Atomic Structure

1.8 recall that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells

The centre of an atom is the nucleus, which is composed of protons and neutrons. Their masses are roughly equal and since the mass of the electron is pretty much negligible, most of the mass of an atom is in the nucleus. 

The electrons are found in a series of energy levels which you call shells at IGCSE. Each 'shell' can only hold a certain number of electrons, these shells can be thought of as getting progressively further from the nucleus. Electrons will always go into the lowest possible energy level, provided there is space. The first shell can only hold 2 electrons, then the shells after that can hold a maximum of 8. 

In a diagram, the electrons are shown on circles around the nucleus. Beware that these circles are just imaginary lines to help you understand that the electrons orbit around the nucleus, at IGCSE level you just need to accept that. 

For those moving on to higher levels of Chem, the 'truth' is different, it's actually not really possible to plot the path of an electron. For more information look at the following link, to be honest I don't really understand it myself, it's so confusing. 

http://www.chemguide.co.uk/atoms/properties/atomorbs.html#top

Later in the course you will come across  dot-and-cross diagrams. Dots or crosses are used to represent electrons, in the above, Carbon has 4 outer shell electrons. They are drawn far apart even though you could draw them close to each other like in the first shell, this is because electrons would repel each other as they have the same negative charge. (Remember like charges repel). So only if you have more than 4 OSE, then do you draw them in pairs. Remember, draw the 4 OSE like in the diagram of carbon above, then pair up any OSE left. The following diagram might help you understand:

This way of drawing electrons is clear and makes it easy to count too. When you learn about ions, dot-and-cross diagrams are useful and they help you see how the electrons are transferred. Like in the above diagram, the Chlorine atom gains one electron (the cross) from the sodium atom to become a chloride ion (Cl^-), whilst the sodium atom becomes a sodium ion (Na⁺). It is positive because it lost one electron, so it has one more proton than electron now. :)

1.9 recall the relative mass and relative charge of a proton, neutron and electron

	Relative mass	Relative charge
Proton	1	+1
Neutron	1	0
Electron	1/1840 (negligible)	-1

Extra (you don't need to know this): 

Protons and neutrons don't actually have exactly the same mass - neither of them has a mass of exactly 1 on the carbon-12 scale (the scale on which the relative masses of atoms are measured). On the carbon-12 scale, a proton has a mass of 1.0073, and a neutron a mass of 1.0087. 

1.10 understand the terms atomic number, mass number, isotopes and relative atomic mass (A_r)

Atomic number is the number of protons there are in the nucleus, it is sometimes called the proton number, though atomic number should be more accurate because atoms are electrically neutral, the number of protons and electrons are equal. (Protons have a charge of +1 whilst electrons are -1, so they cancel each other out.) So the atomic number tells you the number of protons and the number of electrons. 

Mass number is the number of protons and neutrons. It is sometimes known as the nucleon number, because protons/neutrons are nucleons. So if a question asks you for the number of neutrons in an atom, mass number - atomic number = no. of neutrons. 

Isotopes: these are atoms which have the same atomic number but different mass numbers, i.e. same number of protons but different number of neutrons.

The number of neutrons in an atom can vary a little. For instance, there are three kinds of carbon atoms ¹²C, ¹³C and ¹⁴C. Their number of neutrons varies but they have the same number of protons, because each element's atomic number is unique. If it has a different number of protons, it wouldn't be the same element anymore. So these atoms are isotopes of carbon. Bear in mind that the fact that they have varying numbers of neutrons makes no difference whatsoever to the chemical reactions of the carbon. Though their physical properties may vary. 

	Protons	Neutrons	Mass number
Carbon-12	6	6	12
Carbon-13	6	7	13
Carbon-14	6	8	14

Relative atomic mass (web definition): the ratio of the average mass per atom of the naturally occurring form of an element to one-twelfth the mass of an atom of carbon-12. Symbol A_r Abbreviation r.a.m. I find it easier to think of it as:
The number of times the mass of one atom of an element is heavier than 1/12 of the mass of a carbon-12 atom. Anyways, this number is always at the top of an element's symbol, it's always either mass number or RAM at the top, atomic number at the bottom - as shown below:

mass number is always on the top, atomic number at the bottom, don't mix them up. at least you know that the mass number is always greater than atomic number

1.11 calculate the relative atomic mass of an element from the relative abundances of its isotopes
You multiply the relative abundance of each isotope by its mass number, add these together, and divide by 100. It's easier to understand through an example, in this case I'll use chlorine, since it's pretty common.

   and  

Chlorine consists of 75% Chlorine-35 and 25% Chlorine-37. You can think of the data as 100 atoms, 75 having a mass of 35 and 25 with a mass of 37. So the calculation is:
[(75 x 35) + (25 x 37)] / 100 = 35.5
So the RAM of chlorine, or A_r(Cl) is 35. 
(There are tiny percentages of other chlorine isotopes but the two shown above are the most common, and so the rest are ignored at IGCSE level.)

The RAM of an element will be closer to the mass number of the more abundant isotope. For example, the RAM of chlorine is 35.5, which is closer to chlorine-35, because it is the more abundant isotope. Obviously 75% > 25%!.


1.12 understand that the Periodic Table is an arrangement of elements in order of atomic number
The number of protons in the element's atom increases across the Periodic Table as you've probably noticed in yours.


1.13 deduce the electronic configurations of the first twenty elements from their positions in the Periodic Table
To work out the electronic arrangement of an atom:

• Look up the atomic number in the Periodic Table - making sure that you choose the right number if two numbers are given. The atomic number will always be the smaller one and tends to be below the symbol.
• This tells you the number of protons, and hence the number of electrons.
• Arrange the electrons in levels, always filling up an inner level before you go to an outer one. 

e.g. to find the electronic arrangement in oxygen

• the Periodic Table gives you the atomic number of 8.
• Therefore there are 8 protons and 8 electrons.
• The arrangement of the electrons will be 2,6. (First shell only holds 2 electrons, then there's 6 left which occupy the second shell.)

1.14 deduce the number of outer electrons in a main group element from its position in the Periodic Table
If you look at the patterns in the table:

• The number of electrons in the outer level is the same as the group number. (Except with helium which has only 2 electrons. The noble gases are also usually called group 0 - not group 8.) This pattern extends throughout the Periodic Table for the main groups (i.e. not including the transition elements). 
• So if you know that barium is in group 2, it has 2 outer shell electrons (btw, outer shell electrons which I will abbreviate to OSE are also known as valence electrons); iodine is in group 7, so it has 7 OSE, lead is in group 4, so surprise surprise, it has 4 OSE. 
• Noble gases have full outer shells. Thus they are unreactive. More on them in this post:  http://askmichellechemistry.blogspot.com/2012/03/periodic-table.html