How can we determine the number of neutrons in an atom?

[Sithlord586]

OK i know the first shell can hold upt to 2 electrons. What of the other shells? HOw many can the second , third , fourth and so on? And is there an easy way of remembering?

 

[moose]

S,P,D,F
Those are the main electron orbital shapes.
The S has one orbital and can hold two electrons
The P has three orbitals and can hold six electrons
The D has five orbitals, so it can hold ten electrons
the F has seven orbitals, and it can hold fourteen electrons.

The easiest way to remember this is by knowing which part of the periodic table refers to which. A way of remembering the order of increasing electrons can be just thinking of S, and then the file extension of a "pdf" file :P
I mad this image in photoshop in order to help.

http://1337moose.com/school/table.jpg

*if anyone sees something wrong with what i said, please point it out*

 

[cmscoby]

Generally, you can label the shells with an integer L, such that

L = 0 <=> s-shell
L = 1 <=> p-shell
L = 2 <=> d-shell
L = 3 <=> f-shell
L = 4 <=> g-shell ... etc.

Then, the number of electrons held in each shell is given by the formula,

(# of electrons) = 2*(2*L+1)

giving 2 for s, 6 for p, 10 for d, 14 for f, and so on.

For more on why this is true, check out http://hyperphysics.phy-astr.gsu.edu/hbase/atpro.html#c2 .

Cheyne

 

[Sithlord586]

thank u for responding. One more question. How do we know the number of neutrons in the nucleus? For example , LITHIUM has three protons. What of the neutrons?

 

[whozum]

The atomic mass - the number of protons = number of neutrons. Since they are almost equal in mass, knowing the total mass and knowing how many protons will tell you how many neutrons you have.

 

[OlderDan]

thank u for responding. One more question. How do we know the number of neutrons in the nucleus? For example , LITHIUM has three protons. What of the neutrons?

The first reply you received is correct, but it can be expanded. There is not a unique answer for the number of neutrons in the nucleus of an element. In most cases, one number is the most abundant form of the element, but others exist. The difference between the atomic weight and the atomic number will tell you the number of neutrons in the most abundant form of the element.

The different forms of the same element are called isotopes of that element, and generally the number of different isotopes increases as the number of protons increases. Most isotopes are unstable and decay by giving off radioactive particles. There is an extensive list of the isotopes with information about abundance and radioactivity at this site

http://ie.lbl.gov/education/isotopes.htm