查看完整版本 : 如何解釋點解共價鍵強過金屬鍵?

無奈的決定 2013-4-14 05:58 PM

如何解釋點解共價鍵強過金屬鍵?

我記得我答問題:點解一個共價化合物嘅熔沸點高過一種金屬?,
係答因為共價鍵強過金屬鍵。
但比人話錯。
咁我要點解釋點解某啲共價化合物強過某啲金屬化合物?:smile_o16:

dustbus 2013-4-14 06:57 PM

下正常covalent bond既野唔係低過metal咩??

除非佢係giant cova.structure姐

btw寫d野都係話咩atom are held by strong covalent bond之類:smile_41:

jmlo 2013-4-14 11:54 PM

Melting point is related to how atoms/molecules are held in solid state. Except for giant covalent structure (e.g. diamond), covalent molecules are bound by intermolecular forces in solids (e.g. dipole-dipole, van der Waals force). Meanwhile, metal atoms are held by metallic bonds in solid form. Therefore, covalent compounds usually have lower melting points than metals.

無奈的決定 2013-4-15 03:42 PM

[quote]原帖由 [i]jmlo[/i] 於 2013-4-14 11:54 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=358502837&ptid=21762573][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
Melting point is related to how atoms/molecules are held in solid state. Except for giant covalent structure (e.g. diamond), covalent molecules are bound by intermolecular forces in solids (e.g. dipol ... [/quote]
Thanks~
Maybe my question wasn't that clear.

If a question asks why the boiling/melting point of a compound with [b]giant covalent structure[/b] is higher than that of a metal with[b] giant metallic structure[/b], how should I explain it ?  It seems I can't answer "because covalent bond is stronger than metallic bond" as I remember that they are equally strong(correct?).

雲艾法比 2013-4-15 09:14 PM

Interesting.Would like to know the answer.

Zzlaz 2013-4-15 09:23 PM

唔得.
兩隻都係STRONG ELECTROSTATIC FORCE OF ATTRACTION, 平均計應該不相伯仲.

但COVALENT 係DIRECTIONAL, METALLIC 係NON-DIRECTIONAL.

TUNGSTEN VS SILICON:smile_34:

無奈的決定 2013-4-18 03:41 PM

回覆 6# 的帖子

因為共價鍵係directional,所以具有巨型共價結構的化合物熔點高過有巨型金屬結構的金屬?:$

Zzlaz 2013-4-18 08:35 PM

[quote]原帖由 [i]無奈的決定[/i] 於 2013-4-18 03:41 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=358829028&ptid=21762573][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
因為共價鍵係directional,所以具有巨型共價結構的化合物熔點高過有巨型金屬結構的金屬?:$ [/quote]
都話唔係, 兩樣都係勁, 唔可以話邊隻鍵係勁D, 更何況離子鍵, 金屬鍵本身係NON-DIRECTIONAL (一個CATION 痴好多ANION,VICE VERSA / 一個CATION 被大量的DELOCALIZED E- 包圍) 佢的鍵能平均地計係同共價鍵同一個ORDER OF MAGNITUDE

而且GIANT COVALENT STRUCTURE 例子較少.

COUNTER-EXAMPLE
鎢熔點係3千幾度, 而石英只係1700'C

jmlo 2013-4-18 10:48 PM

Interesting to think about the facts:

(1) Carbon is among the element with the highest melting point (>3500C, even higher than the melting point of tungsten, which is around 3400C).

(2) C has the largest tendency of catenation in Group 14, and C-C bond energy is around 350 kJ/mol while Si-Si bond energy is about 220 kJ/mol.

(3) For transition metals, melting point increases down a group and decreases across a period in general. Their melting points are usually higher than main group metals.

They may give you some ideas about how covalent and metallic bond strength can be compared.

James. 2013-4-18 11:38 PM

For the melting point, generally, metals which have giant metallic structure have a lower melting point than Giant covalent ones because melting of metals requires partial breaking of metallic bond.
By the way,
Melting is not only related to the strength of the bonding of them but also we have to consider the packing in most cases. For example, consider the boiling point and melting point of brached chained carbon and straight chained isomers

Zzlaz 2013-4-18 11:50 PM

[quote]原帖由 [i]James.[/i] 於 2013-4-18 11:38 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=358870331&ptid=21762573][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
For the melting point, generally, metals which have giant metallic structure have a lower melting point than Giant covalent ones because melting of metals requires partial breaking of metallic bond.
... [/quote]

GIANT COVALENT, 都唔一定要全部BREAK 哂, 都係PARTIAL BREAKING 可以, complete breaking down 就變了ATOMS 了.

silicon - 1414'C melting point, 3265'C boiling point.

Iron - 1538'C melting point, 2862'C boiling point.


Yes, packing efficiency. 但你例子唔得, PACK 得好, 係effect on van der Waals' forces, not directional covalent bond.

Zzlaz 2013-4-18 11:56 PM

:smile_40:
伽利略MODE.

湯川學送左舊Germanium 比內海

Germanium 有沒有鑽石的properties?

在下,鍺是一種銀白色的元素,硬但易碎[url=http://zh.wikipedia.org/wiki/%E9%94%97#cite_note-nbb-19][19][/url]。這種形式構成一種,技術上叫α鍺,它帶金屬光澤,結構與一樣,為[url=http://zh.wikipedia.org/wiki/%E9%94%97#cite_note-usgs-17][17][/url]。當高於120時,會形成另一種同素異形體,叫β鍺,它的結構與β一樣[url=http://zh.wikipedia.org/wiki/%E9%94%97#cite_note-HollemanAF-20][20][/url]。與、、、與一樣,鍺在熔化態固體化時(即)會膨脹[url=http://zh.wikipedia.org/wiki/%E9%94%97#cite_note-HollemanAF-20][20][/url],而有這種特性的物質並不多。[b]鑽石結構[/b],也叫,空間晶格為,每個晶格點的基元包含兩個相同的原子,分別位在000和。每個原子有四個最鄰近的原子,形成四面體鍵結。[color=#ff0000]"""[b]鑽石結構相當空的,以硬球堆積模型來計算[/b][/color]僅有(34%)。"""[/color][/b][b][/b]
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