Melting and Boiling points? Group I metals Trends Alkali Metal Melting and Boiling points? Reactivity… Densities

What is the trend in density? The alkali metals generally become more dense going down the group, but the trend is not perfect because potassium is less dense than sodium. Element Density (g/dm3) lithium 0.53 sodium 0.97 potassium 0.86 rubidium 1.53 caesium 1.87 Water has a density of 1 g/dm3 (note: dm3 = a volume unit equivalent to a litre). Which elements in group 1 will float on water?

Quick review… The size of the element’s atoms, and the number of full electron shells, increases down the group. Size & # of e shells Li Na This means that, down the group, the electron in the outer shell gets further away from the nucleus and is shielded by more electron shells. K Rb The further away an electron is from the positive attraction of the nucleus, the easier it can be lost in reactions. Cs

What is the trend in melting and boiling points? The melting points and boiling points of alkali metals decrease going down the group. Element Melting point (°C) Boiling point (°C) lithium 181 1342 sodium 98 883 potassium 64 760 rubidium 39 686 caesium 28 671 The size of atom increases down Group I and therefore attraction between positive nucleus and the delocalised electrons is weaker (electrons are further away from the nucleus and therefore less heat energy is required in order for the atom to lose its outer electron)

General Reactive trends Caesium – the most reactive metal available in useful amounts Metals getting more reactive Fluorine – the most reactive non-metal

Electron structure and reactivity The reactivity of alkali metals increases down the group. What is the reason for this? The size of the element’s atoms, and the number of full electron shells, increases down the group. increase in reactivity Li Na This means that, down the group, the electron in the outer shell gets further away from the nucleus and is shielded by more electron shells. K Rb The further away an electron is from the positive attraction of the nucleus, the easier it can be lost in reactions. Cs This means that reactivity increases with the size of the atom.

How do the alkali metals react with oxygen? All alkali metals react with oxygen in the air to form metal oxides. This produces a layer of dull oxide on the surface of the metal, called tarnish. The speed with which alkali metals react with oxygen in the air increases going down the group: lithium – tarnishes slowly sodium – tarnishes quickly potassium – tarnishes very quickly. Photo credit (top and bottom): Dr John Mileham Top image – uncut lithium, highly tarnished on the exposed surface. Why are alkali metals stored in oil? The oil prevents them from reacting with oxygen and tarnishing.

What does the reaction with water produce? All the alkali metals react vigorously with water. The reaction with water becomes more vigorous as you go down the group. It is an exothermic reaction as it releases a lot of heat. The reaction produces a gas that ignites a lighted splint with a squeaky pop. What is this gas? When green universal indicator is added to the reaction mixture, it turns purple. What does this tell you about the products of this reaction? Photo credit: Dr John Mileham Image of lithium in water, with universal indicator added. The indicator was green before the reaction, but has turned purple during the course of the reaction.

What is the equation for the reaction with water? This reaction creates alkaline hydroxide ions. This is why the group 1 elements are called the alkali metals. The general equation for the reaction between an alkali metal reacting with water is: 2M(s) + 2H2O(l)  2MOH (aq) + H2(g) alkali metal + water  alkali metal + hydrogen hydroxide

How does lithium react with water? Lithium is the least reactive of the alkali metals. When added to water, it fizzes and moves around slowly across the surface of the water. What is the equation for this reaction? Photo credit: Dr John Mileham 2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g) lithium + water  lithium + hydrogen hydroxide

How does sodium react with water? When added to water, sodium fizzes more than lithium, and moves quickly across the surface of the water. The sodium melts as it reacts, and it becomes spherical and shiny, like a ball bearing. The hydrogen sometimes catches fire because of the heat from the reaction. What is the equation for this reaction? Photo credit (left and right): Dr John Mileham Left image – sodium reacting with water. It has created enough heat to melt the sodium to a round and spherical shape. Right image – in this image, sodium is reacting so strongly with the water that it has caught fire, burning with the characteristic sodium flame coloured. 2Na (s) + 2H2O (l)  2NaOH (aq) + H2 (g) sodium + water  sodium + hydrogen hydroxide

How does potassium react with water? When added to water, the potassium moves across the surface of the water very quickly. The reaction produces so much heat that the hydrogen given off catches alight. What colour would the flame be? Like sodium, it melts with the heat of the reaction. What is the equation for this reaction? Photo credit: Dr John Mileham 2K (s) + 2H2O (l)  2KOH (aq) + H2 (g) potassium + water  potassium + hydrogen hydroxide

See Group 1 & 7 trends.FLA