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Elements and Compounds

Key Concepts

  • Elements and Compounds are pure substances.

  • An element is a pure substance that cannot be decomposed into simpler substances.

  • Elements are listed in the Periodic Table of the Elements.

  • Each element has a 1 or 2 letter symbol.

  • A compound is a pure substance that can be decomposed into simpler substances.

  • Compounds are made up of two or more elements.


An element is a pure substance that cannot be decomposed (broken down) into simpler substances.

There are 92 naturally occurring elements.

Each element has been given a 1 or 2 letter symbol:

  • the first letter of the symbol is always a capital letter
        eg, H for hydrogen, C for carbon, N for nitrogen

  • if there is a second letter in the symbol it is a lower case letter
        eg, He for helium, Ca for calcium, Ne for neon

Elements can be present in nature as solids, liquids or gases.

Liquid Elements Gaseous Elements Solid elements
2 elements exist in nature as liquids: 11 elements exist in nature as gases: all other elements exist in nature as solids, eg:
mercury (Hg)
bromine (Br)
hydrogen (H)
helium (He)
nitrogen (N)
oxygen (O)
fluorine (F)
neon (Ne)
chlorine (Cl)
argon (Ar)
krypton (Kr)
xenon (Xe)
radon (Rn)
lithium (Li)
carbon (C)
sodium (Na)
magnesium (Mg)
aluminium (Al)
silicon (Si)
phosphorus (P)
sulfur (S)
potassium (K)
calcium (Ca)
zinc (Zn)

The atmosphere is mostly made up of the elements nitrogen (~78%) and oxygen (~21%).

Common elements found in the earth's crust are:

  • oxygen (O)
  • silicon (Si)
  • aluminium (Al)
  • iron (Fe)
  • calcium (Ca)
  • sodium (Na)
  • potassium (K)
  • magnesium (Mg)
  • hydrogen (H)

The most common elements found in living things are:

  • carbon (C)
  • hydrogen (H)
  • oxygen (O)
  • nitrogen (N)
  • phosphorus (P)
  • sulfur (S)

The most common elements found in the universe are:

  • hydrogen (H)
  • helium (He)
  • oxygen (O)
  • carbon (C)
Some Elements
Name Symbol
hydrogen H
helium He
lithium Li
beryllium Be
boron B
carbon C
nitrogen N
oxygen O
fluorine F
neon Ne
sodium Na
magnesium Mg
aluminium Al
silicon Si
phosphorus P
sulfur S
chlorine Cl
argon Ar
potassium K
calcium Ca
scandium Sc
titanium Ti
vanadium V
chromium Cr
manganese Mn
iron Fe
cobalt Co
nickel Ni
copper Cu
zinc Zn
gallium Ga
germanium Ge
arsenic As
selenium Se
bromine Br
krypton Kr
rubidium Rb
strontium Sr
zirconium Zr
silver Ag
tin Sn
barium Ba
tungsten W
platinum Pt
gold Au
mercury Hg
lead Pb
uranium U


Compounds are pure substances made up of 2 or more elements.

Each compound has a formula showing which elements are present in the compound

Examples of some common compounds are shown below.

compound name compound formula elements present
waterH2Ohydrogen (H) and oxygen (O)
ammoniaNH3nitrogen (N) and hydrogen (H)
carbon monoxideCOcarbon (C) and oxygen (O)
carbon dioxideCO2carbon (C) and oxygen (O)
sodium chlorideNaClsodium (Na) and chlorine (Cl)
sodium hydroxideNaOHsodium (Na), oxygen (O) and hydrogen (H)
calcium chlorideCaCl2calcium (Ca) and chlorine (Cl)
calcium carbonateCaCO3calcium (Ca), carbon (C) and oxygen (O)
calcium nitrateCa(NO3)2calcium (Ca), nitrogen (N) and oxygen (O)
calcium phosphateCa3(PO4)2calcium (Ca), phosphorus (P) and oxygen (O)
calcium sulfateCaSO4calcium (Ca), sulfur (S) and oxygen (O)
methaneCH4carbon (C) and hyrogen (H)
ethanolC2H5OHcarbon (C), hydrogen (H) and oxygen (O)

A compound can be decomposed into simpler pure substances.

For example, an electric current can be passed through water to form the elements hydrogen and oxygen.

Water can be decomposed into hydrogen and oxygen.

Water is a compound made up of hydrogen and oxygen.

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