Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Periodic table groups

  1. Oct 10, 2004 #1

    Math Is Hard

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I've got one of those "research in other resources" questions in my chemistry homework. (meaning the info isn't in the lecture or the text, I guess :grumpy: ). It goes like this:

    Consider Group 1A and Group 1B of the periodic table. The text states that although A groups have very regular patterns, the "situation gets a bit more complicated with the B families".

    Researching in other resources, find out which of these predictions becomes more complicated.

    a) predicted number of electrons for each element in each group.

    b) predicted number of outer electrons for each element of each group.

    c) predicted formula when each element combines chemically with chlorine.

    I am not really sure what they mean by "prediction". For part a) if it's a neutral atom, can't I just tell how many electrons by the number of protons in the atomic number?

    For b) I am getting confused about outer electrons. The notes from my lecture sat that the group number tells you how many outer electrons there are. With Group 1A, it is always 1, and those were pretty easy to count(like with Lithium, 2 at the first energy level and then 1 at the second (and outer) level for a total of 3 given by the Atomic number), but how many outer electrons do copper, gold and silver have? I am not sure about this.

    For part c) I don't even know what they are asking. :confused:

    Any help is appreciated. Thanks!
    Last edited: Oct 10, 2004
  2. jcsd
  3. Oct 10, 2004 #2
    Well, I'll try to help a little, from what I remember. I'm assuming you haven't gotten into valence electrons yet, eh? Or the spdf subshells. An atom is made up of subshells which include spdf. Inside these subshells are orbitals. Orbitals can hold up to 2 elections each. An s subshell has one orbital, thus an s subshell can only have at max, 2 electrons in it. P subshell has 3 orbitals, thus it can hold up to 6 electrons. d has 5 orbitals and f has 7 orbitals. I won't go into to much detail partly because I can't remember, lol.

    Here is how they get the valence electrons of an atom, there is a specific order in which you have to arrange this configuration, I'll just show you a configuration of a F atom and explain some things. 1s^2 2s^2 2p^5 ... 1s^2 means that there is 2 electrons in this s subshell, 2 electrons in the 2s subshell and 5 in the 2p subshell. Notice anything? The number of electrons add up to 9 which is shown on the periodic table. Now what does the grp number stand for on the periodic table? Well, it stands for the number of valence electrons in all the atoms of this group. For grp 7, which F is in, there is 7 valence electrons. All the elements in this grp have 7 valence electrons. How do they determine the valence electrons in an element?

    Lets look at the F configuration, 1s^2 2s^2 2p^5. To get the valence electrons we look at the LAST s and p subshell and add their electrons, which is.... 7! The charge of F is -1 because it takes one electron to make F have 8 electrons. Ionic bonds are based on the octet rule, which states that an atom needs 8 electrons to be stable.

    Lets look at H. The electron config for H is 1s^1. It is in grp 1 because it only has 1 electron is it's s subshell. This is also H's valence electron because the last s subshell only has 1 electron, there is no p subshell because H has a total of 1 electron. All it needs is one orbital. Thus, it's in grp 1(every element has 1 valence electron in this grp). For H, it has a +1 charge because it is easier to give away 1 electron than it is to gain 7. Grp 1,2,3 and 4 will all have + charges corresponding to the number of valence electrons(valence electrons = grp number). 5,6,7 have negative charges, however, minus the grp number from 8 and that will be it's charge. So 8-7=1, grp 7 will gain 1 electron making it a -1 charge. Grp 8 is special, these are called noble gases and already have 8 electrons, this is what the octet rule is based on.

    I'll answer some of your questions now, lol. Part A: I'm assuming they just want you to look at each element of grp 1 and 2 and give it's total electrons. Look at your periodic table. See in the box of Li, it says 3? Well that is the number of protons, but also the number of electrons in Li. Protons=electrons, this changes when you have bonding occuring though. Protons stay the same but electrons may change. I can't remember how to do b. C: Cl is in grp 7 that means it has a what charge? It's in the same grp as F. It needs how many electrons to be stable and agree with the octet rule? Grp 7 elements need 1 electron to be stable, thus -1 charge. Li has a +1 so the formula is LiCl ( a very soluble salt :rofl:) How did I get LiCl? Cl only needs 1 electron to get 8 electrons, and Li only has 1 electron to give.. thus there is one Li and one Cl,LiCl. Let's say you had 2 Cl atoms. Now you need Li to give 2 electrons away. So the formula is Li2Cl2, the 2s are subscripts. 2 Li, and 2 Cl atoms. Metals are always written first in a ionic compound.

    Some elements don't have fixed charges, one of them is Cu, which you mentioned.. but that is another story, that you'll learn in school, I hope. I don't really remember the outer electron stuff, may be linked with s and p shells, but I can't remember. Try to digest this, it will help you as you move along this semester.
  4. Oct 10, 2004 #3

    Math Is Hard

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Thanks for your help! ok, here come some questions:

    1) Outer electrons and valence electrons are the same thing, right? (Just making sure).
    2) What is spdf?
    3) Is an orbital the shell that is closest to the nucleus?
    From what I learned, the first "energy level" (shell) can hold a max of two electrons. The next highest can hold a maximum of 8 electrons. If you go to the next highest energy level, can it also hold a maximum of 8 electrons?
    4) For this one:
    "Lets look at the F configuration, 1s^2 2s^2 2p^5. To get the valence electrons we look at the LAST s and p subshell and add their electrons, which is.... 7! The charge of F is -1 because it takes one electron to make F have 8 electrons"
    I thought that there would be no charge associated with an atom as long as it had the same number of protons and electrons (wasn't an ion). I didn't think it had anything to do with the number of valence electrons. I think I am confusing something important here.

    Sorry for the dumb questions. This is my first chemistry class. :blushing:
  5. Oct 10, 2004 #4
    spdf are subshells as I stated above. Each have a set number of orbitals. Yes, you are correct that an element doesn't have a charge until it forms an Ionic bond, making this element an ion.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook