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What you know about Group 2?

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Presentation on theme: "What you know about Group 2?"— Presentation transcript:

1 What you know about Group 2?
True or false? Get a whiteboard and pen and look at your periodic table

2 Boardworks AS Chemistry Trends in Group 2
The electronic configuration for calcium is 1s2 2s22p63s24s23p6 False, 4s2 in wrong place

3 The atomic radius of the group 2 elements increases down the group

4 The charge density increases down group 2

5 Atomic radius is the distance from the first energy level to the outermost energy level

6 You would expect the first ionisation energy of a group 2 metal to be higher than that of a group 1 metal in the same period

7 Group 2 The melting point of the group 2 metals decreases as you descend the group

8 Boardworks AS Chemistry Trends in Group 2

9 Boardworks AS Chemistry Trends in Group 2
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

10 Boardworks AS Chemistry Trends in Group 2
Physical properties Boardworks AS Chemistry Trends in Group 2 The arrangement of the periodic table is such that trends can be analysed both across a period and down a group. Group 2 of the periodic table is shown here. Trends that can be analysed down the group include atomic radius, first ionization energy and melting point. Teacher notes Radium has not been included here because it is very radioactive. This affects its availability, its properties and the experiments that can easily be performed using it. Elements in the same group also undergo similar chemical reactions.

11 Boardworks AS Chemistry Trends in Group 2
Trend in atomic radius Boardworks AS Chemistry Trends in Group 2

12 Explaining the trend in atomic radius
Boardworks AS Chemistry Trends in Group 2 The atomic radius of the elements increases down group 2 from beryllium to barium. Element Atomic radius (nm) The number of protons increases down the group; however, so does the number of shielding electrons. Effective nuclear charge therefore remains approximately constant. beryllium 0.112 magnesium 0.145 calcium 0.194 strontium 0.219 Teacher notes See the ‘Trends in Period 3’ presentation for more information about atomic radius, shielding and effective nuclear charge. barium 0.253 The increase in radius is due to higher principle energy levels being filled, whose orbitals are located further from the nucleus.

13 Trend in first ionization energy
Boardworks AS Chemistry Trends in Group 2

14 First ionization energies in group 2
Boardworks AS Chemistry Trends in Group 2

15 Trend in melting points
Boardworks AS Chemistry Trends in Group 2

16 Explaining the trend in melting points
Boardworks AS Chemistry Trends in Group 2 The melting points of the elements decrease down group 2, with the exception of magnesium to calcium. Element Melting point (K) beryllium 1560 magnesium 923 A metal’s melting point depends on the strength of its metallic bonds. This decreases down the group because the atomic radius increases, resulting in a weaker attraction between the nucleus and delocalized electrons. calcium 1115 strontium 1050 barium 1000 Teacher notes It is not as simple as the crystal packing in magnesium being different to that in the other group 2 metals. In fact, beryllium has the same type of structure as magnesium (hexagonal close-packed), calcium and strontium have another type of structure (face-centred cubic), and barium is different again (body-centred cubic). Melting points are complex phenomena and neither the packing in the crystal nor the increase in atomic radius fully account for the trend observed. See the ‘Bonding and Intermolecular Forces’ presentation for more information about metallic bonding. The melting point of magnesium is lower than expected due to variation in how its atoms pack in the metallic crystal.

17 Physical properties summary
Boardworks AS Chemistry Trends in Group 2

18 First ionization energy of group 2 metals
Boardworks AS Chemistry Trends in Group 2

19 Boardworks AS Chemistry Trends in Group 2
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

20 Method You need to carry out the following tests and record patterns down the group: Flame colours of the group 2 metals (use metal salts) Reaction of the group 2 metals with water Reaction of group 2 oxides with water Reaction of group 2 oxides with acid Reaction of group 2 hydroxides with acid Solubility of group 2 sulphates Solubility of group 2 hydroxides

21 Solubilities of group 2 hydroxides
Boardworks AS Chemistry Trends in Group 2 Teacher notes The beryllium ion is very small and highly charged, and therefore highly polarizing. This gives it and its compounds anomalous properties; for example, the formation of mainly covalent compounds rather than ionic. Beryllium has therefore not been included in the majority of this section.

22 Solubilities of group 2 hydroxides
Boardworks AS Chemistry Trends in Group 2 The solubility of the group 2 hydroxides increases down the group. Magnesium hydroxide is considered to be sparingly soluble and the hydroxides of the lower members of the groups are all considered to be soluble. Group 2 hydroxide Solubility Mg(OH)2 sparingly soluble Ca(OH)2 slightly soluble Sr(OH)2 soluble Ba(OH)2 soluble Teacher notes Beryllium hydroxide is insoluble, following the trend shown. However, it is amphoteric – in some conditions it behaves as a base; in others as an acid. As the solubility of the group 2 hydroxides increases, so does the pH of the solutions formed. This is because the more of the hydroxide that dissolves, the greater the concentration of hydroxide ions (OH-) in the solution formed.

23 Applications of group 2 hydroxides
Boardworks AS Chemistry Trends in Group 2 A suspension of magnesium hydroxide is commonly called milk of magnesia. It is used in medicine as a laxative and to relieve acid indigestion. Calcium hydroxide, also called slaked lime, is used in agriculture to raise the pH of soils. Soil pH is an important factor in agriculture. Photo credit: Russ Munn / Agstockusa / Science Photo Library Teacher notes Regulating soil pH is important because different plants have different pH requirements. Adjusting the soil pH to that at which a particular crop grows best can maximize the yield of that crop.

24 Solubilities of group 2 sulfates
Boardworks AS Chemistry Trends in Group 2

25 Solubilities of group 2 sulfates
Boardworks AS Chemistry Trends in Group 2 The solubility of the group 2 sulfates decreases down the group. Magnesium and calcium sulfate are considered to be soluble, whereas strontium and barium sulfate are considered to be insoluble. Group 2 hydroxide Solubility Note that this decrease in solubility down the group is the opposite of the trend for the solubility of the group 2 hydroxides. MgSO4 soluble CaSO4 slightly soluble Teacher notes Beryllium sulfate is soluble, following the trend shown here. SrSO4 insoluble BaSO4 insoluble

26 Testing for sulfate ions
Boardworks AS Chemistry Trends in Group 2

27 Applications of group 2 sulfates
Boardworks AS Chemistry Trends in Group 2 Barium sulfate is used as a radiocontrast agent to help take X-ray images of the digestive system. It is sometimes known as a ‘barium meal’. Barium sulfate is insoluble, so is not absorbed by the body when swallowed. However, barium is a very good absorber of X-rays and it helps to define structures of the digestive system to aid in diagnosis. Photo credit: Wellcome Photo Library, Wellcome Images Teacher notes Here the X-ray image taken using barium sulfate reveals that the patient has a gastric ulcer.

28 Applications of group 2 compounds
Boardworks AS Chemistry Trends in Group 2

29 Chemical properties summary
Boardworks AS Chemistry Trends in Group 2

30 Boardworks AS Chemistry Trends in Group 2
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

31 Boardworks AS Chemistry Trends in Group 2
Reaction with oxygen Boardworks AS Chemistry Trends in Group 2 Teacher notes The reaction with beryllium is very difficult to carry out because a layer of oxide forms on the surface of the metal preventing the beryllium from burning. The flame colours shown are hard to observe in practice. They are caused by the presence of the metal ions, which are not in very high concentration. There is also likely to be contamination of the sample and flame, which alter its colour.

32 Boardworks AS Chemistry Trends in Group 2
Flame tests Boardworks AS Chemistry Trends in Group 2 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 – bright white calcium – brick red/orange Teacher notes The substance to be tested (often a solution containing unknown ions) is usually held on platinum wire because platinum will not colour the flame itself. Flame tests are vulnerable to contamination, particularly by sodium ions (yellow flame). Viewing the flame through cobalt blue glass can filter out the yellow of sodium contamination. Flame tests produce only qualitative results and rely on experimenters’ experience of the colours. Flame colour is also dependant on the temperature of the flame. strontium – red/crimson barium – pale green/yellow-green

33 Explaining flame tests
Boardworks AS Chemistry Trends in Group 2 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. 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. light heat energy Teacher notes Magnesium does not tend to emit energy of the appropriate frequency to be seen as visible light. The bright white light seen on burning magnesium is to do with the reaction with oxygen producing magnesium oxide, not the electronic transitions discussed here. Burning the metal is not the best way to see the coloured flame because reactions other than electronic transitions take place.

34 Boardworks AS Chemistry Trends in Group 2
Flame test colours Boardworks AS Chemistry Trends in Group 2 Photo credit (calcium and barium): Andrew Lambert Photography / Science Photo Library Photo credit (strontium): Andrew McClenaghan / Science Photo Library

35 Redox reaction with oxygen
Boardworks AS Chemistry Trends in Group 2 When group 2 metals react with oxygen, they form the metal oxide. For example: 2Mg(s) + O2(g) ® 2MgO(s) oxidation states +2 -2 The oxidation state of magnesium has increased from 0 in its elemental form to +2 when it is in magnesium oxide. This means the magnesium has been oxidized. Teacher notes See the ‘Redox Reactions’ presentation for more information about oxidation and reduction. The oxidation state of oxygen has decreased from 0 in its elemental form to -2 when it is in magnesium oxide. This means the oxygen has been reduced.

36 Redox reaction with chlorine
Boardworks AS Chemistry Trends in Group 2 When group 2 metals react with chlorine, they form the metal chloride. For example: Ca(s) + Cl2(g) ® CaCl2(s) oxidation states +2 -1 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. Teacher notes See the ‘Redox Reactions’ presentation for more information about oxidation and reduction. 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.

37 Boardworks AS Chemistry Trends in Group 2
Reaction with water Boardworks AS Chemistry Trends in Group 2 Teacher notes Beryllium does not react with water or steam.

38 Redox reaction with water
Boardworks AS Chemistry Trends in Group 2 When group 2 metals react with water they form the metal hydroxide and hydrogen gas. For example: Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g) oxidation states +1 +2 The oxidation state of strontium has increased from 0 in its elemental form to +2 when it is in strontium hydroxide. This means the strontium has been oxidized. Teacher notes See the ‘Redox Reactions’ presentation for more information about oxidation and reduction. The oxidation state of hydrogen has decreased from +1 in water to 0 when it is in its elemental form. The means the hydrogen has been reduced.

39 Explaining the trend in reactivity
Boardworks AS Chemistry Trends in Group 2 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

40 Reaction of oxides with water
Boardworks AS Chemistry Trends in Group 2 When group 2 metal oxides react with water they form the metal hydroxide. For example: SrO(s) + H2O(l) ® Sr(OH)2(aq) Similar to the reaction between the metal and water, the resulting solution has high pH due to the hydroxide ions from the metal hydroxide. Reactivity is as follows: Oxide Reaction beryllium does not react reacts slowly to form alkaline suspension magnesium calcium reacts to form alkaline suspension strontium, barium react to form alkaline solutions

41 Decomposition of group 2 carbonates
Boardworks AS Chemistry Trends in Group 2 When heated, the group 2 metal carbonates decompose to form the metal oxide and carbon dioxide gas. Splitting compounds using heat is called thermal decomposition. MCO3(s) ® MO(s) CO2(g) The group 2 carbonates become more stable to thermal decomposition going down the group: magnesium carbonate: MgCO3 Teacher notes Note that the ‘M’ in the equation represents any group 2 metal. increasing stability calcium carbonate: CaCO3 strontium carbonate: SrCO3 barium carbonate: BaCO3

42 Decomposition of group 2 nitrates
Boardworks AS Chemistry Trends in Group 2 Thermal decomposition of group 2 metal nitrates forms the metal oxide, nitrogen dioxide and oxygen. 2M(NO3)2(s) ® 2MO(s) + 4NO2(g) + O2(g) Like the group 2 metal carbonates, the nitrates become more stable to thermal decomposition down the group. magnesium nitrate: Mg(NO3)2 Teacher notes Note that the ‘M’ in the equation represents any group 2 metal. increasing stability calcium nitrate: Ca(NO3)2 strontium nitrate: Sr(NO3)2 barium nitrate: Ba(NO3)2

43 Explaining the trend in thermal stability
Boardworks AS Chemistry Trends in Group 2 Metal ions become larger down group 2 but have the same charge. This means their charge density is reduced. A metal ion with a high charge density has strong polarizing power. It can therefore polarize the carbonate ion, making it more likely to split into O2- and CO2 when heated. polarization A metal ion with a low charge density has weak polarizing power, meaning the carbonate ion is less polarized and therefore more thermally stable.

44 Equations for reactions
Boardworks AS Chemistry Trends in Group 2

45 Stability of group 2 carbonates
Boardworks AS Chemistry Trends in Group 2

46 Boardworks AS Chemistry Trends in Group 2
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

47 Boardworks AS Chemistry Trends in Group 2
Glossary Boardworks AS Chemistry Trends in Group 2 Teacher notes atomic radius – Half the shortest internuclear distance found in the structure of an element. Other values often used to represent the atomic radius are covalent radius, van der Waals radius and metallic radius. charge density – The charge-to-size ratio of an ion. Ions with greater charge-to-size ratio have more polarizing power. effective nuclear charge – The resultant attraction from the nucleus experienced by the outermost electrons, after taking shielding effects into account. first ionization energy – The amount of energy required to remove the first electron from an atom. More accurately, it is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms. flame test – A test for the presence of particular metal ions, involving holding the substance in a flame. Different ions produce different coloured flames due to a different pattern of electron transitions and corresponding energy emissions on heating. metal – A substance formed from a regular lattice consisting of positively-charged metal ions surrounded by a sea of delocalized electrons and held together by the attraction between them (metallic bonds). oxidation state – The charge a particular atom in a compound would have if the compound consisted entirely of separate ions. oxidation – The process during which an atom, molecule or ion loses one or more electrons and increases in oxidation state. periodicity – A repeating pattern in certain properties of elements across different periods of the periodic table. polarization – The process in which the electron density of an atom, molecule or ion is distorted so that it develops uneven charge distribution, or becomes polar. principle energy level – One of seven main energy levels in an atom in which an electron may reside, which consist of one or more sub-levels. They are analogous to electron shells. redox – Short for reduction/oxidation. A chemical reaction in which the reacting species have their oxidation states changed. A redox reaction is one in which both oxidation and reduction take place. reduction – The process in which an atom, molecule or ion gains one or more electrons and decreases in oxidation state. shielding – An effect where some of the attraction of the nucleus experienced by the outermost electrons in an atom is cancelled by repulsion from electrons in the lower energy levels. sparingly soluble – A substance of which only a very small amount will dissolve. thermal decomposition – The process of splitting compounds using heat. thermal stability – The degree of resistance to thermal decomposition a compound has.

48 Boardworks AS Chemistry Trends in Group 2
What’s the keyword? Boardworks AS Chemistry Trends in Group 2

49 Boardworks AS Chemistry Trends in Group 2
Multiple-choice quiz Boardworks AS Chemistry Trends in Group 2


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