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

 

Protons = Electrons ⇒ Atom. Neutral.

Protons ≠ Electrons ⇒ Ion. Charged.

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

 

Element

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.