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Atoms can be considered the smallest units of matter keeping its specific chemical properties.
In fact, if we divide further every atom in its fundamental items, (electrons, protons and neutrons), these particles don't represent anymore any element of the periodic table.

Every atom has a "nucleus" formed by a certain number Z of "protons" (p+), particles with a conventionally (+) charge of 1.602 x 10^(-19) Coulombs and the "neutrons" (N), with the same mass of p+ , but without electrical charge and not always in the same number of p+.

Neutrons have the task of keeping strictly tied together the p+ that, being charges of the same sign, would violently reject one another.

The "atomic mass", another typical feature of every element, is measured as a multiple of a conventional atomic mass unit (a.m.u) that is 1/12 of the mass of the stable C-12 isotope of carbon (with 6 p+ and 6 N in its nucleus).

From the Periodic Table, we see that the atomic mass is not an entire number, as it could be from the simple sum of p+ and N, because every element is formed by a constant mix of isotopes, each of one has the same number of p+, but a slightly different number of N.
Every isotope has a different mass and their average is the "official" atomic mass of the given chemical element.

Many isotopes are stable but others not and tend to decay, more or less slowly, emitting particles and energy (radioactivity).
Some elements, beyond the Z of Bismuth, have only unstable isotopes (like Uranium and Plutonium) because their nuclei are "too big" to be stable.

Around the nucleus, there are some "electrons"(e-), negative charges, equal in the number (Z) and in the value of the charge to the p+, but opposite in sign, to keep the atom electrically neutral and stable.
Every chemical element is distinguished from all the others by its Z, named "atomic number" and this can be considered one of the pillars of Chemistry.

The e- has a mass of 9,1095 x 10^(-31) Kg, 1837 times minor than that of a p+ and it rotates at an incredible speed around the nucleus not following a precise "planetary" orbit, as it was believed until 1920, according to the Niels Bohr model (1913), but in precise and constant energy level around the nucleus.

The "Uncertainty Principle", expressed by W. Eisenberg in 1927, in fact, affirms it's not possible to determine, in the same time, the exact position and speed of the e- around the nucleus.
So, if we find exactly its position d(x), we lose precision on its speed

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