Question Video: Explaining Why the Melting Point of the Period Three Metals Increases from Group 1 to Group 13 | Nagwa Question Video: Explaining Why the Melting Point of the Period Three Metals Increases from Group 1 to Group 13 | Nagwa

Question Video: Explaining Why the Melting Point of the Period Three Metals Increases from Group 1 to Group 13 Chemistry • Second Year of Secondary School

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Which of the following statements does not in part explain why the melting point of the period 3 elements increases from Na to Al? [A] From Na to Al, the charge on the metal ion decreases from 3+ to 1+. [B] All three elements are metals, so they exhibit metallic bonding. [C] The number of delocalized electrons increases from Na to Al. [D] The strength of the metallic bonding increases.

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Video Transcript

Which of the following statements does not in part explain why the melting point of the period three elements increases from sodium to aluminum? (A) From sodium to aluminum, the charge on the metal ion decreases from three plus to one plus. (B) All three elements are metals, so they exhibit metallic bonding. (C) The number of delocalized electrons increases from sodium to aluminum. Or (D) the strength of the metallic bonding increases.

Moving across a period or down a group on the periodic table, we find periodic trends or patterns occurring in the properties of the elements, for example, in atomic radius and in melting point, as well as many other properties. The question specifically asks us about the period three elements, from sodium to aluminum. These elements are specifically sodium, magnesium, and aluminum. The question asks us which statement does not explain why the melting point trend between these three elements increases. So we can write on our diagram increasing melting point from sodium to aluminum.

We could answer this question by first identifying which statements do correctly but partially describe why the melting point increases from sodium to aluminum. And then we can eliminate the correct statements to find the incorrect statement.

Sodium, magnesium, and aluminum are all metals, and so they all have metallic bonding. Metallic bonds are strong electrostatic forces of attraction between positively charged metal cations and their delocalized valence electrons. Because this electrostatic interaction is an attractive force, it will influence the melting point of a metal, since energy is required to overcome this attractive force during melting. Therefore, statement (B) — all three elements are metals, so they exhibit metallic bonding — does partially explain why there is an increase in the melting point trend from sodium to aluminum, but it does not explain it fully.

In the metallic lattice of sodium, sodium exists as Na+ cations. Each sodium particle donates one valence electron to the sea of delocalized electrons. So the metallic bonding diagram on the right is a good representation of metallic bonding in sodium. Magnesium metal, however, consists of Mg2+ cations and two valence electrons donated to the sea of delocalized electrons per magnesium atom. So we need to adjust our metallic bonding diagram on the right for magnesium. A stronger attractive force will exist between the cations and the delocalized electrons. Therefore, metallic bonding in magnesium is stronger than in sodium. And therefore, the melting point of magnesium is higher than that of sodium.

The same trend applies as we move to aluminum. Aluminum metal consists of Al3+ ions. And each aluminum atom donated three electrons to the sea of delocalized electrons to form these aluminum ions. We can see this in the metallic bonding diagram on the right. Now we have even stronger attractive forces between the delocalized valence electrons and the ions. This increased metallic bonding gives an even higher melting point, because more energy is required to overcome these strong electrostatic interactions. We can now see that statement (C) — the number of delocalized electrons increases from sodium to aluminum — is true. And it partially explains why the melting point increases across this period.

Statement (D) is also true; it is factually correct. The strength of the metallic bonding increases from sodium to aluminum. However, the question asked which statement does not in part explain this melting point trend. We can now change our checks to crosses and eliminate (B), (C), and (D). “(A) From sodium to aluminum, the charge on the metal ion decreases from three plus to one plus” is factually incorrect, and therefore this is our answer.

Finally, which of the following statements does not in part explain why the melting point of the period three elements increases from sodium to aluminum? The answer is (A). From sodium to aluminum, the charge on the metal ion decreases from three plus to one plus.

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