写一写。

我是一粒（lì）种子。春天到了，我才发芽（yá）。谢谢太阳和雨水，是它们帮助我长大。现在，我已经长得很高了。到了秋天，我就会结（jiē）出肥肥果实,农民看了可高兴啦!

    If you do not use your arms or your legs for some time, they become weak; when you start using them again, they slowly become strong again. Everybody knows that. Yet many people do not seem to know that memory works in the same way.

    When someone says that he has a good memory, he really means that he keeps his memory in practice by using it. When someone else says that his memory is poor, he really means that he does not give it enough chance to become strong.

    If a friend says that his arms and legs are weak, we know that it is his own fault. But if he tells us that he has a poor memory, many of us think that his parents are to blame, and few of us know that it is just his own fault.

    Have you ever found that some people can't read or write but usually they have better memories? This is because they cannot read or write and they have to remember things; they cannot write them down in a little notebook. They have to remember days, names, songs and stories; so their memory is the whole time being exercised.

    So if you want to have a good memory, learn from the people: practise remembering.

（1）如图1是部分主族元素第一电离能梯度图，图中，a点对应的元素为氢，b、c两点对应的元素分别为　 ____　、　 ____　（填元素符号）．

（2）将CrCl₃•6H₂O溶解在适量水中得到深绿色溶液，溶液中Cr³⁺以Cr（H₂O）₅Cl²⁺形式存在．

①上述溶液中，不存在的微粒间作用力是　 ____　（填标号）．

A．离子键   B．共价键   C．金属键    D．配位键    E．范德华力

②Cr（H₂O）₅Cl²⁺中Cr³⁺的轨道杂化方式不是sp³，理由是 ____

（3）下面关于晶体的说法不正确的是　 ____　．

A．晶体熔点由低到高：CF₄＜CCl₄＜CBr₄＜CI₄

B．硬度由大到小：金刚石＞碳化硅＞晶体硅

C．熔点由高到低：Na＞Mg＞Al

D．晶格能由大到小：NaF＞NaCl＞NaBr＞NaI

（4）硼化镁晶体在39K时呈超导性．在硼化镁晶体中，镁原子和硼原子是分层排布的，图2是该晶体微观结构的透视图，图中的硼原子和镁原子投影在同一平面上．则硼化镁的化学式为　 ____　．

（5）铁的金属密堆积方式为体心立方结构，结构如图3所示．  则晶胞中铁原子的配位数为　 ____　．若铁原子的半径为r pm，写出该堆积方式的空间利用率的表达式（不要求算出结果）　 ____　．