Periodic Relationships Among the Elements

• atomic masses- they devised the periodic table using their knowledge of _____, In the nineteenth century, when chemists had only a vague idea of atoms and molecules and did not know of the existence of electrons and protons,
• true-Arranging elements according to their atomic masses in a periodic table seemed logical to those chemists
• John Newlands- In 1864 the English chemist, noticed that when elements were arranged in order of atomic mass, every eight element had similar properties.
• law of octaves-Newland referred to this peculiar relationship as the
• law of octaves-this “law” turned out to be inadequate beyond calcium, and Newlands’ work was not accepted by the scientific society.
• Russian chemist Dmitri Mendeleev and the German chemist Lothar Meyer - independently proposed a much more extensive tabulation of the elements based on the regular, periodic recurrence of properties
• Mendeleev proposed the existence of an unknown element that he called eka- aluminum and predicted a number of its properties.
• Gallium was discovered four years later, its properties matched the predicted properties of eka-aluminum.
• Mendeleev’s periodic table included 99 known elements. By 1900, some 30 elements were added to the list.
• Henry Mosely, an English physicist, found that atomic number increases in the same order as atomic mass.
• According to the type of subshell being filled, the elements can be divided into categories- the representative elements, the noble gases, the transition metals, the lanthanides, and the actinides.
• The representative elements (also known as main group elements) are the elements in Groups 1A through 7A, all of which have incompletely filled s or p subshells of the highest principal quantum number.
• With the exception of helium, noble gases (the Group 8A elements) all have a completely filled p subshell.
• The transition metals are the elements in Groups 1B and 3B through 8B, which have incompletely filled d subshells, or readily produce cations with incompletely filled d subshells.
• The lanthanides and actinides are sometimes called f-block transition elements because they have incompletely filled f subshells.
• representative elements have the same valence electron, that is, the outer most electrons, which are involved during chemical bonding
• krypton and xenon- these elements all have completely filled ns and sp subshells, a condition that represents stability.
• • Atomic Radius-A number of physical properties, including density, melting point, and boiling point, are related to the sizes of atoms, but atomic size is difficult to define.
• the electron density extends far beyond the nucleus
• we define the size of an atom in terms of its atomic radius,
• • Ionic radius is the radius of a cation of anion. 
• • When a neutral atoms is converted to an ion, we expect a change in size. If an atom forms into an anion, its size increases, because the nuclear charge remains the same but the repulsion resulting from the additional electron(s) enlarges the domain of the electron cloud.
• When a neutral atoms turns to a cation, upon the removal of electron(s), the electron repulsion is reduced, but the nuclear charge remains the same, so the electron cloud shrinks, and the cation is smaller than the atom.