The atomic number is the number of protons in an element’s nucleus, determining its unique properties. Mass number is the sum of protons and neutrons, and isotopes have varying numbers of neutrons. The periodic table groups elements by atomic number, with columns based on electron arrangement.
The atomic number is the number of protons – positively charged particles – in the nucleus of an atom of a chemical element. Elements are distinguished from each other by the number of these particles they have, and therefore each element has its own unique atomic number. The chemical properties of an element are determined by its number of electrons, but in a neutral atom it is equal to the number of protons. Atoms can, however, gain or lose electrons to form negatively or positively charged ions, so the atomic number is defined as the number of protons, as this is always the same for any given element.
Atomic number, mass number and atomic weight
It is possible to confuse these values, but they are quite distinct from each other. Atoms consist of a nucleus containing positively charged protons and electrically neutral neutrons, with electrons orbiting some distance apart. Protons and neutrons are relatively heavy and of similar weight, but electrons are much lighter and contribute very little to the weight of an atom. The mass number of an atom is the number of protons plus the number of neutrons and is nearly equal to the weight of the atom.
The number of neutrons in an element can vary. Forms of an element with different numbers of neutrons are known as isotopes. For example, the most common form of hydrogen has one proton and no neutrons, but there are two other isotopes of hydrogen, deuterium and tritium, with one and two neutrons, respectively. Natural elements are often mixtures of different isotopes. Carbon is another example, made up of isotopes with mass numbers 12, 13, and 14. These all have six protons, but have six, seven, and eight neutrons, respectively.
Although 19th-century chemists had established good approximations of the atomic weights of known elements, precise calculations are not always simple due to the presence of different isotopes in varying proportions. Often, atomic weight is determined as an average, based on the relative abundance of isotopes. Because some isotopes are unstable, changing over time into other elements, atomic weights can vary and can be represented as a range, rather than a single value. Isotopes are usually represented by the atomic number in the lower left of the chemical symbol and the mass number, or approximate atomic weight, in the upper right. For example carbon 19 would be shown as 13C6.
The periodic table
In the 1860s, Russian chemist Dimitri Mendeleev worked on a table of elements known at that time, initially listing them in order of atomic weight and arranging them in rows grouping elements with similar chemical properties. It had previously been noted by other chemists that the properties of the elements, ordered by weight, tended to repeat themselves at more or less regular intervals. For example, lithium, sodium, and potassium are all reactive metals that combine with nonmetals in similar ways, while helium, neon, and argon are all completely nonreactive gases. For this reason, Mendeleev’s list became known as the periodic table.
Mendeleev’s first draft worked well, but there were some inconsistencies. For example, listed in order of weight, iodine came before tellurium. The problem was that this grouped iodine with oxygen, sulfur and selenium and tellurium with fluorine, chlorine and bromine. According to their chemical properties, it should have been the other way around, so before publishing his table in 1869, Mendeleev simply swapped these elements. It was not until the early 20th century, however, that the reason for these inconsistencies was revealed.
In 1913, physicist HGJ Moseley established a relationship between the wavelengths of X-rays produced by different elements and their sequence in the periodic table. As the structure of the atom was revealed by other experiments around that time, it became clear that this relationship depended on the number of protons in an element’s nucleus—in other words, its atomic number. The periodic table could then be sorted by this number, placing the observed chemical properties of the elements on a solid theoretical basis. The occasional inconsistencies in the original table were due to the fact that changes in neutron numbers could sometimes result in one element having a higher atomic weight than another element having a higher atomic number.
The modern periodic table shows the elements in boxes arranged in rows and columns, with ascending atomic number along each row. Each column groups elements with similar chemical properties. The columns are determined by the number and arrangement of electrons in the atoms, which in turn is determined by the number of protons. Each box normally contains the element’s chemical symbol, with the atomic number above it.
Protect your devices with Threat Protection by NordVPN
