Why are phycial and chemical property trends of the periodic table seen? In other words, why do they occur?
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the short answer is valence shell electrons.
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This applies for all groups.\nThe general trend for atoms as you move down a group is for the size of the atoms to increase.\nThis is partially accounted for by the shielding of outer electrons by inner electrons. Also because atoms further down a group have more energy levels creating more distance between the nucleus and valence electrons.
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the periodic table is great because it ditinguishes the different shells (s, p, d and f) that are filled as you add electrons. eg. He has 2 s electrons (full shell)\nNe has 1s2, 2s2 and 2p6 (full shell).\nIt gets a little more complicated at d electrons (transition metals) but Mendeleev did a great first shot.
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The atoms increase in size as you move down the group, because each atom has an additional energy level or shell that contains the valence electron.\n\nWhen atom of group one metals form ions, they lose the valence electron, which causes them to decrease in size.\n\nThe smallest ion will be at the top of the column, since it starts out as the smallest atom,  and the largest ion will be at the bottom.
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go to www.google.com them write periodic table and than go to images and you'll see
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It has to do with periodic table trends. In Metals the melting point generally decreases as you go down a group. Non-metals the the melting point generally increases as you go down a group.\nNa is a Metal, K is a Metal, therefore Na possesses a higher metling point because it is above K on the group.
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The position of the elements in the periodic table gives you clues about the valences of each element. Also, if you remember the formulae for familiar substances (like carbon dioxide, water, sulfur dioxide, hydrochloric acid, etc) that will also give you clues about possible or likely valences. \n\nThe final answer is:\n\nmemorize.
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though there are many, one of them is:\niceland spar crystal\none reason can be due to the harmonicity of vibrations of the atoms of the substance when their kinetic energy increases
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http://chemlab.pc.maricopa.edu/PERIODIC/periodic.html\n\nHere you can click on an element and see facts on it.\n\nCa<Cr<W
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the study of metals. their characteristics, and behaviours, etc.
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As you move down a family in the periodic table, the number of shells of electrons increases  (draw the Bohr- Rutherford diagrams), and the atoms increase in size.\n\nThe larger the atom, the further the valence electrons are from the nucleus, and the weaker the hold it has on them. So, it becomes easier to remove a valence electron.\n\nSince IE measures the amount of energy needed to remove an electron from an atom, this value will decrease, moving down the family from He to Rn.
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Transition elements have some anomolous properties . The transition shows horizontal similarity and they have variable valencies. Instead of the gradation in properties, they show variety in properties. The groups like lanthanides and actinides are separated from the transition metals due to their unique properties. The transition elements includes differents  group of elements with  unique properties
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British chemist, John Newlands had the idea of arranging chemical elements in order of their relative atomic masses and he arranged his elements in columns. However, Russian chemist, Dmitri Mendeleev greatly improved on Newlands' idea and convinced other chemists to use it, so Mendeleev has been credited with the invention of the periodic table. Dmitri Mendeleev came up with his first version of the periodic table in 1864, since then more than 700 versions of the table have been proposed.\n\nNo chemistry textbook, classroom, lecture theatre or research laboratory is complete without a copy of the periodic table of the elements. Since the earliest days of chemistry, attempts have been made to arrange the known elements in ways that revealed similarities between them. However, it required the genius of Mendeleev to see that arranging elements into patterns was not enough; he realised that there was a natural plan in which each element has its allotted place, and this applies not only to the known elements but to some that were still undiscovered. Today we have the so-called long form of the table. This has emerged supreme from well over 100 designs that have been proposed since the time of Mendeleev. With the advantage of hindsight we can now see why this form of the table was bound to succeed.\n\nToday there are 111 elements recognised by IUPAC, and these are usually displayed in the form of a matrix called a periodic table. The term periodic came from the regular occurrence of certain chemical properties in the list of known elements when these are arranged in order of increasing relative mass. The common form, complete with the new group numbers (1-18) were finally agreed by the International Union of Pure Applied Chemistry, IUPAC, in 1985, after years of wrangling. In truly Parkinsonian fashion, this least important of changes has probably consumed the most effort!\n\nSince Antoine Lavoisier first defined a chemical element and drew up a table of 33 of them for his book 'Traité Elémentaire de Chimie' (Treatise on the Chemical Elements) published in 1789, there have been attempts to classify them. Lavoisier himself grouped them into four categories on the basis of their chemical properties: gases, nonmetals, metals and earths. In the first category he listed substances that we now know as oxides but which at the time had defeated all attempts at separation.\n\nThis desire to identify and classify elements continued in chemistry for another 80 years until Mendeleev stumbled upon the correct classification - the periodic table. Today the periodic table is securely based on the properties of atomic number of the nucleus and the electron energy levels which surround it. Both of these concepts postdate Mendeleev by several decades. He, however perceived them indirectly through the related properties of atomic weight and chemical valency and arrived at the periodic table in 1869.\n\nBecause atomic weight, relative atomic mass, is roughly proportional to atomic number, and because valency, which manifests itself in the chemical composition, is based on the outermost electrons of an atom, Mendeleev had chosen the two properties that in his day most nearly reflected the fundamental principles on which the table today is based. Consciously or subconsciously, he arrived at the idea that a table existed with positions that were to be occupied by the elements, rather than the other way round - that the known elements determined the arrangement of the table, as others imagined.\n\nThis being so, Mendeleev then put the 65 elements he knew into his table and at the same time pointed out the many unoccupied positions in the overall scheme. He took the further and much bolder step of predicting the properties of these missing elements. Moreover, the gap in atomic weights between cerium (140) and tantalum (182) suggested to him that a whole period of the table remained to be discovered. Later in the century many of these elements,
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Color, shape, hardness, melting point, boiling point, density
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Not sure what you mean by "active", but  elements vary in how reactive they are (like how easily they bond with other elements) according to their position in the periodic table. \n\nLook up "Chemical Reactivity" in your text or on the internet. There's a fairly clear explanation here:\nhttp://www.sciencebyjones.com/chemical_reactivity.htm
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The elements in a group on the periodic table all have the same number of valence electrons and therefore generally have similar properties.
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goto http://www.webelements.com/ or better stillhttp://www.schoolscience.co.uk/periodictable.html run over the element with your mouse it tells you what it is