What are Ions?
Ions are atoms that have either lost (given away) or gained (accepted) electrons.
- Atoms that give away electrons become POSITIVELY charged. We call them "cations."
The positive charge results because now you have more protons (+) than electrons (-).
- Atoms that gain electrons become NEGATIVELY charged. We call them "anions."
The negative charge results because now you have more electrons (-) than protons (+).
Why do we have ions?
Every element dreams of looking like a Noble Gas. Ah, stability! It's the atomic equivalent of a restful vacation on a tropical island. To "look like" a Noble Gas, atoms need to have full outer orbitals (or as full as they can get). When we get to electron configuration you'll see just how that process happens, but for now just understand this:
- If an atom has a few valence electrons in an outer oribital it costs less energy to give them away than to attract more to become stable. These are your CATIONS and are typically formed by metals.
- If an atom needs just a few valence electrons to fill an outer oribital it costs less energy to grab them to become stable. These are your ANIONS and are typically formed by non-metals.
Cool trick: The number of valence electrons for atoms/elements in Groups 1A-8A match the Group A number! Group 1A atoms all have 1 valence electron; Group 2A atoms all have 2 valence electrons; Group 8A atoms all have 8 valence electrons.
How can you tell if an atom will give away or grab electrons?
We'll study the Periodic Table in more detail soon, but for now think about it this way. Whether you give away or grab electrons you need to spend energy, and the "magic number" needed to fill your s- and p- orbitals is 8 (focus on the number 8 not the s-, p- thing).
- If you have 1, 2, or even 3 valence electrons it's easier to give them away because otherwise to be stable you'd need to grab 5 or more electrons! Group 1A, 2A and most 3A elements are givers.
- If you have 5, 6, or 7 valence electrons it's easier to grab a few more because otherwise to be stable you'd need to find new homes for up to 7! Group 5A, 6A and 7A elements are grabbers.
- And if you have 4 valence electrons? You'll probably share and not form ions.
Take a look at the picture in the left margin (you'll see the full version in one of the websites below). Why are anions (-) larger than cations (+)?
Think about it - a cation gives away valence electrons so it doesn't need to support a partially empty orbital. Once the electrons are gone - POOF - bye-bye orbital. The cation is SMALLER than the atom. On the flip side an anion grabs as many electrons as it need to fill that partially empty outer orbital. The orbital remains, except now it's happily full.
BOTH ions end up looking like a Noble Gas (ahhhhhh....stability!) but they do it in opposite ways. When sodium (Na) gives away its valence electron it looks like Neon, the Noble Gas in the previous row on the Periodic Table. If chlorine grabs that extra electron it looks like Argon, the Noble Gas in the same row on the Periodic Table.
These resources will help aquaint you with ions:
How do you write the symbol for ions?
To write the symbol for an isotope you just add to the the basic symbol for an isotope, namely the charge..
Notice the 2+ in the upper right corner of the ion symbol? That is theion's CHARGE. The symbol tells you 1) how many eletrons were transferred and 2) whether they were lost or gained.
The "2" means two electrons were transferred. Because the charge is POSITIVE (+) you know electrons were lost. If the charge had been 2- then two electrons would have been gained.
How much do you need to know about ions?
- Know the definition of an ion.
- Be able to predict which elements will give away and which elements will grab electrons based on their location in the Periodic Table.
- Know how to write and identify names and symbols of ions.
- Know that an ion's charge is determined by the number of ELECTRONS transferred.