Competing claims to element names continued for decades, in a period of discord known to nuclear chemists as the Transfermium Wars. For 40 years, labs across the US, Europe, and the Soviet Union all continued to name elements without regard for one another. It wasn't until 1997 that a group of chemists negotiated consensus names for elements 102-109. However, that group proved weak and easily manipulated by the lobbying of labs looking name elements 110 and higher. In 1999, a new committee ended the Transfermium Wars with a structure backed by the world community of chemists and physicists.
The atomic weight is basically a measurement of the total number of particles in an atom's nucleus. In reality, it isn't that clean cut. The atomic weight is actually a weighted average of all of the naturally occurring isotopes of an element relative to the mass of carbon-12. Didn't understand that? Doesn't matter. All you really need to find is something called the mass number. Unfortunately, the mass number isn't listed on the Table of Elements. Happily, to find the mass number, all you need to do is round the atomic weight to the nearest whole number.
The table has 7 periods, or horizontal rows, and 18 groups or families. The groups are the vertical columns. If you look closely at the periods, you will see that all of the elements in a period have the same number of electron energy levels. The first period has 1 electron energy level, the second period has 2 electron energy levels, etc. There is a pattern in the groups as well. All of the elements in group 1 have 1 electron in their outer energy level. All of the elements in group 2 have 2 electrons in their outer energy level; the elements in group 13 have 3 electrons in their outer energy level; the elements in group 14 have 4 electrons in their outer energy level, etc. You might have noticed that groups 3-12 were not included in the trend. That is because they are the transition elements, and the transition elements follow a slightly different trend due to the overlap of energy levels and the way those energy levels fill. The number of electrons in the outer energy level determines the element's chemical properties.
An element is the smallest part of matter that cannot be broken down into simpler forms by chemical means. In the early 19th century, scientists began to arrange elements according to similar physical and chemical properties. The scientist who had the greatest success with this was Dmitri Medeleev, a Russian chemist. He arranged the elements according to increasing atomic mass, as well as in columns according to similar properties. In this way, he was able to predict elements that were as yet undiscovered through the gaps in his chart. In the early 1900's, Henri Mosely improved Medeleev's periodic table by arranging it according to similar properties as well as increasing atomic number. Today's periodic table is arranged according to increasing atomic number and increasing atomic mass (for the most part).
By arranging all of the 63 elements then known by their atomic weights, [Mendeleev] managed to organize them into groups possessing similar properties. Where a gap existed in the table, he predicted a new element would one day be found and deduced its properties. And he was right.
[Mendeleev's] greatest accomplishment, however, was the stating of the Periodic Law and the development of the Periodic Table. From early in his career, he felt that there was some type of order to the elements, and he spent more than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some of the chaos in the field for his students. Mendeleev was one of the first modern-day scientists in that he did not rely solely on his own work but rather was in correspondence with scientists around the world in order to receive data that they had collected. He then used used their data along with his own data to arrange the elements according to their properties.
In 1856, Mendeleev returned to St. Petersburg and defended his master's thesis: "Research and Theories on Expansion of Substances due to Heat." Following his masters program, Dmitri focused his life on his career of teaching and research. He was essentially a teacher devoted to his work and to his students; he was next a lover of his country and of his fellow men. The first led to his books and the periodic table, while the latter gave rise to his studies of chemical technology and the organization of Russia's industries, agriculture, transport meteorology and metrology.
The elements are the gateway between the mathematical purity of physics and the messy reality of life. Between you and the ancient supernova in which the stuff of your body was created, there is one link: your elements (really just the nuclei of your elements-the electrons got swapped out long ago). So to celebrate the astonishing diversity of nature, I decided that after four years of collecting and photographing the elements, Iwas ready to make the very thing I started by rebelling against: a periodic-table poster to hang on the wall.
Some of the groups [in the periodic table] have common names. Group 1A elements are the alkali metals and group 2A are the alkaline earths. Group 7A are the halogens and group 8A are the noble gases, so called because they are extremely unreactive.
When the elements are arranged in order of increasing atomic number it is observed that certain properties seem to repeat in a regular way. We start a new row (a period) so that elements with similar properties fall in the same column (a group). This arrangement is called the periodic table.