Structure of the Atom, and the Periodic Table: Alkali metals (Group I elements)

1.0 Structure of the Atom, and the Periodic Table

2.1 Alkali metals (Group I elements)

In Topic 4 of Module 1 we learnt about bases. Bases which dissolve in water are called alkalis. In fact, the change of litmus from red to blue occurs with soluble bases (alkalis) only. Sodium hydroxide is an example, and sodium metal is in Group I of the Periodic Table. Other Group I elements also show this behavior and are therefore called alkali metals. They include lithium (Li), sodium (Na) and potassium (K), among others.

2.1.1 Physical properties of alkali metals

Figure 2.1 shows the appearance of alkali metals.

Observe the video demonstration of physical properties of alkali metals.

Many properties of alkali metals are easy to understand if we consider their atomic structures and electronic configurations, E.C. (Figure 2.1).

The distance between the nucleus and the outermost electron in an atom is called atomic radius, represented by r1, r2, and r3 in Figure 2.1(b). From the video demonstration and Figure 2.1(b), answer Questions 2.1(a).

Questions 2.1(a)

1. How does the atomic radius change as we move down Group I, from lithium to potassium?
2. What causes this change in atomic radius?
3. What can you say about the texture (hardness or softness) of alkali metals, compared to other metals such as iron or steel? How do you know that?
4. How do the atomic structures differ, and how are they similar to one another?
5. From the texture, would you expect the melting and boiling points of alkali metals to be low or high? Explain your answer.
6. Normally smaller particles pack more closely and are therefore more strongly held together than larger particles. Which of the three alkali metals do you expect to have the highest melting and boiling points?
7. Describe the appearance of fresh surfaces of alkali metals.
8. Do the metals conduct electricity or not?

Answers to Questions 2.1(a)

Atomic radius of alkali metals increases down the group (Table 2.1). This is because of the increase in number of energy levels as we go down the group from lithium to potassium. But this leads to a decrease in melting point because the bigger atoms become less closely packed and therefore weakly held together.

Table 2.1 Atomic radii and melting points of alkali metals

Element	Atomic radius (nanometers, nm)	Melting point (oC)
Lithium	0.133	180
Sodium	0.157	98
Potassium	0.203	54

Questions 2.1(b)

Study Figure 2.1 then answer the following questions.

1. Alkali metals form ions by losing the outermost energy level and the electron in it. How does ionic radius change as we move from lithium to sodium? Why?
2. All electrons, being negatively charged, are held by the positively charged nucleus attracting them.
   1. In which of the three elements is the outermost electron most weakly held? Why?
   2. In which of the atoms is the outermost electron most strongly held? Why?
   3. Which of the three atoms is the most reactive? Why?
   4. The energy required to knock off the outermost electron is called ionization energy. Which of the elements lithium, sodium, and potassium have the highest and lowest ionization energies respectively? Explain your answers.

Answers to Questions 2.1(b)

2.1.2 Chemical properties of alkali metals

Observe the video demonstration of chemical properties of alkali metals below and answer Questions 2.1(c).

(courtesy Youtube-Lithium, Sodium, Potassium react with water by SquiggleMom)

Questions 2.1(c)

1. What happens to the fresh surfaces of alkali metals when left exposed to the air? Explain your observation.
2. Why are sodium and potassium stored in oil? Why is oil used instead of water?
3. What is the sign and quantity of charge on ions formed by alkali metals? Why?
4. What is the valency of alkali metals?
5. Write the formulae of lithium, sodium and potassium ions.
6. Describe what is observed when sodium or potassium is added to water, and the solution formed is tested with red litmus paper.
7. Write a chemical equation for the reaction between potassium and water.

Answers to Questions 2.1(c)

Alkali metals react vigorously with cold water to produce metal hydroxides and hydrogen gas. The equations are as follows.

2Li(s)  +   2H₂O(l)  ⟶   2LiOH(aq)  +  H₂(g)

2Na(s)  +   2H₂O(l)  ⟶   2NaOH(aq)  +  H₂(g)

2K(s)  +   2H₂O(l)  ⟶   2KOH(aq)  +  H₂(g)

This reaction becomes more vigorous down the Group as we move from lithium to potassium, and further to rubidium, cesium and francium (not shown).

NB: Alkali metals undergo the reactions of other metals; but their vigorous reaction with cold water is unique. Other metals either do not react with cold water or do so very slowly.

Reaction with chlorine

Observe the video demonstration of the burning of alkali metals in chlorine gas below. Then answer Questions 2.1(d).

(courtesy Youtube-Reacting alkali metals with chlorine by Cowen Physics)

Questions 2.1(d)

1. Describe the observations made when a burning piece of sodium or potassium metal is lowered into a gas jar full of dry chlorine gas.
2. What is the effect of adding water to the product and shaking the mixture?
3. What is the pH of the mixture (neutral, acidic or basic)?
4. Write chemical equations for the reactions between
   1. lithium and chlorine
   2. sodium and chlorine
   3. potassium and chlorine

Answers to Questions 2.1(d)

Alkali metals combine directly with chlorine to form metal chlorides which are white, soluble and form neutral solutions.

Reaction with oxygen

Observe the video demonstration of the burning of alkali metals in oxygen gas below. Then answer Questions 2.1(e).

NB:Forward the video to begin at 5:40th minute (to 6:20).

(courtesy Youtube-Reactions of group 1 metals by Royal Society of Chemistry)

Questions 2.1(e)

1. Describe the observations made when a burning piece of sodium or potassium metal is lowered into a gas jar full of dry oxygen gas
2. Write chemical equations for the reactions between
   1. lithium and oxygen
   2. sodium and oxygen
   3. potassium and oxygen

Answers to Questions 2.1(e)

Formation of soluble oxides and hydroxides is a unique property of alkali metals. Other metal oxides and hydroxides are either insoluble or only sparingly (not much) soluble. Because they are highly soluble, natural compounds of alkali metals mainly occur as solutions in water. Sodium chloride, for example, exists mainly as a solution in sea water, from which it is usually extracted.

Caution: Alkali metals, especially sodium, potassium and heavier members react explosively with steam and dilute acids. These reactions must therefore not be attempted.

Summary about alkali metals

1. Alkali metals have a shiny metallic lusture (appearance).
2. Atomic and ionic radii increase down the group because of increasing number of energy levels.
3. Hardness, melting point and boiling point decrease down the group because atoms become more loosely held together down the group.
4. React by losing one electron; so they form cations (Li⁺, Na⁺, K⁺)
5. Ionization energy decreases down the group because, with increasing energy levels, the outermost electron is farther from the nucleus, loosely held, and requires less energy to knock off
6. Reactivity increases down the group because, with increasing energy levels, the outermost electron is farther from the nucleus, loosely held, and readily knocked off.
7. Alkali metals are highly reactive. They react vigorously with oxygen, chlorine, water and many other non-metals.