Group 2 The Alkaline Earth Metals

Where are the alkaline earth metals? The elements in group 2, on the left of the periodic table, are called the alkaline earth metals. Ra Ba Sr Ca Mg Be beryllium magnesium calcium strontium barium radium Called alkaline earth metals, after their oxides, the alkaline earths. Obtained by electrolysis of the molten earths. 2

Ionisation Energies What is the definition for the first ionisation energy? Energy required to remove one mole of electrons from one mole of atoms in the gaseous state to form one mole of positively charged ions: What would be the equation for the second ionisation energy?

Ionisation Energies What three factors do you think the value of the first ionisation energy depend on? The distance between the outer electron and the nucleus (the atomic radius) The effective nuclear charge The shielding produced by inner electron shells

What is the trend and can you explain it? Both the table and graph above also shows a specific trend in ionisation energies down the group. What is the trend and can you explain it?

Ionisation Energy Trends Down the group: Ionisation energies decrease down the group There is an increasing effective nuclear charge down the group. However, the atomic radius increases too, so the outermost electrons experience this less strongly. Also, there is an increased shielding effect down the group too.

What is the trend and can you explain it? Here you can see that there is a massive jump between the second and third ionisation energies Both the table and graph above also shows a specific trend in ionisation energies down the group. What is the trend and can you explain it?

Ionisation Energy Trends Successive ionisation energies: two electrons in the outer shell. Losing two electrons leave them with noble gas configurations. So readily form 2+ ions. However, removing the third would mean breaking into a new shell, and so requires great energy.

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Identifying Elements – Flame Tests In a flame test, the sample is converted to a chloride because these are more volatile than other salts. However, many cations have no effect on the flame. While, cations like sodium produce such an intense flame, they can mask the presence of other cations.

Why do we get these colours? Electrons occupy certain discrete energy levels. When an electron is promoted from its usual energy level to a higher one, the atom becomes excited. When the electron drops back down to its ground state (usual energy level), the atoms emits a photon of light in the visible range Meaning a certain amount of energy that has a wavelength corresponding to a certain colour.

Why do we get these colours? Atoms or ions that give out energy in regions of the spectrum outside the visible range do not show a characteristic colour. For some the difference between the energy levels is too large for the ion to become excited in a Bunsen burner flame.

A More Accurate Test A flame photometer is used to make a quantitative measurement of the wavelength of light given out by ions in a sample by analysing the flame produced. Used to work our sodium and potassium levels in blood samples in hospitals. Also used to monitor the concentration of sodium, potassium and calcium during the manufacture of wines.

The Reactivity of Group 2 Elements

Reactions with oxygen Reactivity increases down the group. So Barium is the most reactive, and is often stored under oil like the alkali metals, as a result! Metals burn in oxygen to form a simple metal oxide. To be able to make any sensible comparison, you would have to have pieces of metal which were all equally free of oxide coating, with exactly the same surface area and shape, exactly the same flow of oxygen around them, and heated to exactly the same extent to get them started. It can't be done! CaO is called quick lime and is used in farming to counteract soil acidity! It is made commercially by the thermal decomposition of limestone! 16

Strontium - richer red tinge to the flame. Beryllium – sparkly white flame . Magnesium - intense white flame Calcium - intense white flame with a tinge of red at the end. Strontium - richer red tinge to the flame. Barium - pale green flame

Reaction with Chlorine

This is due to a thick oxide layer on the surface Reactions with Water This is due to a thick oxide layer on the surface Beryllium has no reaction with water or steam even at red heat…but why?

This is a more complete reaction! Magnesium has a very slight reaction with cold water. The reaction soon stops because magnesium hydroxide is almost insoluble in water and forms a barrier preventing further reaction. Water: (l) Steam This is a more complete reaction!

The hydroxides aren't very soluble. calcium hydroxide shows up mainly as a white precipitate. Get less precipitate as you go down the group because more of the hydroxide dissolves in the water. These all react with cold water with increasing vigour to give the metal hydroxide and hydrogen. Ca(OH)2 is called slaked lime and is commonly used in farming to counteract acidity of soil! e.g.

Reaction with Water - Summary

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Trends in Group 2 Compounds Progressing down group 2 the atomic radius increases due to the extra shell of electrons for each element. Going down the group the first ionisation energy decreases there is more shielding between the nucleus and the outer electrons and the distance between the nucleus and the outer electron increases and therefore the force of attraction between the nucleus and outer most electrons is reduced. Generally the melting point of the metals decreases down the group this is because as the metal ions get larger the distance between the bonding electrons and the positive nucleus gets larger and reduces the overall attraction between the two. For similar reasons the electronegativity decreases. The reactions of the elements with water become more vigorous down the group. When they do react they produce hydroxides and hydrogen. The solubilities of the hydroxides of the elements increase going down the group. The solubilities of the sulphates of the elements decreases down the group. Barium sulphate is insoluble and is used as a qualitative test to identify sulphate ions.