Metallic bonding

In nineteen hundred a physicist named Paul Drude tried to explain how a solid lump of metal carries electricity. What did he picture moving through it? The tempting answer is the atoms themselves, sliding along and carrying the charge with them. But the particles in a solid metal stay in their places. Drude pictured something far smaller drifting between them: electrons, loose from their atoms, moving about like a gas. The electron had been discovered only three years earlier. His picture of loose electrons among fixed particles survives in the model of a metal taught today. How those electrons carry a current is a later video's job; the model itself comes first.

Start with the whole lump, not one atom. A metal is a giant structure: millions upon millions of atoms packed in regular rows and layers, repeating until the lump ends. Inside a metal, each atom's outer electrons do not stay with that atom. They leave it, and become free to move anywhere in the whole structure. Those freed electrons are called delocalised. Delocalised means not located anywhere in particular: no electron belongs to any one atom. What each atom leaves behind is a positive metal ion. An ion is a charged particle: this one has lost negative electrons, so it is positive. It is still the same element, because its protons never moved. The delocalised electrons form a sea that fills the spaces between the ions. Every ion shares the whole sea, and no two atoms share a pair of their own. The metallic bond is the strong electrostatic attraction between the positive metal ions and the sea of delocalised electrons. It acts in every direction and holds the whole structure together. This model is for Foundation and Higher alike. Three possible labels for the particles sitting in the rows. The first: atoms. The second: protons. The third: positive metal ions. Which of those three labels belongs on the particles in the rows? The third label. Each atom has given its outer electrons to the sea, so what sits in each place in the rows is a positive ion of the metal. Protons are only part of its nucleus.

How many electrons each atom gives to the sea depends on its group in the periodic table. A sodium atom, in group 1, gives one electron and becomes a sodium ion, one plus. A magnesium atom, in group 2, gives two and becomes a magnesium ion, two plus. Aluminium is in group 3, and its foil is a metal like any other. What charge do the ions in solid aluminium carry? Three plus. Each aluminium atom gives its three outer electrons to the sea and is left as an aluminium ion, three plus. One examiner's report on a Foundation paper, about aluminium, notes: "Many stated that aluminium, rather than aluminium ions, is positive." In plain words, students called the metal positive. Say positive metal ions, in full, every time: the metal as a whole is not charged.

A question about a metal's structure wants the lump described, so here is one to describe. Describe the structure and bonding in solid magnesium. Name the particles, describe what happened to the outer electrons, and name what the metallic bond is an attraction between. All three parts belong together in one answer. Have a go at this one before I show you. Solid magnesium is a giant structure of magnesium ions, two plus, in regular rows. Each atom lost its two outer electrons, which are now delocalised. The metallic bond is the strong electrostatic attraction between those positive metal ions and the sea of delocalised electrons. A report on a Higher paper puts the slip this way: "When candidates did not gain credit it was usually because they described the atomic structure of lithium, often also describing how it bonds with other elements, rather than the metallic structure." Students described one atom instead of the metal. The way to avoid it is to ask first: is this question about one atom, or about the lump?

A metallic diagram is a different kind of picture from a dot-and-cross diagram. A dot-and-cross diagram shows where particular electrons came from and went. A metallic diagram cannot, because no electron belongs anywhere, so it shows rows of positive ions in a sea. Asked to draw the bonding in sodium, a student draws two sodium atoms sharing an electron as dots and crosses. The circles are labelled protons, and the middle is labelled shared electrons. Nothing about that drawing fits the metal it was meant to show. What should the diagram have been, and what should the two labels say? A metallic diagram: rows of circles labelled sodium ions, one plus, in a sea labelled delocalised electrons. Sodium makes no pairs, so no dots and crosses. Another report on a Higher paper lists this under misconceptions: "the drawing of a variety of covalent and ionic bonding dot and cross diagrams and the labelling of protons rather than positive metal ions." Students reached for the wrong kind of diagram, and named the wrong particle. The habit that fixes both: rows of positive metal ions, a sea of delocalised electrons, and every circle labelled with its charge.

One more question follows from the model itself, and it is a fair one to ask. A block of magnesium is made of magnesium ions, and every one of them is two plus. So why does the block as a whole carry no charge? Because the electrons that left the atoms are still inside the block, in the sea. Each ion is two plus, matched by two delocalised electrons, so the charges cancel. Nothing has left the metal. The electrons have only stopped belonging to one atom each. Not every exam asks you to describe this model by name. It is still what the answers about metal properties are built on, so it is worth having.

Over to you, to rebuild the model from memory. What is the metallic bond an attraction between? The positive metal ions and the sea of delocalised electrons around them. Now a metal the video has not used, from group 1. Potassium: what are the particles in the solid, with their charge? Potassium ions, one plus, in rows, in a sea of delocalised electrons, one from each atom. Try this student sentence about the copper in a wire. In copper, each outer electron stays with its own atom. What is wrong with that? The outer electrons are delocalised. They belong to no one atom and move through the whole structure. One more. This one goes back to Drude, in the year nineteen hundred. In Drude's picture, what do we call the fixed particles today, and what do the loose electrons form? The fixed particles are positive metal ions in regular rows. The loose electrons form the sea of delocalised electrons that holds them together.

That is the third and last bond, the one that holds every metal together. If this one's yours, thumbs it up and it stays ticked off. If not, save it. The next chapter leans on this model to explain how metals conduct and bend, so it comes round again.

Next in the chapter: What our models of structure leave out. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh four, oh six.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.