The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group. The alkali metals show a number of trends when moving down the group - for instance, decreasing electro negativity, increasing reactivity, and decreasing melting and boiling point. Density generally increases, with the notable exception of potassium being less dense than sodium, and the possible exception of francium being less dense than cesium.
The alkali metals are all highly reactive and are rarely found in elemental form in nature. As a result, in the laboratory they are stored under mineral oil. They also tarnish easily and have low melting points and densities. Potassium and rubidium possess a weak radioactive characteristic (harmless) due to the presence of long duration radioactive isotopes. Alkali metals are famous for their vigorous reactions with water, and these reactions become increasingly violent as one moves down the group. The modern explanation of the pattern of the periodic table is that the elements in a group have similar configurations of the outermost electron shells of their atoms: As most chemical properties are dominated by the orbital location of the outermost electron. There are three ways of numbering the groups of the periodic table.
Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electro negativity. Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Similarly, electro negativity will increase in the same manner as ionization energy because of the amount of pull that is exerted on the electrons by the nucleus. Electron affinity also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period) with the exception of the noble gases. In groups, reactivity of metals increases with atomic number because the ionization energy decreases.
The alkali metals are all highly reactive and are rarely found in elemental form in nature. As a result, in the laboratory they are stored under mineral oil. They also tarnish easily and have low melting points and densities. Potassium and rubidium possess a weak radioactive characteristic (harmless) due to the presence of long duration radioactive isotopes. Alkali metals are famous for their vigorous reactions with water, and these reactions become increasingly violent as one moves down the group. The modern explanation of the pattern of the periodic table is that the elements in a group have similar configurations of the outermost electron shells of their atoms: As most chemical properties are dominated by the orbital location of the outermost electron. There are three ways of numbering the groups of the periodic table.
Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electro negativity. Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Similarly, electro negativity will increase in the same manner as ionization energy because of the amount of pull that is exerted on the electrons by the nucleus. Electron affinity also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period) with the exception of the noble gases. In groups, reactivity of metals increases with atomic number because the ionization energy decreases.