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Atom
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: Different [[atom]]s can have different numbers of [[proton]]s and [[neutron]]s.
: The simplest [[atom]] is [[Hydrogen]] which has 1 [[proton]] and 1 [[electron]] and no [[neutron]]s.
==Key Stage 4==
===Meaning===
An [[atom]] is a very small [[particle]] made of [[proton]]s, [[neutron]]s and [[electron]]s that can join with other [[atom]]s to make [[molecule|molecules]].
===About Atoms===
: [[Atom]]s consist of a small, central [[Atomic Nucleus|nucleus]] containing [[proton]]s and [[neutron]]s surrounded by [[electron]]s [[Electron Orbital|orbiting]] the [[Atomic Nucleus|nucleus]].
: The [[electron]]s [[orbit]] the [[Atomic Nucleus|nucleus]] in so called '[[Electron Orbital|electron shells]].
{| class="wikitable"
|-
|[[File:AtomDiagram.png|center|400px]]
|-
| style="height:20px; width:200px; text-align:center;" |A [[diagram]] of an [[atom]].
|}
: In an [[atom]] there is always the same number of [[proton]]s as [[electron]]s. If any [[electron]] is added or removed the [[atom]] becomes an [[ion]].
: [[Atom]]s can be different [[element]]s depending on the number of [[proton]]s.
{| class="wikitable"
| style="height:20px; width:200px; text-align:center;" |'''Hydrogen'''
| style="height:20px; width:200px; text-align:center;" |'''Helium'''
| style="height:20px; width:200px; text-align:center;" |'''Lithium'''
| style="height:20px; width:200px; text-align:center;" |'''Beryllium'''
|-
|[[File:Hydrogen.png|center|200px]]
|[[File:Helium.png|center|200px]]
|[[File:Lithium.png|center|200px]]
|[[File:Beryllium.png|center|200px]]
|-
|[[File:HydrogenSymbol.png|center|200px]]
|[[File:HeliumSymbol.png|center|200px]]
|[[File:LithiumSymbol.png|center|200px]]
|[[File:BerylliumSymbol.png|center|200px]]
|-
| style="height:20px; width:200px; text-align:center;" |[[Hydrogen]] always has 1 [[proton]].
| style="height:20px; width:200px; text-align:center;" |[[Helium]] always has 2 [[proton]].
| style="height:20px; width:200px; text-align:center;" |[[Lithium]] always has 3 [[proton]].
| style="height:20px; width:200px; text-align:center;" |[[Beryllium]] always has 4 [[proton]].
|}
: [[Atom]]s of the same [[element]] can have different numbers of [[neutron]]s so they can be different [[isotope]]s of the same [[element]].
===History of Atoms===
: The existence and structure of [[atom]]s was not always known.