### Bohr model (1913)

Particle | Symbol | Relative Charge | Mass Unit
Electron | e- | -1 | 5.4858 x 10-4
Proton | p+ | +1 | 1.0073
Neutron | n | 0 | 1.0073

### Element

Protons = Electrons ⇒ Atom. Neutral.

Protons ≠ Electrons ⇒ Ion. Charged.

Electrons are surrounding the nucleus, consisting of protons and neutrons.

Mass number are the number of protons and neutrons in the nucleus.

Atomic number are the number of protons in nucleus.

For example, 42He, where mass number is written on top left, and atomic number on bottom left.

### Periodic Table

The elements are arranged in the order of increasing atomic number, from left to right, from up to down.

Each element has a unique atomic number (or the number of proton).

### Electronic arrangement

The maximum number of electron that a shell can hold is 2(n)2

N | Max e-
1 | 2
2 | 8
3 | 18
4 | 32

### Electronic configuration

There are subshells inside the shell.

For example, Cl-17, instead of written 2, 8, 7, we can write more clearly with 2, 2+6, 2+5, that is 1s2 2s2 2p6 3s2 3p5

Subshell | Orbitals | Max e- | Name of orbital | Historical Name
s | 1 | 2 | s | Sharp
p | 3 | 6 | px, py, pz | Principal
d | 5 | 10 | dxy, dxz, dyz, dx2-y2, dz2 | Diffuse
f | 7 | 14 |  | Fundamental
g | 9 | 18 |  |

Orbitals | Quantum Number | Specify
n | Principal | The shell of electron belong to
l | Angular Momentum | The subshell of electron occupies
m | Magnetic | The orientation of orbital
s | Spin | If the electron is spin up or down

Electrons in boxes diagram

⇡⇣ |  | ⇡⇣ |  | ⇡⇣ | ⇡⇣ | ⇡⇣ |  | ⇡⇣ |  | ⇡⇣ | ⇡⇣ | ⇡
1s |  | 2s |  | 2p |  | 3s |  | 3p

### Aufbau principle

The lowest energy subshell or orbital will be filled first

1s < 2s < 2p < 3s < 3p < 4s < 3d

Imagine drawing a line from right to left.

You should be able to find out that 4s subshell has a lower energy than 3d subshell this way.

n |  |  |  | 1 |  |  |  |  |  |
|  |  | ⁄ |  |  |  |  |  |  |
1 |  | 1s |  | 2 |  | 3 |  |  |  |
| ⁄ |  | ⁄ |  | ⁄ |  |  |  |  |
2 |  | 2s |  | 2p |  | 4 |  | 5 |  |
| ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |  |  |
3 |  | 3s |  | 3p |  | 3d |  | 6 |  | 7
| ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |
4 |  | 4s |  | 4p |  | 4d |  | 4f |  | 8
| ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |
5 |  | 5s |  | 5p |  | 5d |  | 5f |  |
| ⁄ |  | ⁄ |  | ⁄ |  | ⁄ |  |  |

### Pauli's exclusion principle

Each orbital can be occupied by no more than 2 electrons.

If 2 electrons occupy the same orbital, they must have opposite spins.

### Hund's rule

Electron must occupy each orbital singly before pairing.

### Exception: Half and full filled orbital

The electronic configuration of Cr (Chromium) and Cu (Copper) do not follow the order. The outer subshell become half or fully filled to gain extra stability.

It is also because that 4s and 3d energy difference is small.

### Periodic table groups

Group 1 - Alkali metal

Group 2 - Alkaline earth metal

Group 17 (7) - Halogen

Group 18 (8) - Noble gas

Noble gases are stable in atomic form because the outermost shell are fully filled.

Other element tend to lose, gain or share electron to achieve nearest fully filled electronic structure, that is duplet or octet structure.

### Cation

Atom that loses e-, +ve ions

Usually group 1 and 2. Like Li+, Na+, Mg2+

For transition metal, 4s electron will be lost first. Like Ti2+, Cu+

| Name | Color (Water)
Li+ | Lithium ion | Colorless
Na+ | Sodium ion
K+ | Potassium ion
Ag+ | Silver ion
Mg2+ | Magnesium ion
Ca2+ | Calcium ion
Zn2+ | Zinc ion
Pb2+ | Lead (II) ion
Al3+ | Aluminium ion
NH4+ (Polyatomic) | Ammonium ion
H+ | Hydrogen ion | Combined
Cu+ | Copper (I) ion | Insoluble
Cu2+ | Copper (II) ion | Greenish blue
Fe2+ | Iron (II) ion | Pale green
Fe3+ | Iron (III) ion | Yellow
Ni2+ | Nickel (II) ion | Green
Cr3+ | Chromium (III) ion | Green

### Anion

Atom that gains e-, -ve ions

Usually group 5 to 7. Like N3-, S2-, Cl-

| Name | Composition | Color (Water)
O2- | Oxide ion | Monoatomic | React
S2- | Sulphide ion
N3- | Nitride ion
F- | Fluoride ion | Colorless
Cl- | Chloride ion
Br- | Bromide ion
I- | Iodide ion
OH- | Hydroxide ion | Polyatomic
NO2- | Nitrite ion
NO3- | Nitrate ion
SO32- | Sulphite ion
SO42- | Sulphate ion
HSO4- | Hydrogensulphate ion
CO32- | Carbonate ion
HCO3- | Hydrogencarbonate ion
MnO4- | Permanganate ion | Purple
Cr2O72- | Dichromate ion | Orange

### Isoelectronic

If they have the same number of electrons

| Name
Na+ | 10e-
Mg2+
N3-
S2- | 18e-
Cl-

### Isotopes

If they have the same number of protons but different amount of neutrons

For example, 126C (Carbon-12) and 146C (Carbon-14)

The chemical properties will be the same as they have the same number of electron.

The physical property will be slightly different.

Iodine-131 is used in the detection and therapy of thyroid tumours.

Iron-59 is used to monitor red blood cell production in bone marrow.

Fluorine-18 is used for Positron Emission Tomography (PET) imaging tumors in oncology.

Technetium-99m is used as a radioactive tracer and can be detected in the body by medical equipment (gamma cameras).