Things to note for (N02) Atomic Structure:

1. Sub-atomic particles: Proton, Neutron, Electron
The symbol for electron is 'e-', as each electron has a relative charge of 1-. 

(i) Proton number/ Atomic number
It is the number of protons present in an atom. It determines the identity of the element.
(ii) Nucleon number/Atomic mass number/ Mass number:
It is the total number of protons and neutrons present in an atom.
(iii) Atomic mass:
It is the mass of the atom. Calculation: Sum up the relative mass of all the protons, neutrons and electrons present in the atom.
(iv) Relative atomic mass:
Calculation: Average mass of all the isotopes of the element, the relative abundance of the isotopes will be required for the calculation.


2. Drawing of the atomic structure of an atom or ion: 
If you are asked to draw 
(i) all sub-atomic particles
draw all electrons, label to no. of protons and no. of neutrons in the nucleus.
(ii) all the electrons
draw only the electrons, replace the nucleus with the symbol of the element. 
(iii) only the valence electron
draw only the electrons in the outermost/valence shell, do not reflect any of the inner shells in your diagram, the nucleus will be replaced with the symbol of the element.

For all diagrams, please use either dot or cross as the symbol of electrons. A legend must be provided. If you are drawing a negatively charged ion, the electrons which the atom gained should be indicated with a different symbol. If you are drawing a positively charged ion, the electrons which are given away should no longer be shown in the diagram, as they have been given away. 


3. Symbol, Chemical formula:
(i) Symbol:
It is the only the symbol as shown in the Periodic Table.
(ii) Chemical formula:
It depends if the elements exist as individual particles or as diatomic molecules. Most of the elements in the Periodic Table exist as individual particle, while all Group VII, Nitrogen, Oxygen and Hydrogen exist as diatomic molecules.
(iii) Symbol/notation showing the number of sub-atomic particles present:
It will include the symbol of the element and on the top left position, it will be the nucleon number, the bottom left position will be the proton number. 
(No examples are provided, see A02.2 Q6 answer (d) for the correct notation.)

Example 1: Fluorine
(i) Symbol of fluorine will be F.
(ii) Chemical formula will be F2. 
(the number '2' should be at the bottom right position, subscript position.since fluorine exists as a diatomic molecule.

Example 2: Sodium
(i) Symbol of sodium will be Na.
(ii) Chemical formula will be Na.
since sodium exists as an individual particle.

Other forms of representation also include, for example, C-12, which represents the element Carbon with total number of protons and neutrons (nucleon number) being 12. 

More examples:
C-13: Carbon with nucleon number 13;
O-18: Oxygen with nucleon number 18;
N-15: Nitrogen with nucleon number 15 and so on.


4. Word, Chemical, Half- equations:
All equations must include an ARROW and at least a '+' sign.

(i) Word equations: 
They consist of only the chemical names or words in them.
(ii) Chemical equations: 
They consist of only the chemical formula in them.
(iii) Half equations/ Chemical equation for the formation of an atom or ion: 
They consist of only the chemical formula and electron(s) in them.

Example 1: Formation of sodium ion.
(i) Word equation: sodium 
→ sodium ion + electron
(ii) Chemical equation/Half-equation: Na → Na+ + e-           
(the '+' change should be at the top right position, superscript position)


5. Formation of ions: 
Atoms either gain or lose electron(s) in order to obtain a stable electronic configuration.

Rule for formation of an ion:
(i) Less than 4 valence: LOSE all the valence electrons.
(ii) More than 4 valence electrons: GAIN electrons to make it stable.

Example 1: Formation of the ion of aluminium.
Aluminium atom has 3 valence electrons, thus it will lose all 3 valence electrons to become a stable ion. 

Half-equation: Al → Al3+ + 3e-   
(the '3+' charge should be at the top right position, superscript position)

Example 2: Formation of the ion of bromine.
Bromine atom, a Group VII element, has 7 valence electrons, thus it will gain 1 electron to become a stable ion. The atom must accept the electron before forming the ion.

Half-equation: Br + e- → Br-     
(the '-' charge should be at the top right position, superscript position)


Note: If the electron is taken in order to form the ion, it will appear before the arrow. However, if it is lost in order to form the ion, it will appear after the arrow as it is given away by the atom. The symbol for electron should always be written as e-. It should never be written as e+ to represent electron gained. 


6. Isotopes: 

The are atoms with the same number of protons but different number of neutrons.


credit: 
most of the above points came from questions or comments contributed by students, from 3E4 and 3E5, who came for consultation on Saturday (27 March '10).



------------------------------ Part 1: HISTORY OF AN ATOM ---------------------------------------


Video 1: Our Friend the Atom 1/5 (8:10 ~ End; Democritus + Aristotle)
Video 2: Our Friend the Atom 2/5 (3:10 ~ 4:50 min; Dalton)
Video 3: Our Friend the Atom 3/5 (3:50 ~ 7:40 min; Rutherford)

* To view more Science related videos from Disney, click here: Banyt's Channel

--------------------------- Part 2: STRUCTURE OF AN ATOM: -------------------------------------


Video 1: A is for Atom Part 1 (start ~ 5:43 min; structure of atom and isotopes)
Video 2: A is for Atom Part 3 (2:22 ~ 3.25 min; uses of radioactive isotopes)

* Part 2 of the video and start of Part 3 are on radioactivity. 
   To view, click here: Part 2
 


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