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Thunder was given its name by dividing the hexagram of same name 震zhèn, shock, thunder, earthquake, in the half, and is named 震zhèn the thunder. The trigram 火huǒ fire was given by the attributes of light, fire, of the hexagram 離 lí, that also means separate, a net. The hexagram 兑duì is generally translated as 悦yuè happiness, was given the symbol of 澤zé marsh, translated in the west as a tranquil lake, because a marsh is where the Chinese grow rice, and a lot of food means happiness. 乾qián, creating, powerful, was given the element of 天tiān the heaven because of its highness and power. The trigram 巽xùn gentleness, penetrating, was given the symbol of 风fēng wind because of its softness, also the penetrating soft sprouts of a plant of wood 木mù. The trigram 水shuǐ water, was given its name because doubling it you obtain the hexagram 坎kǎn pit, abyss, because these places accumulate water, and water means danger like in these places, because of the fear of the sea the Chinese had, also the floods and storms, and crossing a river (like seem in the Yi jing text). The 艮gèn stopping hexagram gives name to the 山shān mountain, because of its solidness, Sun Tzu in the art of war wrote “have the immovability of a mountain”. The 坤kūn field, earth hexagram names the 土tǔ earth, that even has its character as a component of the hexagram name, because the acceptance and softness of earth that embrace everything.

leandroscardoso said:

The hexagram 兑duì is generally translated as 悦yuè happiness, was given the symbol of 澤zé marsh, translated in the west as a tranquil lake, because a marsh is where the Chinese grow rice, and a lot of food means happiness.

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That is an interesting point.

it's not mine, I read somewhere that I can't remember, but too me, as you can see, make a lot of sense

Five elements:
qian乾 dui 兑 （metal)
kan 坎 （water)
gen 艮 kun 坤（earth)
xun巽 zhen震（wood)
li离 （fire)