One day three Frenchmen who did not know English very well were talking about the English language.
The first Frenchman said, ‘I heard someone shout ‘look out’ ,I put my head out of a window and a basin of water fell on me. It seems that ‘look out’ may mean ‘don't look out!’
    The second one followed, ‘ I was once on a ship and heard the captain（船长） shout, ‘All hands on the deck(甲板)’ I put my hands on the deck and someone trod(踩)on them.’
    The third one told his friends his experience too, ‘One early morning I called on my English friend and the maid(女佣) said, ‘He is not up yet, come back in half an hour.’ When I went again for him, she said, ‘He’s not down yet.’I said, ‘If he’s not up and he is not down, where is he then?    What a strange language it is!’

用所学知识回答下列问题．

（1）氟化钠是某些牙膏的添加剂，能有效预防龋齿．如图是氟元素在周期表中的相关信息及钠原子结构示意图，则氟元素的原子核外电子数是 ____，钠原子在化学反应中易 ____（填“失”或“得”）电子．氟化钠可以用钠与氟气（F₂）反应制得．反应的化学方程式是 ____．

（2）氢、碳、氧、钙等元素能组成多种物质．A的化学式是CaO，俗称 ____，A与B（胃液中含有的酸）反应放出大量的热，该反应的化学方程式是 ____．用化学式表示3个碳酸根离子 ____．

1. 在磁体的周围有着一种我们看不见的特殊物质，叫做 ____。磁场对放入其中的磁性物体产生 ____的作用．磁感线是为了形象地描述磁场而引人的理想模型，在客观上 ____。磁感线上，任何一点的 ____方向表示该点磁场的方向，曲线分布的密疏程度表示磁场的 ____。磁场越强的地方，磁感线分布越 ____；磁场越弱的地方，磁感线分布越 ____。