Lesson 2

3 05 2012

Click this link to download: Lesson 2: Atomic Structure, Isotopes and Electronic Configuration

Objective: To understand the basics of atomic structure, Isotopes and electronic configuration.

Atomic Structure

Atoms are made up of sub-atomic particles called protons (+), neutrons (0 hence being neutral) and electrons (-).

Particles Mass Charge
p 1 +1
n 1 0
e 0 -1

Example:

7

   Li

3

In Lithium there are 3 protons, 3 electrons, and 4 neutrons.

The number below the elemental symbol (in this case 3) is the atomic number; this indicates the number of protons in the nucleus of the atom.

The number above (in this case 7) is the atomic mass number; this represents the number of protons + the number of neutrons.

We already know from the atomic number that the number of protons is 3, so if we subtract 3 from 7 we are left with the number of neutrons, 4.

Remember: the number of protons = the number of electrons

(If we assume the element to be electrically neutral, or uncharged. If dealing with questions regarding charge, ions etc, then further considerations would need to be made.)

Test yourself:

39

     K

19

p = ?

n = ?

e = ?

(See answers at the end of this document)

Isotopes

These are atoms of the same element that have the same atomic number, but different mass number.

In other words, Isotopes have the same amount of protons but a differing amount of neutrons.

Example:

35                        37

     Cl                         Cl

17                        17

n = 18                 n = 20

Both Isotopes of Chlorine have 17 protons (and electrons), but as you can see a different amount of neutrons.

Electrons exist around the nucleus of atoms in different energy levels called electron shells, of which there are several.

Sticking with Chlorine, we see that its 17 electrons are spread throughout the layers of electron shells that make up an atom of this element.

The centre is of course the nucleus, where protons and neutrons reside, and the grey dots surrounding it are the electrons.

Those nearest to the nucleus are in the first shell, moving out one is the second shell, and the outer third shell.

In the first shell there are 2 electrons, in the second shell there are 8 electrons and in the third outermost shell there are 7 electrons (hence 2, 8, 7). It is important to know how many electrons each shell can physically hold; as there is a reason every element follows these basic rules, so onto our next heading….

Electronic Configuration

The way in which electrons are arranged within an atom.

Shell

name

Subshell

name

Subshell

max

electrons

Shell

max

electrons

K

1s

2

2

L

2s

2

2+6=8

2p

6

M

3s

2

2+6+10

=18

3p

6

3d

10

N

4s

2

2+6+

+10+14

=32

4p

6

4d

10

4f

14

http://en.wikipedia.org/wiki/Electron_shell

As you can see each shell has a sub-shell, with shell 2 being made up of 2s and 2p, shell 3 being made up of 3s, 3p and 3d. Each sub-shell has a limit as to the number of electrons it can hold, which dictates how many the entire shell can hold.

Returning to our example Chlorine, we see that its outermost shell (3rd) contains 7 electrons, but has the ability to hold 8 (2s holds 2, 2p holds 6). It is important to note that Chlorine wants 8 electrons in its outer shell, to achieve a ‘stable octet’, so is reactive.

Observe the Noble gases on your periodic table, Group 0: helium, neon, argon, krypton, xenon and radon. The reason these elements have a very low chemical reactivity is because they each have a ‘stable octet’ of 8 electrons in their outer shell. To put it simply, they are content, they do not want to give away an electron or receive one, which is what chemical reactions are all about; the sharing, taking or giving of electrons from the outer shell (the ‘valence electrons’).

Think of the noble gases as being antisocial snobs who think they’re too upper-class to associate with the other elements (just don’t write that in your exam paper). This is not to say they never react, but it is difficult and requires more energy.

Looking at the periodic table below you see how the energy levels and electron shells are structured. When asked for the electronic configuration of an element, you are basically being asked how many electrons are in each sub shell, so chlorine which in simple form is 2, 8, 7, would be written as 1s2 2s2 2p6 3s2 3p5. (The numbers in bold are the amount of electrons, the regular numbers are which shell they are located in, and the small letters s, p, d or f refer to the sub shells). 3s2 and 3p5 are both Chlorine’s outer shell.

Example: you can see why 1s2 is filled, there are 2 ‘1s’ boxes on the periodic table, 2s fills 2 boxes so is 2s2, 2p consists of 6 boxes so can hold 6 electrons, hence 2p6. 3s has 2 boxes so 3s2, 3p has 6 boxes so 3p6, and so on.

When asked for the electronic configuration of an element, one method is to ‘tick off’ each box from left to right across the periodic table until you get to your chosen element, you will see that Chlorine is the 17th box, hence having 17 electrons, so tick off 1s2, 2s2, 2p6, 3s2, 3p5.

http://www.800mainstreet.com/33/0003-000-spdf-bloc.gif

 Another method is to look at a diagram of the order in which the shells are filled:

http://www.meta-synthesis.com/webbook/34_qn/qn2.jpg

This is not the method I use, but it is a simple way of visually showing the sequence. 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p etc.

 

Answers to Test Yourself Question: p = 19, n = 20, e = 19

Click this link to download: Lesson 2: Atomic Structure, Isotopes and Electronic Configuration

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One response

10 05 2012
Ammonia « ruthlearns

[…] Ammonium is isoelectronic CH4 (Methane) so has the same shape, tetrahedral. Remember ‘iso’ in Greek means ‘equal’, so in chemistry it means ‘the same’ or ‘no change’, remember we studied Isotopes of elements in Lesson 2. […]

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