They are pulled out in order to make the table itself fit more easily onto a single page. A group is a vertical column of the periodic table, based on the organization of the outer shell electrons. There are a total of 18 groups. There are two different numbering systems that are commonly used to designate groups and you should be familiar with both.
The traditional system used in the United States involves the use of the letters A and B. The first two groups are 1A and 2A, while the last six groups are 3A through 8A. The middle groups use B in their titles. Unfortunately, there was a slightly different system in place in Europe. To eliminate confusion the International Union of Pure and Applied Chemistry IUPAC decided that the official system for numbering groups would be a simple 1 through 18 from left to right.
Many periodic tables show both systems simultaneously. Skip to main content. The Periodic Table. Search for:. How has the English dictionary evolved over time? Summary The periodic table is arranged in order of atomic number A period is a horizontal row of the periodic table.
A group is a vertical row of the periodic table. Why did you choose that table? Explain your answer. Why are there two different numbering systems for groups? CC BY 2. Licenses and Attributions. So all of these elements in orange right over here, they have one valence electron, and they're trying to get to the octet rule, this kind of stable nirvana for atoms, and so you can imagine is that they're very reactive, and when they react, they tend to lose this electron in the outermost shell, and that is the case.
These alkali metals are very very reactive, and actually, they have very similar properties. They're shiny and soft, and actually, because they're so reactive, it's hard to find them where they haven't reacted with other things. Well, let's keep looking at the other groups. Well, if we move one over to the right, this group two right over here, these are called the alkaline earth metals. Alkaline, alkaline earth metals.
And once again, they have very similar properties, and that's because they have two valence electrons, two electrons in their outermost shell, and also for them, not quite as reactive as the alkali metals, but let me write this, alkaline earth metals, but for them it's easier to lose two electrons than to try to gain six to get to eight, and so these tend to also be reasonably reactive, and they react by losing those two outer electrons.
Now something interesting happens as you go to the D-block, and we studied this when we looked at electron configurations, but if you look at the electron configuration for say, scandium right over here, the electron, let me do it in magenta, the electron configuration for scandium, so scandium, scandium's electron configuration is going to be the same as argon, it's going to be argon.
The aufbau principle would tell us that the electron configuration, we would have the 4s2 just like calcium, but by the aufbau principle, we would also have one electron in 3d.
So it would be argon, then 3d1 4s2. And to get things in the right order for our shells, let me put the 3d1 before the 4s2. And so when people think about the aufbau principle, they imagine all of these d-block elements as somehow filling the d-block.
Now as we know in other videos, that's not exactly true, but when you're conceptualizing the electron configuration it might be useful. Then you come over here and you start filling the p-block. So for example, if you look at the electron configuration for, let's say carbon, carbon is going to have the same electron configuration as helium, as helium, and then you're going to fill your s-block 2s2, and then 2p one 2. So 2p2. So how many valence electrons does it have?
Well, in its second shell, its outermost shell, it has two plus two, it has four valence electrons, and that's going to be true for the things in this group, and because of that, carbon has similar bonding behavior to silicon, to the other things in its group. And we can keep going on, you know, for example, oxygen, oxygen and sulfur, these would both want to take two electrons from someone else because they have six valence electrons, they want to get to eight, so they have similar bonding behavior.
You go to this yellow group right over here, these are the halogens. So there's a special name for them. These are the halogens. And these are highly reactive, because they have seven valence electrons. They would love nothing more than to get one more valence electron, so they love to react, in fact, they especially love to react with the alkali metals over here. And then finally, you get to kind of your atomic nirvana in the noble gases here. And so the noble gases, that's the other name for the group 18 elements, noble gases.
And they all have the very similar property of not being reactive. Why don't they react? They have filled their outermost shell. They don't find the need, they're noble, they're kind of above the fray, they don't find the need to have to react with anyone else. Understand: the periodic table and properties of elements.
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