Here's why:
* Valence electrons are the electrons in the outermost shell of an atom. These are the electrons involved in chemical bonding.
* Periods on the periodic table represent the number of electron shells an atom has.
* Groups (columns) represent the number of valence electrons an atom has.
As you move from left to right across a period:
1. Protons in the nucleus increase: This increases the positive charge of the atom.
2. Electrons are added to the same shell: The added electrons are attracted to the increased positive charge of the nucleus.
3. The number of valence electrons increases: This is because the new electrons are added to the outermost shell, making them valence electrons.
For example, let's look at the second period:
* Lithium (Li): 1 valence electron
* Beryllium (Be): 2 valence electrons
* Boron (B): 3 valence electrons
* Carbon (C): 4 valence electrons
* Nitrogen (N): 5 valence electrons
* Oxygen (O): 6 valence electrons
* Fluorine (F): 7 valence electrons
* Neon (Ne): 8 valence electrons (a full outer shell)
Therefore, the number of valence electrons increases as you move from left to right across the periodic table, reflecting the increasing positive charge of the nucleus and the addition of electrons to the outermost shell.
