How Atoms Bond: Ionic Bonds

Air Date: 01/10/2012
Source:
NBC Learn
Creator:
Beth Nissen
Air/Publish Date:
01/10/2012
Event Date:
01/10/2012
Resource Type:
Science Explainer
Copyright:
NBCUniversal Media, LLC.
Copyright Date:
-
Clip Length:
00:06:28

This NBC Learn video uses common table salt to explain and illustrate what happens between the electrons and protons in atoms of the element sodium and atoms and the element chlorine to make crystals of sodium chloride.

How Atoms Bond: Ionic Bonds


BETH NISSEN, reporting:


Over the course of time, a few students may have asked, “Why does chemistry matter?”


And the answer, of course, is matter: all matter in the universe.


Anything that can be physically touched, anything living, is made up of atoms – which join together to form molecules, which join together into compounds, until you’ve got, say, a spoon, or a tree, or a hedgehog.


How does this happen – how do atoms join together, or bond, to form molecules?


Let’s take a close look – really close: atoms aren’t even big enough to be microscopic.


As eensy as atoms are, they’re made up of even smaller particles: neutrons, protons, and electrons. A diagram of an atom looks like this: “shells” surrounding a center, or nucleus. Inside the nucleus are all of that atom’s protons (and neutrons). Moving all around the shells are that atom’s electrons.


Let’s focus on protons first. The protons in the atom’s nucleus stay there – an atom of a particular element will always have, and keep, the same number of protons.


That number of protons is so permanent (memory trick: ‘proton’ and ‘permanent’ both start with ‘p’) that it’s part of the information in the Periodic Table: that number, usually at the top of each square, is the atomic number – the number of protons in the nucleus of an atom of this element.


Look at sodium (abbreviated Na). A sodium atom has 11 protons in its nucleus. The atomic number also tells you how many electrons there are: individual atoms of every element have the same number of protons and electrons. So a sodium atom has 11 protons, 11 electrons.


Pop Quiz #1: How many electrons are there in an atom of chlorine (abbreviated Cl)?


Answer: 17 – same as the number of protons.


While the number of protons and electrons in each atom is the same, protons and electrons are very different…with different ‘electric’ charges. Protons have a positive ‘electric’ charge. (Add to the earlier memory trick: ‘p’ for ‘proton,’ ‘permanent’ and ‘positive’ and ‘plus.’).


Electrons (which, by the way, are much smaller than protons) are the opposite: they have a negative charge.


And atoms themselves? Heads up – this next part contains information that might prove useful in a later quiz: Because atoms have equal numbers of positive protons and negative electrons, the electric charges cancel each other out. Net charge: zero.


OK, now back to electrons. While the positive protons are permanently in the atom’s nucleus, the negative electrons in an atom are not: electrons move around in the shells. And -- this is key – unlike protons, the number of electrons in an atom can change…and do, when and if atoms bond.


To explain this step by step, let’s use a simple everyday example: how atoms of the element Na, sodium…and atoms of Cl, chlorine…form a bond – an ionic bond – to make NaCl, the abbreviation for sodium chloride, more commonly known as salt.

Atoms bond by ‘swapping’ or transferring electrons from their outermost shells, called valence shells. It’s a little like an atomic version of the card game Go Fish: each one wants to let go of or pick up enough electrons so it ends up with a particular number of electrons in its outermost shell – very often a total of 8.
This is so common, it’s called the Octet Rule, or Rule of 8.


Look how many electrons Chlorine has in its outer shell: 7. Sodium? One. Sodium gives up one electron, which chlorine picks up. That gives chlorine the ‘magic 8’ electrons in its valence shell.


Giving up that one lone electron empties sodium’s outermost shell – think of it as a “Go Fish” player with no cards to trade, who has to drop out. The 2nd shell now becomes the outermost ‘valence’ shell. It still has electrons to ‘trade’ – actually, 8 of them. So sodium, too, now has its ‘magic 8.’


Pop Quiz #2 – NOW how many electrons are in each sodium atom? (To remind you, this is sodium’s atomic number.)


Answer: 10 – because it gave one of its 11 electrons to chlorine.


The positive protons in the atom now ‘outnumber’ the negative electrons, 11 to 10. So sodium is no longer neutral – it now has a positive charge, indicated with a ‘plus’ sign, and is a sodium ion (that’s the term for an atom with an unequal number of protons and electrons).


Pop Quiz #3 – How many electrons are in chlorine now?


Answer: 18 – which means the chlorine atom is now an ion, too: 18 negative electrons outnumber 17 positive protons, which gives chlorine a negative charge, and changes its abbreviation to Cl-minus. (And also changes ‘chlorine’ to ‘chloride.’)


You probably know: opposites attract. Positive sodium and negative chloride ions are strongly attracted to each other – and that attraction is what bonds Na+ and Cl- ions together form NaCl: sodium chloride.


Most chemical bonds are ionic, like the ones that forms salt, or covalent – that’s where atoms don’t give up and pick up electrons, but share them to fill their outer shells…H2O is a good example of that.


There’s much more to know about all this – but now you have at least an “elementary” understanding of how atoms bond.


(And the smart answer to the question: what’s the matter?)

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