X、Y均为元素周期表中前20号元素，其简单离子的电子层结构相同，下列说法正确的是（ ）
A．由mXa＋与nYb－得，m＋a=n－b
B．X2－的还原性一定大于Y－
C．X、Y一定不是同周期元素
D．若X的原子半径大于Y，则气态氢化物的稳定性HmX一定大于HnY
答案：B
题目分析：mXa+与nYb-电子层结构相同，即m-a=n+b，A项错；B项中两者都带负电荷，且X带两个，Y是一个时，X2-的还原性强于Y-，B项正确；X、Y可能在同一周期，也可能不在同一周期，若同为金属或非金属元素即为同一周期，若为金属和非金属两种元素，则为不同周期，C项错；若X的原子半径大于Y，则X的非金属性弱于Y，其气态氢化物的稳定也弱于Y，D项错。故本题选B。
点评：本题考查元素在元素周期表中的位置关系以及元素周期律，可根据所学知识作答，题目难度中等。
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| Bill White's father is a lawyer. In his 1 time, he likes to get out in the country and take some 2 of animals. For a long time, Bill had wanted to go with his father on one of his 3 . But his father didn't take Bill along with him 4 he was fifteen. He and Bill drove to the farm which 5 to his uncle in the afternoon. They 6 the night there, so that they could 7 out early the next morning. Uncle Steve and his son Larry, who was sixteen, were going to go out with them. It was still dark when Bill heard the alarm 9 the next morning. He wondered why it was running so early. He couldn't remember 9 he was. He turned over 10 and looked at the clock. It was 5 o'clock. Then he 11 that he was at the 12 and that he was going out to take pictures with his father. He jumped out of bed and got 13 quickly. He ran downstairs. The others were already in the 14 . Uncle Steve was cooking bacon and eggs. There was a pot of coffee on the stove. It 15 good. They ate breakfast quickly. They didn't talk 16 , because they didn't want to 17 up the other people in the house. They filled a thermos bottle 18 hot coffee and took some sandwiches 19 Aunt Grace, Uncle Steve's wife, had made for them. They gathered their camera 20 and started out. | ||||
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