn’t change Mg reacts slowly with water – on descending the groups reactions get more vigorous.

Redox reaction with chlorine When group 2 metals react with chlorine, they form the metal chloride. For example: Ca(s) + Cl 2(g) ® Ca. Cl 2(s) 0 0 +2 -1 What are the oxidation states? The product is a white metal chloride. The oxidation state of calcium has increased from 0 in its elemental form to +2 when it is in calcium chloride. This means the calcium has been oxidized. The oxidation state of chlorine has decreased from 0 in its elemental form to -1 when it is in calcium chloride. This means the chlorine has been reduced.

Groups 1 and 2 elements Lesson Objective: To know the chemical and physical properties of the Group 2 elements and to know the flame tests for Groups 1 and 2 Success criteria: • understand reasons for the trend in ionisation energy down Group 2 • understand reasons for the trend in reactivity of the Group 2 elements down the group • know the reactions of the elements Mg to Ba in Group 2 with oxygen, chlorine and water • understand the formation of characteristic flame colours by Group 1 and 2 compounds in terms of electron transitions • know the flame colours for Groups 1 and 2 compounds • understand experimental procedures to show flame colours in compounds of Group 1 and 2 elements

Flame tests practical When metals are burned in oxygen they burn with characteristic flame colours. We are going to investigate the colours of groups 1 and 2 metals. 1. Mix a small amount of the compound you are testing with a few drops of concentrated hydrochloric acid 2. Heat a piece of nichrome wire in a hot Bunsen flame to clean it. 3. Dip the wire into the compound/acid mixture. Hold it in a very hot flame and note the colour produced.

Results • What results did we get? • Why do we get the different coloured flames?

Flame tests When group 2 metals are burned in oxygen, coloured flames are produced. This is due to the presence of metal ions. Flame tests exploit this fact. The presence of certain metal ions can be identified by noting the characteristic flame colour that results from burning. The colours for group 2 metal ions are: magnesium – burns with a bright white light, not classed as a flame colour calcium – brick red strontium – bright red barium – pale green

Group 1 flame tests It's possible to test a compound to detect the presence of an alkali metal ion. Different metals give different coloured flames.

Explaining flame tests When heated, some electrons in an atom or ion are excited to higher energy levels. When they fall back to their initial levels, energy is emitted; sometimes seen as visible light heat energy Electrons may be excited by different amounts into different energy levels and drop back at different times. The colour of the flame is a combination of all these energy emissions.

Groups 1 and 2 elements Lesson Objective: To know the chemical and physical properties of the Group 2 elements and to know the flame tests for Groups 1 and 2 Success criteria: • understand reasons for the trend in ionisation energy down Group 2 • understand reasons for the trend in reactivity of the Group 2 elements down the group • know the reactions of the elements Mg to Ba in Group 2 with oxygen, chlorine and water • understand the formation of characteristic flame colours by Group 1 and 2 compounds in terms of electron transitions • know the flame colours for Groups 1 and 2 compounds • understand experimental procedures to show flame colours in compounds of Group 1 and 2 elements

Practice questions 1) Use the electron configurations of magnesium an calcium to help explain the difference in their first ionisation energies. (5 marks) Mg 1 s 22 p 63 s 2 Ca 1 s 22 p 63 s 23 p 64 s 2 (1 mark) First ionisation energy of Ca is smaller (1 mark) because Ca has (one) more electron shell(s) (1 mark). This reduces the attraction between the nucleus and the outer electrons because it increases the shielding effect (1 mark) and because the outer shell of Ca is further from the nucleus (1 mark)

Practice questions 2) Calcium can be burned in chlorine gas. a) Write an equation for the reaction (1 mark) Ca (s) + Cl 2 (g) Ca. Cl 2 (s) b) State the change in oxidation state of calcium (1 mark) From 0 to +2 c) Predict the appearance of the product (2 marks) White (1 mark) solid (1 mark) d) What type of bonding would the product have? (1 mark) Ionic

Practice questions 3) The table shows the atomic radii of three elements from Group 2. Element X Y Z Atomic radius (nm) 0. 089 0. 198 0. 176 a) Predict which element would react most rapidly with water (1 mark) Y b) Explain your answer (2 marks) Y has the largest radius (1 mark) so it will have the smallest ionisation energy (1 mark)

Practice questions flame tests a) When a substance is heated, what changes occur within the atom that give rise to a coloured flame? (2 marks) Energy is absorbed and electrons move to higher energy levels (1 mark). Energy is released in the form of coloured light when the elctrons fall nack to the lower levels (1 mark) b) A compound gives a lilac flame in a flame test. What s block metal ions might this compound contain? (1 mark) Potassium

Questions 1. Mg + 2 HCl Mg. Cl 2 + H 2 is a redox reaction a) Identify the changes in oxidation number b) Which species are being oxidised and which are being reduced? c)Which species are the oxidising and reducing agents? 2. a) Write the equations for these reactions; i) Barium with water ii) Strontium with oxygen b) Using oxidations numbers, identify what has been oxidised and reduced

Groups 1 and 2 elements Lesson Objective: To know the chemical and physical properties of the Group 2 elements and to know the flame tests for Groups 1 and 2 Success criteria: • understand reasons for the trend in ionisation energy down Group 2 • understand reasons for the trend in reactivity of the Group 2 elements down the group • know the reactions of the elements Mg to Ba in Group 2 with oxygen, chlorine and water • understand the formation of characteristic flame colours by Group 1 and 2 compounds in terms of electron transitions • know the flame colours for Groups 1 and 2 compounds • understand experimental procedures to show flame colours in compounds of Group 1 and 2 elements