The fourth shell can hold 8 electrons until the fifth shell has 2 electrons, then another 10 (total of 18) Memorising the first 20 elements This trend occurs for shells 3/4 and 4/5. After Zinc, electrons fill the outermost shell again until Krypton ( $ 2,8,18,8 $ ), after which the fifth shell starts filling. After the 4th shell has 2 electrons, they begin to fill from the 3rd shell until Zinc, which fills the third shell to its capacity of 18 ( $ 2,8,18,2 $ ). In the fourth period, Potassium has an electron configuration of Potassium is $ 2,8,8,1 $ This concept is a bit confusing at first.

The third shell can hold 8 electrons until the fourth shell has 2 electrons, then another 10 (total of 18).

The second shell can hold a maximum of 8 electrons.

The first shell can hold a MAXIMUM of 2 electrons.

Electron shells fill from the innermost available space:.

Atoms have a nucleus (which contains their protons and neutrons) and orbiting shells (which contain electrons in fixed rings).

An atom is the smallest sample of an element.

An element is a substance made out of the SAME TYPE OF ATOM.

An atom is a particle containing protons and electrons (and generally neutrons too).

NOTE: ATOM and ELEMENT do NOT mean the same thing!.

The periodic table is a table listing the fundamental building blocks of the universe: elements.

Acid-Metal reaction: when an acid and metal react to produce a metallic salt and Hydrogen.

Precipitation: formation of an insoluble solid when two soluble solutions are combined (product is known as a precipitate).

oxidisation: corrosion reaction where the gas/liquid is Oxygen (O).

decomposition: when a single compound breaks down into two or more compounds.

Combustion: exothermic reaction between a fuel and oxidiser, which produces heat, light and gaseous products.

Corrosion: a gas or liquid chemically attacking an exposed surface.

Neutralisation: mixing an acid and a base to create water.

Atomic mass: number of protons + number of neutrons.

Catalyst: something that causes and/or speeds up chemical reactions.

Octet: 8 electrons in the valence shell (2 if the element is Hydrogen or Helium).

Valence shell: Outermost electron shell.Ion: An atom which is electrostatically charged.

Hsc chemistry tutorial