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That phrase had me slightly puzzled - I thought chemical formulas were for molecules, i.e. combinations of elements that actually react with each other and form

by rob74 7y ago

That phrase had me slightly puzzled - I thought chemical formulas were for molecules, i.e. combinations of elements that actually react with each other and form bonds, like H2O or CO2? While a neodymium magnet is actually an alloy of mostly two metals, plus a little bit of boron, but without chemical bonds?

pjc50 7y ago
Most metals are crystalline, so while the atoms are bonded together they're all bonded into big lattice structures rather than neat little ones with covalent bonds. (Changing the distribution of crystal size and shape is an important part of metal forging as it affects strength and ductility)
rcxdude 7y ago
The notation is also used to describe the ratio of elements used in alloys and crystals. In such a case the elements are also bonded together, but form much larger structures so writing out a specific number for each 'molecule' becomes meaningless (the numbers are huge and also vary massively).
peter_d_sherman 7y ago
Before this point in time, I always thought that Neodymium magnets were made of... well, Neodymium <g> (and yes, I claim complete and total ignorance! <g>)... it never occurred to me that they were mostly Iron... Amazing just how wrong my thinking about them was, prior to this point... (but I forgive myself... as JFK so eloquently put, "An error does not become a mistake -- unless you refuse to correct it."...)
pedrow 7y ago
With alloys, the formula given is akin to the 'empirical formula' for molecules, i.e. the simplest integer ratio. For the mass fraction, the relative masses of atoms of iron:neodymium:boron are approx 56:144:11, so Nd2 Fe14 B should be 27% Nd, 72% Fe and 1% B by mass, I think.
mncharity 7y ago
> but without chemical bonds? Metallic bonds. My understanding is all bonding is atoms sticking together electrostatically, with (insanely fast) "slow" lumbering nuclei, nudged around by their local e-field, and wacky electrons, adopting weird spatial distributions based on the current nuclei positions, setting that field. The bond taxonomy descriptively sorts by characteristics. Here, one is delocalization of valence electrons. Perhaps picture https://www.youtube.com/watch?v=oSCX78-8-q0&list=PLaFe0BJiho2ofZqEVA5H6RZMJiusN-WSL&index=1 https://www.youtube.com/watch?v=oSCX78-8-q0&list=PLaFe0BJiho... , STM imaging of cold CO molecules sitting on a copper surface, IIRC. You can't see the individual copper atoms, and there are extended ripples in surface electron density.

by rob74 7y ago

pjc50 7y ago

Most metals are crystalline, so while the atoms are bonded together they're all bonded into big lattice structures rather than neat little ones with covalent bonds. (Changing the distribution of crystal size and shape is an important part of metal forging as it affects strength and ductility)

rcxdude 7y ago

The notation is also used to describe the ratio of elements used in alloys and crystals. In such a case the elements are also bonded together, but form much larger structures so writing out a specific number for each 'molecule' becomes meaningless (the numbers are huge and also vary massively).

peter_d_sherman 7y ago

Before this point in time, I always thought that Neodymium magnets were made of... well, Neodymium <g> (and yes, I claim complete and total ignorance! <g>)... it never occurred to me that they were mostly Iron... Amazing just how wrong my thinking about them was, prior to this point... (but I forgive myself... as JFK so eloquently put, "An error does not become a mistake -- unless you refuse to correct it."...)

pedrow 7y ago

With alloys, the formula given is akin to the 'empirical formula' for molecules, i.e. the simplest integer ratio. For the mass fraction, the relative masses of atoms of iron:neodymium:boron are approx 56:144:11, so Nd2 Fe14 B should be 27% Nd, 72% Fe and 1% B by mass, I think.

mncharity 7y ago

> but without chemical bonds? Metallic bonds. My understanding is all bonding is atoms sticking together electrostatically, with (insanely fast) "slow" lumbering nuclei, nudged around by their local e-field, and wacky electrons, adopting weird spatial distributions based on the current nuclei positions, setting that field. The bond taxonomy descriptively sorts by characteristics. Here, one is delocalization of valence electrons. Perhaps picture https://www.youtube.com/watch?v=oSCX78-8-q0&list=PLaFe0BJiho2ofZqEVA5H6RZMJiusN-WSL&index=1 https://www.youtube.com/watch?v=oSCX78-8-q0&list=PLaFe0BJiho... , STM imaging of cold CO molecules sitting on a copper surface, IIRC. You can't see the individual copper atoms, and there are extended ripples in surface electron density.