Gold, silver, copper, and iron are all metals. Why do magnets only attract iron?
It is a well-known phenomenon that a magnet will attract iron. I believe that everyone has tried to use a magnet to approach other metals to see if the magnet can attract them. However, the “experimental results” are often disappointing. Because common metals such as gold, silver, and copper are not attracted by magnets at all.
Gold, silver, copper, and iron are all metals. Why do magnets only attract iron? This needs to start from the internal structure of the atom.
We know that atoms are composed of nuclei and electrons. The nuclei are positively charged and have spins, while the electrons are negatively charged, and the electrons are spinning in the space outside the nucleus. According to Maxwell’s equation, the change An electric field creates a magnetic field, so atomic nuclei and electrons generate tiny magnetic fields.
Relatively speaking, the strength of the magnetic field generated by electrons is much higher than that of the nucleus, generally up to 1000 times. Therefore, whether an atom can exhibit magnetism as a whole depends on the effect of the superposition of the magnetic field generated by the electrons inside the atom.
Inside the atom, the arrangement of electrons is very regular. We can simply understand that there are several “shells” outside the nucleus. The closer to the “shell” of the nucleus, the lower its energy level , and electrons will always tend to fill the “shell” with a lower energy level.
Each “shell” can only accommodate a certain number of electrons, so when the lowest energy “shell” is filled, the rest of the electrons will fill the outer “shell”. When it is full, the rest of the electrons can only fill the outer “shell”…
According to the description of quantum mechanics, in the “shell” filled with electrons, electrons are always arranged in pairs, but due to the restriction of “Pauli exclusion principle”, they are not allowed to be in exactly the same state , in this case, the directions of the magnetic fields generated by them are opposite, and the superposition effect is to “cancel each other out”.
Therefore, it can be said that if an atom wants to exhibit magnetism as a whole, its outermost “shell” cannot be full of electrons, and among the many known elements, only some of them can meet this condition, such as the element The “transition elements” in the periodic table.
It can be seen that gold (Au), silver (Ag), copper (Cu) and other metal elements are “transition elements” like iron (Fe), so why is only iron attracted by magnets? Let’s move on.
When iron atoms form a crystal, there will be a special quantum effect called “exchange interaction” between the electrons of adjacent iron atoms. The effect is: make the electrons of adjacent iron atoms The magnetic field directions are aligned in approximately the same direction.
Since this quantum effect can only make the direction of the atomic magnetic field “approximately the same”, if the number of iron atoms is too large, they can only form a small area of “the direction of the atomic magnetic field is basically the same”. The area is called “Magnetic Domain”.
In fact, common iron objects belong to the situation of “too many atoms”. After all, iron atoms are too small (the number of iron atoms contained in a mere 1 cubic centimeter of iron is about 8.5 x 10^ 22), so we can simply think that common iron objects actually contain a large number of “magnetic domains”.
In most cases, a large number of “magnetic domains” gather together, and the magnetic field directions of each “magnetic domain” cannot be kept consistent as a whole, so their superimposed effects will “cancel each other out”, so common iron objects Usually also does not exhibit magnetism.
However, “magnetic domains” have an important feature, that is, their magnetic field directions are easily affected by external magnetic fields. Therefore, when ferrous objects approach magnets, the magnetic field directions of many “magnetic domains” inside them will be affected by the magnetic field. Under the influence of a magnetic field, it becomes “uniform”, and then exhibits magnetism on a macroscopic level, which is also called “magnetization”.
After being “magnetized”, there will be electromagnetic interaction between the iron object and the magnet, and thus attract each other, so the phenomenon of “iron will be attracted by the magnet” appears.
Except for a very few elements such as iron, nickel, and cobalt, most of the other “transition elements” (including gold, silver, and copper) do not have the quantum of “exchange interaction” when they form crystals. effect, so in the metal composed of these elements, the directions of the magnetic fields of each atom are chaotic, so the superposition effect of these atomic magnetic fields appears to “cancel each other”, so that there is no magnetism at the macroscopic level, and there is no will be attracted by a magnet.