What is an atomic number?

Atomic number is the number of protons - positively charged particles - in the core of the atom of the chemical element. The elements differ from each other by the numbers of these particles they have, and therefore each element has its own unique atomic number. The chemical properties of the element are determined by its number of electrons, but in a neutral atom it is the same as the number of protons. However, atoms can obtain or lose electrons to create negative or positively charged ions, so the atomic number is defined as the number of protons because it is always the same for the element.

Atomic number, mass number and atomic weight

It is possible to confuse these values, but are completely different apart. Atoms consist of a core containing positive protons and electrically neutrons, with electrons orbiting a certain distance. Protons and neutrons are relatively heavy and similar to weight, but the electrons are much lighter and very little contribute to the weight of the atom. The mass number atom is the number of protonPlus the number of neutrons and is almost the same as the weight of the atom.

The number of neutrons in the element may vary. The forms of element with different number of neutrons are known as isotopes . For example, the most common form of hydrogen has one proton and no neutrons, but there are two more isotopes of hydrogen, deuterium and tritium, with one and two neutrons. Naturally occurring elements are often a mixture of different isotopes. Carbon is another example consisting of isotopes with weight numbers 12, 13 and 14. All have six protons, but have six, seven and eight neutrons.

Although chemists from the 19th century have introduced good approximation of atomic weights of known elements, precise calculations and always direct, due to the occurrence of different isotopes in different proportions. Atomic weight is often determined as an average, based on the relative abundance of isotopes. Because some isotopes are unstable, over time it turns to other elements, atomovThe weights may vary and can be represented as a range rather than a single value. Isotopes are usually represented by an atomic number in the lower left left chemical symbol and a mass number or approximate atomic weight at the top right. For example, carbon 13 would be displayed as _{6 c}

13 .

Periodic table

In the age of 20. Other chemists have previously noticed that the characteristics of the elements, when they were ordered, had a tendency of repeat at more or less regular intervals. For example, lithium, sodium and potassium are all reactive metals that combine with non -metal in a similar way, while helium, neon and argon are completely unrelated gases. For this reason, the Mendeleev list has become known as a periodic table.

The first design of Mendeleev worked well, but there were several discrepancies. For example, in the order of weight, iodine came before Tellurium. The problem was that this grouped iodine with oxygen, sulfur and selenium and telurium with fluorin, chlorine and bromine. According to their chemical properties, it should be the opposite, so before the release of his table in 1869 Mendeleev simply replaced these elements. Until the beginning of the 20th century, however, the reason for these discrepancies was revealed.

in 1913, physicist H.G.J. Moseley created the relationship between wavelengths of X -rays produced by different elements and their sequences a votiodic table. Since the structure of the atom was revealed by other experiments around this time, it was clear that this relationship was dependent on the number of protons in the core of the element, in other words, its atomic number. The periodic table could then be ordered by this number, thereby giving the chemical properties of the elements on a sound theoretical basis. Occasional inconsistenciesThe original table was due to the fact that changes in neutrons could sometimes lead to a higher atomic weight than another element with a higher atomic number.

Modern periodic table shows elements in boxes arranged in rows and columns, while the atomic number rising along each row. Each column groups elements with similar chemical properties. The columns are determined by the number and arrangement of electrons in atoms, which in turn are determined by the number of protons. Each box usually contains a chemical symbol for the element, with the above atomic number.