题目

The USA is a land of immigrants. Between 1815 and 1914, the world witnessed the greatest peaceful migration in its history: 35 million people, mostly Europeans, left their homelands to start new lives in America. Why did these people risk everything by leaving their homes and families? First, what forced emigrants to make the decision to leave? One major cause for European farmers to leave was the rise in population which in turn led to land hunger. Another was politics. There was an increased taxation（税收） and the growth of armies, and many young men fled eastern Europe to avoid设X、Y、Z代表三种元素．已知： ①和两种离子具有相同的电子层结构； ②Z元素原子核内质子数比Y元素原子核内质子数少9个； ③Y和Z两种元素可以形成4核42个电子的负一价阴离子． 据此，请填空： (1)Y元素是______，Z元素是______ (2)由X、Y、Z三种元素所形成的含68个电子的盐类化合物的化学式是______．