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Isotopes | Matter | Physics | FuseSchool

The periodic table divides the world into just over one hundred ​elements​, sorted by their chemical
properties. But did you know that each kind of ​atom is made up of a different combination of just three sub-atomic
particles.​ ​Everything​ ​around​ ​you​ ​made​ ​from​ ​just​ ​three​ ​ingredients!

You’ve​ ​looked​ ​at​ ​atoms​ ​from​ ​a​ ​chemistry​ ​point​ ​of​ ​view,​ ​but​ ​we​ ​also​ ​cover​ ​them​ ​in​ ​physics​ ​too. In​ ​this​ ​video,​ ​we’re​ ​going​ ​to​ ​look​ ​at​ ​different​ ​forms​ ​of​ ​an​ ​atom,​ ​called​ ​isotopes. Before​ ​we​ ​start,​ ​let’s​ ​quickly​ ​recap​ ​the​ ​general​ ​structure​ ​of​ ​an​ ​atom. The nucleus is made up of ​neutrons and positively charged ​protons. The total number of these particles is called the ​mass number​; they each have a relative mass of one. Orbiting the nucleus in specific patterns are negatively charged ​electrons​. These are much lighter, in fact, their mass is so small we​ ​can​ ​usually​ ​ignore​ ​it.

All atoms of a particular element will always have the same number of protons in the ​nucleus​; this is called the ​atomic or ​proton number​. Helium atoms always have two protons; never more, never less. Lithium​ ​has​ ​three​ ​protons,​ ​so​ ​the​ ​proton​ ​number​ ​is​ ​three. But​ ​neutrons​ ​are​ ​a​ ​little​ ​different.​ ​The​ ​number​ ​of​ ​neutrons​ ​in​ ​an​ ​atom​ ​​can​ ​change.

The​ ​differences​ ​in​ ​mass​ ​numbers​ ​is​ ​because​ ​this​ ​one​ ​has​ ​one​ ​more​ ​​neutron​ ​in​ ​its​ ​nucleus. These different forms are called ​isotopes​. So isotopes of an atom always have the same number of
protons, but different numbers of neutrons. The different isotopes do however have identical chemical properties.

The mass shown in the periodic table is a ​weighted average of all the atoms of that element.​ ​So​ ​of​ ​all​ ​the​ ​different​ ​isotopes. If​ ​it’s​ ​close​ ​to​ ​a​ ​whole​ ​number,​ ​that​ ​usually​ ​means​ ​there’s​ ​only​ ​one​ common​ ​isotope. For​ ​example,​ ​around​ ​99%​ ​of​ ​carbon​ ​atoms​ ​have​ ​six​ ​neutrons. Which​ ​means​ ​about​ ​99%​ ​of​ ​them​ ​have​ ​a​ ​mass​ ​number​ ​of​ ​12. But​ ​around​ ​1%​ ​have​ ​an​ ​extra​ ​neutron.​ ​We​ ​call​ ​this​ ​isotope​ ​Carbon-13. And​ ​there​ ​is​ ​Carbon-14​ ​which​ ​is​ ​even​ ​rarer,​ ​with​ ​eight​ ​neutrons. Because most of the carbon isotopes have a mass of 12, but some have 13 and even fewer have 14, this is why the mass number on the periodic table is just a little higher than 12. Only very precise measurements​ ​can​ ​show​ ​this.

Some combinations of protons and neutrons are stable. But some are unstable, which means the nucleus doesn’t last very long. These are often called ​radioisotopes because when they break down they​ ​give​ ​out​ ​​nuclear​ ​radiation​,​ ​which​ ​we’ll​ ​look​ ​at​ ​in​ ​the​ ​next​ ​video. For now, you should be happy explaining what isotopes are and linking the numbers in the periodic table
to​ ​the​ ​sub-atomic​ ​particles​ ​in​ ​each​ ​atom.



CREDITS
Animation & Design: Waldi Apollis
Narration: Dale Bennett
Script: Ian Horsewell

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