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Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table)
American physicist Glenn Seaborg led the research team that discovered plutonium in 1940, and in 1941 isolated Uranium-233. He oversaw plutonium manufacturing and enrichment research for the Manhattan Project, culminating in the development of atomic weapons. Seaborg and his colleagues also discovered americium, berkelium, californium, curium, einsteinium, fermium, mendelevium, and nobelium, and identified more than 100 element isotopes throughout the periodic table.
{http://www.aip.org/history/curie/images/photos/1103-matte.gif}

1864
Dmitri Ivanovich Mendeleev produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights.
====understanding=understanding of how on the table.====table.
{http://www.aip.org/history/curie/images/table1869-reprint.gif} Mendeleev’s table as published in 1869, with many gaps and uncertainties
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1894
William Ramsay discovered the Noble Gases.

1864
Dmitri Ivanovich Mendeleev produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights.
understanding====understanding of how Principles of Chemistry, MendeleevChemistry,Mendeleev created a on the table. table.====
{http://www.aip.org/history/curie/images/table1869-reprint.gif} Mendeleev’s table as published in 1869, with many gaps and uncertainties
||
1894
William Ramsay discovered the Noble Gases.
Marie and Pierre Curie isolated radium and polonium from pitchblende.
Marie Sklodowska Curie (1867–1934) was the first person ever to receive two Nobel prizes: the first in 1903 in physics, shared with Pierre Curie (her husband) and Henri Becquerel for the discovery of the phenomenon of radioactivity, and the second in 1911 in chemistry for the discovery of the radioactive elements polonium and radium.
.) {http://www.aip.org/history/curie/images/table-Ostwald.gif}
1900
Ernest Rutherford discovered the source of radioactivity as decaying atoms.

Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table)
American physicist Glenn Seaborg led the research team that discovered plutonium in 1940, and in 1941 isolated Uranium-233. He oversaw plutonium manufacturing and enrichment research for the Manhattan Project, culminating in the development of atomic weapons. Seaborg and his colleagues also discovered americium, berkelium, californium, curium, einsteinium, fermium, mendelevium, and nobelium, and identified more than 100 element isotopes throughout the periodic table.
The valence electrons are the electrons in the last shell or energy level of an atom. They do show a repeating or periodic pattern. The valence electrons increase in number as you go across a period. Then when you start the new period, the number drops back down to one and starts increasing again.
Elements in a vertical column of the Periodic Table typically have many properties in common. After all, Mendeleeve used similarities in properties to construct a periodic table in the first place. Because they show common characteristics, elements in a column are known as a family. Sometimes a family had one very important characteristic many chemists knew about: that characteristic became the family name. Four important chemical family names of elements still widely used are the alkali metals, the alkaline earths, the halogens, and the noble gases. The alkali metals are the elements in Group 1, excluding hydrogen, which is a special case. These elements—lithium, sodium, potassium, rubidium, cesium and francium—all react with water to give solutions that change the color of a vegetable dye from red to blue. These solutions were said to be highly alkaline or basic; hence the name alkali metals was given to these elements.
The elements of Group 2 are also metals. They combine with oxygen to form oxides, formerly called "earths," and these oxides produce alkaline solutions when they are dissolved in water. Hence, the elements are called alkaline earths.
The name for Group 17, the halogens, means salt former because these elements all react with metals to form salts.
The name of Group 18, the noble gases, has changed several times. These elements have been known as the rare gases, but some of them are not especially rare. In fact, argon is the third most prevalent gas in the atmosphere, making up nearly 1% of it. Helium is the second most abundant element in the universe—only hydrogen is more abundant. Another name used for the Group 18 family is the inert gases.

The name for Group 17, the halogens, means salt former because these elements all react with metals to form salts.
The name of Group 18, the noble gases, has changed several times. These elements have been known as the rare gases, but some of them are not especially rare. In fact, argon is the third most prevalent gas in the atmosphere, making up nearly 1% of it. Helium is the second most abundant element in the universe—only hydrogen is more abundant. Another name used for the Group 18 family is the inert gases.

Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table)
American physicist Glenn Seaborg led the research team that discovered plutonium in 1940, and in 1941 isolated Uranium-233. He oversaw plutonium manufacturing and enrichment research for the Manhattan Project, culminating in the development of atomic weapons. Seaborg and his colleagues also discovered americium, berkelium, californium, curium, einsteinium, fermium, mendelevium, and nobelium, and identified more than 100 element isotopes throughout the periodic table.
The valence electrons are the electrons in the last shell or energy level of an atom. They do show a repeating or periodic pattern. The valence electrons increase in number as you go across a period. Then when you start the new period, the number drops back down to one and starts increasing again.
Elements in a vertical column of the Periodic Table typically have many properties in common. After all, Mendeleeve used similarities in properties to construct a periodic table in the first place. Because they show common characteristics, elements in a column are known as a family. Sometimes a family had one very important characteristic many chemists knew about: that characteristic became the family name. Four important chemical family names of elements still widely used are the alkali metals, the alkaline earths, the halogens, and the noble gases. The alkali metals are the elements in Group 1, excluding hydrogen, which is a special case. These elements—lithium, sodium, potassium, rubidium, cesium and francium—all react with water to give solutions that change the color of a vegetable dye from red to blue. These solutions were said to be highly alkaline or basic; hence the name alkali metals was given to these elements.
The elements of Group 2 are also metals. They combine with oxygen to form oxides, formerly called "earths," and these oxides produce alkaline solutions when they are dissolved in water. Hence, the elements are called alkaline earths.
The name for Group 17, the halogens, means salt former because these elements all react with metals to form salts.
The name of Group 18, the noble gases, has changed several times. These elements have been known as the rare gases, but some of them are not especially rare. In fact, argon is the third most prevalent gas in the atmosphere, making up nearly 1% of it. Helium is the second most abundant element in the universe—only hydrogen is more abundant. Another name used for the Group 18 family is the inert gases.

Periodic Table Timeline
1864
Dmitri Ivanovich Mendeleev produced atomic weights.
understanding of how the elements are related to each other and why they exhibit their particular chemical and physical properties was slow in coming. Between 1868 and 1870, in the process of writing his book, The Principles of Chemistry, Mendeleev created a table or chart that listed the known elements according to increasing order of atomic weights. When he organized the table into horizontal rows, a pattern became apparent--but only if he left blanks in the table. If he did so, elements with similar chemical properties appeared at regular intervals--periodically--in vertical columns on the table.
1894
William Ramsay discovered the Noble Gases.
The Scottish chemist William Ramsay (1852–1916) is known for work that established a whole new group in the periodic table, variously called over time the inert, rare, or noble gases.
1898
Marie and Pierre Curie isolated radium and polonium from pitchblende.
Marie Sklodowska Curie (1867–1934) was the first person ever to receive two Nobel prizes: the first in 1903 in physics, shared with Pierre Curie (her husband) and Henri Becquerel for the discovery of the phenomenon of radioactivity, and the second in 1911 in chemistry for the discovery of the radioactive elements polonium and radium.
1900
as decaying atomsatoms.
A consummate experimentalist, Ernest Rutherford (1871–1937) was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics. He discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. Most important, he postulated the nuclear structure of the atom: experiments done in Rutherford's laboratory showed that when alpha particles are fired into gas atoms, a few are violently deflected, which implies a dense, positively charged central region containing most of the atomic mass.
1913
Henry Moseley determined the atomic number of each of the elements and modified the 'Periodic Law'.
The value of the table gradually became clear, but not its meaning. Scientists soon recognized that the table's arrangement of elements in order of atomic weight was problematic. The atomic weight of the gas argon, which does not react readily with other elements, would place it in the same group as the chemically very active solids lithium and sodium. In 1913 British physicist Henry Moseley confirmed earlier suggestions that an element's chemical properties are only roughly related to its atomic weight (now known to be roughly equal to the number of protons plus neutrons in the nucleus). What really matters is the element's atomic number--the number of electrons its atom carries, which Moseley could measure with X-rays. Ever since, elements have been arranged on the periodic table according to their atomic numbers. The structure of the table reflects the particular arrangement of the electrons in each type of atom. Only with the development of quantum mechanics in the 1920s did scientists work out how the electrons arrange themselves to give the element its properties.
1940
uranium fission.
1940
Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table)
American physicist Glenn Seaborg led the research team that discovered plutonium in 1940, and in 1941 isolated Uranium-233. He oversaw plutonium manufacturing and enrichment research for the Manhattan Project, culminating in the development of atomic weapons. Seaborg and his colleagues also discovered americium, berkelium, californium, curium, einsteinium, fermium, mendelevium, and nobelium, and identified more than 100 element isotopes throughout the periodic table.

Periodic Table Timeline
1864
===DmitriDmitri Mendeleev produced atomic weights.
=
1894
William Ramsay discovered the Noble Gases.