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Alkali metals


The elements in Group 1 of the Periodic Table are called the ALKALI METALS(碱金属 The most 碱金属). 碱金属 common alkali metals are lithium (Li), sodium (Na) and potassium (K).

Position of the alkali metals

Appearance
Lithium, sodium and potassium are all soft metals that are easily cut with a scalpel(解剖刀) or knife. The freshly cut surface is a shiny, silver colour, but this tarnishes(失去光泽) quickly to a dull grey as the (失去光泽) metal reacts with oxygen and water in the air. Pieces of such metals are stored in oil to prevent these reactions. The shiny surface of sodium tarnishes more quickly than that of lithium. And potassium tarnishes more quickly than sodium. This shows the increasing reactivity of the metals as we go down the group.

Handling alkali metals
Because the alkali metals are so reactive, care has to be taken when using them. They must not be touched because they will react with the water in sweat on the skin. Gloves may be used, and goggles(护目镜) should be worn.

Physical properties of the alkali metals
Melting point The alkali metals have low melting and boiling points compared to most other metals. As you can see from the next slide , lithium, at the top of Group 1, has the highest melting point in the group. The melting points then decrease as you go down the group(due to the size of the atoms increasing and the resulting metallic bonding decreasing ).

Melting points of Group 1 metals

Density The density of a substance is a measure of how much mass it has for its size. It is measured in grams/cubic centimetre. For example gold and lead are very dense metals - even a small lump of either of them can still feel heavy. The alkali metals have low densities compared to most other metals. (They feel lighter.) You can see from the next slide that lithium, at the top of Group 1, has the lowest density in the group. The densities then generally increase as you go down the group.

Densities of Group 1 metals

Hardness The alkali metals are very soft. Lithium is the hardest alkali metal and they become softer as you go down the group.

Chemical properties Reaction with cold water
All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group. This shows that the reactivity of the alkali metals increases as you go down Group 1.

Reaction with cold water
Lithium When lithium is added to water, lithium floats. It fizzes(发嘶嘶声) steadily and becomes smaller, until it eventually disappears. lithium + water → lithium hydroxide + hydrogen 2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)

Reaction with cold water
Sodium When sodium is added to water, the sodium melts to form a ball that moves around on the surface. It fizzes rapidly, and the hydrogen produced may burn with an orange flame before the sodium disappears. sodium + water → sodium hydroxide + hydrogen 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Reaction with cold water
Potassium When potassium is added to water, the metal melts and floats. It moves around very quickly on the surface of the water. The hydrogen ignites (着火,开始燃烧)instantly. The metal is also set on fire, with sparks and a lilac flame(浅紫色 的火焰). There is sometimes a small explosion at the end of the reaction. potassium + water → potassium hydroxide + hydrogen 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)

Reaction with cold water
Strong alkalis The hydroxides formed in all of these reactions dissolve in water to form alkaline solutions. These solutions turn universal indicator purple, showing they are strongly alkaline(碱性的). Strong (碱性的) alkalis are corrosive(有腐蚀性的), so care must be taken when they are used - for example, by using goggles and gloves.

Reaction with chlorine
All of the alkali metals react vigorously with chlorine gas. Each reaction produces a white crystalline salt. The reaction gets more violent as you move down Group 1, showing how reactivity increases down the group.

Reaction with chlorine
Lithium If a piece of hot lithium is lowered into a jar (广口瓶) of chlorine, white powder is produced and settles on the sides of the jar. This is the salt lithium chloride. lithium + chlorine → lithium chloride 2Li(s) + Cl2(g) → 2LiCl(s)

Reaction with chlorine
Sodium If a piece of hot sodium is lowered into a jar of chlorine, the sodium burns with a bright yellow flame. Clouds of white powder are produced and settle on the sides of the jar. This is the salt sodium chloride. The reaction of sodium with chlorine is similar to the reaction with lithium, but more vigorous. sodium + chlorine → sodium chloride 2Na(s) + Cl2(g) → 2NaCl(s)

Reaction with chlorine
Potassium Potassium reacts more violently with chlorine than sodium does. potassium + chlorine → potassium chloride 2K(s) + Cl2(g) → 2KCl(s)


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