0:00:07.027,0:00:09.404
Most atoms don't ride solo,

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instead they bond with other atoms.

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And bonds can form between atoms

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of the same element

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or atoms of different elements.

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You've probably imagined bonding as a tug of war.

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If one atom is really strong,

0:00:21.966,0:00:24.110
it can pull one or more electrons

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off another atom.

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Then you end up with one negatively charged ion

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and one positively charged ion.

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And the attraction between these opposite charges

0:00:33.811,0:00:35.975
is called an ionic bond.

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This is the kind of sharing

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where you just give away your toy to someone else

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and then never get it back.

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Table salt, sodium chloride,

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is held together by ionic bonds.

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Every atom of sodium gives up one electron

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to every atom of chlorine,

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ions are formed,

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and those ions arrange themselves

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in a 3D grid called a lattice,

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in which every sodium ion

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is bonded to six chloride ions,

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and every chloride ion is bonded

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to six sodium ions.

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The chlorine atoms never give

0:01:07.472,0:01:09.925
the sodium atoms their electrons back.

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Now, these transactions aren't always so cut-and-dried.

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If one atom doesn't completely overwhelm the other,

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they can actually share each other's electrons.

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This is like a pot luck

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where you and a friend each bring a dish

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and then both of you share both dishes.

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Each atom is attracted to the shared electrons

0:01:27.353,0:01:28.652
in between them,

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and this attraction is called a covalent bond.

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The proteins and DNA in our bodies,

0:01:33.757,0:01:34.584
for example,

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are held together largely by these covalent bonds.

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Some atoms can covalently bond

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with just one other atom,

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others with many more.

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The number of other atoms

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one atom can bond with

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depends on how its electrons are arranged.

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So, how are electrons arranged?

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Every atom of a pure, unbonded element

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is electrically neutral

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because it contains the same number

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of protons in the nucleus

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as it does electrons around the nucleus.

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And not all of those electrons are available for bonding.

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Only the outermost electrons,

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the ones in orbitals furthest from the nucleus,

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the ones with the most energy,

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only those participate in bonding.

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By the way, this applies to ionic bonding too.

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Remember sodium chloride?

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Well, the electron that sodium loses

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is the one furthest from its nucleus,

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and the orbital that electron occupies

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when it goes over to chlorine

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is also the one furthest from its nucleus.

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But back to covalent bonding.

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Carbon has four electrons

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that are free to bond,

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nitrogen has three,

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oxygen two.

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So, carbon is likely to form four bonds,

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nitrogen three,

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and oxygen two.

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Hydrogen only has one electron,

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so it can only form one bond.

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In some special cases,

0:02:44.890,0:02:46.432
atoms can form more bonds

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than you'd expect,

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but they better have a really good reason to do so,

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or things tend to fly apart.

0:02:52.077,0:02:53.099
Groups of atoms

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that share electrons covalently with each other

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are called molecules.

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They can be small.

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For example, every molecule of oxygen gas

0:03:00.612,0:03:03.087
is made up of just two oxygen atoms

0:03:03.087,0:03:04.523
bonded to each other.

0:03:04.523,0:03:06.192
Or they could be really, really big.

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Human chromosome 13 is just two molecules,

0:03:09.379,0:03:13.022
but each one has over 37 billion atoms.

0:03:13.022,0:03:14.210
And this neighborhood,

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this city of atoms,

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is held together by the humble chemical bond.