Topic 2 - Atomic Theory

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[edit] 2.1: The atom

[edit] 2.1.1

Core: Protons and Neutrons form the nucleus of the atom, electrons orbit the nucleus in electron shells.

Expansion: If we let the earth represent an atom, size-wise, a nucleus is roughly a soccer ball. And electrons (the space of their movement) occupy the majority of an atom, while the nucleus resides at the center. So it wouldn't be surprising that electrons determine the chemical properties of elements.

[edit] 2.1.2

Core:

Particle	Mass (amu)	Charge
Protons	1	+1
Neutrons	1	0
Electron	1/1840 5 x 10^-4	-1

Expansion:

Since the mass of electrons are negligible and the mass of a proton is roughly the same as a neutron, the mass of an atom can be relatively represented as the number of protons and neutrons combined. And such standard - the mass of a proton/neutron ( 1/12 of the mass of a carbon-12 atom in practice) is called 'amu', atomic mass unit.
Also, we designate the charge found on protons as positive, and electrons as negative. The two objects possessing different charges attract and balance each other, while objects of the same charge repel. The net charge is the number of + charge subtracted from the number of - charge. When net charge equals zero (charges are balanced), we call it 'neutral'. Elements (atoms of the same number of protons) tend to be in neutral state. Therefore the number of protons = the number of electrons for a certain atom. (and since electrons determine chemical properties, we can see why only protons matter in determination of the types of elements.)
Then why a bunch of protons can pack together? There are other kinds of forces (interactions between matters) making this happen. But the repulsion resulted from protons having the same charge is one of the reasons we can't randomly creat new elements!

1 amu = 1.66053886 × 10^-24 g approximately

[edit] 2.1.3

Mass number (A): Number of protons + neutrons.
Atomic number (Z): Number of protons.
Isotope: Atoms with same atomic number but different mass number (i.e. different numbers of neutrons)

[edit] 2.1.4

^A _Z X
A = mass number, Z = atomic number, X = atomic symbol.

Note! mass number on top left, atomic number (number of protons) at the bottom. The number of neutrons is not explicitly stated. That's the convention. Don't mix up!

[edit] 2.1.5

number of protons = Z
number of neutrons = A - Z
number of electrons = Z - charge numeber (the sign of charge must be included)

[edit] 2.1.6

Isotopes may differ in physical properties (mass/density) and radioactivity but not generally in chemical properties.
Separating isotopes: various approaches are available. Ex. mass accelerator. Yet all of them are essentially exploiting the difference in mass among different isotopes. Imagine you throw stones of different weight away with roughly the same strength. The lighter one would travel further while the heavier one relucts on flying for long. The separation of isotopes is the same idea.

[edit] 2.1.7

The radioactive isotopes are used to determine the date of a certain object (ex.¹⁴C carbondating), perform radiotherapy (ex.⁶⁰Co), and tracing molecules for medical or laboratory needs.

[edit] 2.2: The mass spectrometer

[edit] 2.2.1

[edit] 2.2.2

[edit] 2.2.3

Atomic masses are the average of the actual atomic mass of each isotope (isotopic mass) times the isotope’s relative abundance.

For example:
Bromine has 2 stable isotopes: ⁷⁹Br & ⁸¹Br. Their natural abundances are 50.69% and 49.31% respectively. Then the average atomic mass of bromine can be approximated (using the isotopic mass numbers) as 79.99. (79×0.5069 + 81×0.4931 = 79.9862 ≈ 79.99).

[Supplemental knowledge]The 1:1 ratio also gives a signature feature of bromine containing compound in mass spectrometry - two equal height peaks differing by 2 m/z number (when z=1).

[edit] 2.3: Electron arrangement

[edit] 2.3.1

Continuous spectrum goes continuously through red, orange, yellow, green, blue, indigo, violet. A line spectrum contains only some individual lines from this spectrum.

[edit] 2.3.2

Electrons are excited (usually by running an electric current through them). This causes electrons to 'jump' into higher electron shells ( X -> X^* ) this state is only temporary, however, and the electron falls back to it's ground state. This change decreases the energy of the electron, and this energy is emitted in the form of a photon. If this photon falls into the visible spectrum of light, then it produces a visible spectrum. As electrons move further away from the nucleus, the electron shells become closer together in terms of space and energy, and so lines converge towards the end of the spectrum.

[edit] 2.3.3

The main electron levels go : 2, 8, 18 etc…2n² for n₁, n₂ and n₃...

[edit] 2.3.4

Electrons are added from the left…after each shell is filled, move to the next…2, 8, 18…only up to Z = 20 is required.