题目

The passengers on the subway who caught a glimpse of me may have thought I was strange．In particular a gentleman sitting opposite me was always staring at me，looking at the cheese bread on the floor in front of me and then staring back at me．A passenger probably dropped it by mistake and got off at a previous stop．but the gentleman might not think so．Next stop，St．Patrick Station-my stop was quickly coming up．I had few minutes to either take the cheese bread，which nobody else was claiming，or left it there．      In those few minutes I felt my pride getting in （2007?资阳）元素周期表是科学家根据各元素的原子结构规律编排的，每一种元素都在不同的横排（周期）和纵行（族）所对应的位置上，周期数代表了该元素原子核外的电子层数，主族（ⅠA族-ⅦA族）数代表了该元素原子最外电子层上的电子数．某元素R在周期表中的位置为：4周期、ⅠA族，则下列关于R元素及其原子的叙述正确的是（　　）A．R原子核内有19个质子B．R原子核外的电子层比O原子核外电子层多一层C．R原子失去1个电子形成阳离子，其结构示意图为D．元素R在化合物中的化合价通常为+2价