Navigation
Learn why atoms want to bond and how valence electrons determine an atom's reactivity.
Why does pure Sodium explode when it touches water, while the Neon in a glowing sign stays perfectly calm? The secret lies in an atom's 'social' life—how many electrons it has in its outermost shell.
In chemistry, the only electrons that really matter for bonding are the ones on the very outside. These are called valence electrons. Think of them as the 'hands' an atom uses to grab onto others. For most elements, you can find this number just by looking at their Group Number (the vertical columns) on the periodic table. For Groups 1 and 2, the number is simply 1 or 2. For Groups 13 through 18, just subtract 10 from the group number. For example, Group 15 has valence electrons. These electrons determine how an atom will react with the world around it.
Let's find the valence electrons for Oxygen and Aluminum. 1. Locate Oxygen: It is in Group 16. Subtract 10: . Oxygen has 6 valence electrons. 2. Locate Aluminum: It is in Group 13. Subtract 10: . Aluminum has 3 valence electrons.
Quick Check
How many valence electrons does an atom in Group 17 (the Halogens) have?
Answer
7
Why do atoms bond at all? They are searching for stability. Most atoms are 'unhappy' unless they have a full outer shell, which for almost every element means having 8 valence electrons. This is known as the Octet Rule (think of an octopus with 8 legs). Noble Gases in Group 18 already have 8 electrons, which is why they are 'noble' and rarely react with anyone else. Other atoms will steal, share, or give away electrons just to reach that magic number of 8.
Sodium () is in Group 1, so it has 1 valence electron. 1. To reach a full octet, it could either gain 7 electrons or lose its 1 outer electron. 2. Losing 1 is much easier! 3. When loses that electron, the shell underneath (which is already full) becomes the new outer shell. This makes very reactive because it is desperate to get rid of that one extra electron.
Quick Check
According to the Octet Rule, why is Neon (Group 18) considered non-reactive?
Answer
It already has a full outer shell of 8 valence electrons, so it is already stable.
Atoms take the path of least resistance. Metals (left side of the table) usually have 1, 2, or 3 valence electrons. It is easier for them to lose those few electrons than to find 5 or 6 more. When they lose electrons, they become positive ions. Non-metals (right side) usually have 5, 6, or 7. It is easier for them to gain electrons to reach 8. If an atom has exactly 4 (like Carbon), it often chooses to share instead of giving or taking.
Nitrogen () is in Group 15. 1. It has valence electrons. 2. To reach 8, it needs 3 more (). 3. Because gaining 3 is easier than losing 5, Nitrogen will typically gain 3 electrons during a chemical reaction, forming an ion with a charge of .
An atom is in Group 14. How many valence electrons does it have?
Which of these atoms is most likely to LOSE 2 electrons to become stable?
Atoms with 7 valence electrons are highly reactive because they only need to gain 1 more electron to reach an octet.
Review Tomorrow
In 24 hours, try to list the number of valence electrons for Groups 1, 2, 13, 14, 15, 16, 17, and 18 from memory.
Practice Activity
Find a periodic table and pick five random elements from the main groups. For each, decide if they would rather 'give' or 'take' electrons to reach 8.