Bohr model (1913)

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

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

Electrons in boxes diagram

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.
 

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⁺

Anion

Atom that gains e-, -ve ions

Usually group 5 to 7. Like N^3-, S^2-, Cl^-

Isoelectronic

If they have the same number of electrons