Topic 3: Periodicity
a) What trends are found in the periodic table? (Families, groups, properties)
The periodic table contains vertical and horizontal trends. It is kind of like people you can see various amount of trends, so that can be seen among elements. As you know, in the periodic table, elements are placed side by side in order of increasing atomic number. This arrangement makes it possible to view certain important trends among them. There are vertical trends within groups and there are horizontal trends that you can spot so you look across the table from left to right.
Here is a site of a periodic table and it shows you how the periodic table is broken down to groups. <http://environmentalchemistry.com/yogi/periodic/>
Atomic Radius increases within a family. Chemists calculate atomics in several ways. The radii of some atoms are determined by measuring the distance between centers of like atoms that are joined together in a diatomic molecule. Generally atomic radius increases as you progress down through the elements in each group. Atomic size decreases from left to right across a period. In crossing a period from left to right, each atom gains one more proton and one more electron. The number of protons increases across a period, the positive charge of the nucleus increases.
Ionization energy follows a periodic trend. Recall that atoms are normally electrically neutral. However, an atom may lose or gain electrons to become an electrically charged ion. Imagine you can reach into an atom, hold the nucleus with one hand, and remove the outermost electron, creating an ion. The energy you would use to remove the electron is the ionization energy of the atom.
Electron affinity decreases within a family and increases within a period. The ability of an atom to attract and hold an extra electron is its electron affinity. Electron affinity is measured as the energy change that occurs when an electron is added to an atom. Note that when an atom gains an extra electron, it acquires a negative charge and becomes a negative ion.
Electronegativity decreases within a family and increases within a period. The electronegativity of an atom is the tendency of an atom to attract electrons to it when itself when it is chemically combined with another element.
b) How are
elements created? (Origins of naturally occurring and synthetic elements)
Elements are naturally occurring elements (atomic numbers 1-92) were formed in the interior of stares. Synthetic elements are research scientists make atomic numbers above 93. Synthetic elements are made in particle accelerators, which launch particles at speeds fast enough to generate nuclear reactions. All elements larger than helium are formed by nuclear reaction in which nuclei gain or lose protons to become different elements.
"(Ken
Gregorich, the nuclear chemist who led the discovery team, said: "Our
unexpected success in producing these superheavy elements opens up a whole world
of possibilities using similar reactions: new elements and
isotopes.")"
c) How can atoms be counted or measured? (Atomic mass, mole)
Atoms
can be counted or measured by an atomic mass unit that is 1/12 of the mass of
carbon-12 atom. The atomic mass of an atom is the mass of that atom expressed
in atomic mass units. Scientists
represent large collections of atoms using moles.
One mole is equal to 6.022 x 1023 particles.
The mass in grams of one mole of a substance is called the molar mass.
"Amadeo
Avogadro (1776-1856) 
was the author of Avogadro's
Hypothesis in 1811, which, together with Gay-Lussac's
Law of Combining Volumes, was used by Stanislao
Cannizzaro to elegantly remove all doubt about the establishment of the
atomic weight scale at the Karlsruhe Conference of 1860.
The name "Avogadro's Number" is just an honorary name attached to the calculated value of the number of atoms, molecules, etc. in a gram mole of any chemical substance. Of course if we used some other mass unit for the mole such as "pound mole", the "number" would be different than 6.022 x 1023.
The first person to have calculated the number of molecules in any mass of substance was Josef Loschmidt, (1821-1895), an Austrian high school teacher, who in 1865, using the new Kinetic Molecular Theory (KMT) calculated the number of molecules in one cubic centimeter of gaseous substance under ordinary conditions of temperature of pressure, to be somewhere around 2.6 x 1019 molecules. This is usually known as "Loschmidt's Constant." (This value, no, is now listed at the NIST web site as 2.686 7775 x 1025 m-3)"