In the periodic table of the elements, elements are arranged in a series of rows (or periods) so that those with similar properties appear in vertical columns. Elements of the same period have the same number of electron shells; with each group across a period, the elements have one more proton and electron and become less metallic. This arrangement reflects the periodic recurrence of similar properties as the atomic number increases. For example, the alkaline metals lie in one group (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration. The periodic table of elements has a total of 109 elements.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown below. The filling of each shell corresponds to a row in the table.

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d

7s 7p

8s

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods (particularly the lanthanides).

Periods

Seven periods of elements occur naturally on Earth. For period 8, which includes elements which may be synthesized after 2010, see the extended periodic table.

A group in chemistry means a family of objects with similarities like different families.

Chemical elements in the first period

The first period contains fewer elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule. Chemically, helium behaves as a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its nuclear structure it belongs to the s block, and is therefore sometimes classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.

• Hydrogen (H) is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass. Ionized hydrogen is just a proton. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane. Hydrogen can form compounds with most elements and is present in water and most organic compounds.
• Helium (He) exists only as a gas except in extreme conditions. It is the second lightest element and is the second most abundant in the universe. Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars. On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements. Such 'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume.

Chemical elements in the second period

Period 2 elements involve the 2s and 2porbitals. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.

• Lithium is the lightest metal and the least dense solid element. In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally in compounds. It is the heaviest primordial element forged in large quantities during the Big Bang.
• Beryllium has one of the highest melting points of all the light metals. Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further within stars to create larger nucleii, like carbon, nitrogen or oxygen. Beryllium is classified by the From Yahoo Answers
  Question:we are supposed to draw arrows on a periodic table to indicate the 4 different trends. for example the atomic radius would have and arrow pointing down to indicate the increase in size as well as an arrow pointing to the right. however i cant find the trend for reactvity so it would be great if someone could help me out. thanks =)
  
  Answers:Reactivity - Reactivity refers to how likely or vigorously an atom is to react with other substances. This is usually determined by how easily electrons can be removed (ionization energy) and how badly they want to take other atom's electrons (electronegativity) because it is the transfer/interaction of electrons that is the basis of chemical reactions. Metals Period - reactivity decreases as you go from left to right across a period. Group - reactivity increases as you go down a group Why? The farther to the left and down the periodic chart you go, the easier it is for electrons to be given or taken away, resulting in higher reactivity. Non-metals Period - reactivity increases as you go from the left to the right across a period. Group - reactivity decreases as you go down the group. Why? The farther right and up you go on the periodic table, the higher the electronegativity, resulting in a more vigorous exchange of electron
  

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  Question:What is activity and how does it change across and up the periodic table? And also what is the diff between activity and reactivity?
  
  Answers:Visit these sites to learn more about chemistry formulas and concepts that can help you in regards to your problem: http://www.webelements.com/ http://chemistry.about.com/od/homeworkhelp/Chemistry_Homework_Help.htm
  

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  Question:I am pretty sure that as you move from left to right across the periodic table, the electron affinity gets more negative, but what is the basic rule when moving through the groups(columns). THANKS ALOT!!
  
  Answers:hi Adam - sorry did not see you had posted a ?; I take care of my buddies - just shot me an e-mail in the future. (by 5PM EST) first electron affinity is defined in such a way as that it is positive. It is the amount of energy released when A + e- ---> A- so opposite of delta H. the *general* trend is that affinity increases left to right (except nobel gases are close to zero) and the *general* trend (and not much of a trend actually ) is that affinities decrease as you go down - but there are a lot of exceptions.
  

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  Question:As you move across a period on a periodic table what is the trend in atomic radius?
  
  Answers:On the Rutherford model, the radius would be the same, since you are populating the outer "shell" with electrons, when you move down, you add a shell, and the radius increases. (Well, it did when I was at school..... but then again, Pluto was a planet back then)
  

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  From Youtube

  Periodic Trends :We examine trend in the periodic table for metals
  
  
  Periodic Trends :Very basic talk about atomic radii, ionization energy, electron affinity, & electronegativity. I do not go into multiple ionization energies. I also address a little of the historical development of the periodic table.