Which of the following is a correct procedure for the operation of a microscope? a. first, put the specimen into focus using a high power lens, then switch to a lower power lens if needed. b. remove and add slides while the high power lens is selected. c. first, put the specimen into focus using a low power lens, then switch to a higher power lens if needed. d. use the fine focus knob with the low power lens and use the coarse focus knob with the high power lens

- calcium has greater affinity to bond with electron receptors than hydrogen does and hence, it has the ability to replace hydrogen in most of its compounds.

- lithium has 2 valence electrons in the product because it can make only 1 bond with hydrogen. beryllium has 4 valence electrons in the product because it can only make 2 bonds. it has fewer electrons than neon and can react with electron donors. each hydrogen has 2 valence electrons, the same as helium.

- the covalent bond formed between carbon and hydrogen is polar because carbon is more electronegative than hydrogen and it attracts the bonded electron pair closer to itself, thus giving hydrogen a partial positive charge.

- a hydrogen bond is a relatively weak bond that hydrogen atoms make with the electronegative atoms nitrogen, oxygen or fluorine.

so the best choice is a.) 2 calcium (ca) atoms