Electron Arrangement and Valence Electrons

Elements located more towards the left on the periodic table are considered metals.
Elements located more towards the right on the periodic table are considered nonmetals.

Protons: Carry a +1 charge and reside in the nucleus.
Neutrons: Carry no charge (neutral) and reside in the nucleus.
Electrons: Carry a -1 charge and are found in an "electron cloud" surrounding the nucleus.
- Abbreviated as e^{-}.
For a neutral atom, the number of protons equals the number of electrons.
- Example: Carbon-12
  - Protons: 6
  - Neutrons: 6
  - Electrons: 6

Significance: The physical and chemical properties of an element are largely determined by the arrangement of electrons in the atom.
Electron Energy: Electrons in an atom can only occupy specific, allowed energy levels.
- These are referred to as "energy levels" or "electron shells."
Ground State vs. Excited State Electron Configuration:
- Ground State: The lowest and most stable electron configuration.
- Excited State: When electrons temporarily jump to higher energy levels (e.g., by absorbing light/laser energy).
  - Fluorescence Example: Light or a laser can excite ground state electrons to higher energy levels. When these excited electrons relax back down to the ground state, they emit light, leading to fluorescence.

Principal Energy Level (n): Represented by n = 1, 2, 3, 4, … (with an infinite number of possible levels).
Periodic Table Connection: The row number on the periodic table generally indicates the highest occupied principal energy level for an element in its ground state.
- Energy Level 1 (First Row): Contains 2 elements (Hydrogen, Helium), accommodating 2 electrons.
- Energy Level 2 (Second Row): Contains 8 elements, accommodating 8 electrons.
- Energy Level 3 (Third Row): Contains 8 main group elements.
  - Important Note for Transition Metals: When you reach the transition metals (d-block elements in rows 4 and higher), they actually drop down an energy level. For example, the d-block elements in the 4^{th} row correspond to the 3^{rd} energy level for their d-electrons.
  - Inner Transition Metals: The f-block elements (lanthanides and actinides) drop down two energy levels (e.g., $6^{th} row f-block metals are actually 4^{th} energy level electrons).
- Class Simplification: For this course, electron arrangement typically focuses on elements up to element number 20 to simplify the understanding of transition metal rules.

Valence Electrons:
- These are the outermost electrons of an atom.
- They are in the highest energy level and are farthest from the nucleus.
- They are the primary determinants of the physical and chemical properties of an element.
- Elements in the same column (group) of the periodic table have the same number of valence electrons, which explains their similar chemical properties.
Core Electrons:
- These are all the electrons that are not valence electrons.
- They are the innermost electrons, in the lowest energy levels, and closest to the nucleus.

Carbon (C, Atomic Number = 6):
- Total electrons: 6
- Energy Level 1: 2 electrons
- Energy Level 2: 4 electrons
- Valence Electrons: 4 (from energy level 2)
- Core Electrons: 2 (from energy level 1)
Silicon (Si, Atomic Number = 14):
- Total electrons: 14
- Energy Level 1: 2 electrons
- Energy Level 2: 8 electrons
- Energy Level 3: 4 electrons
- Energy Level 4: 0 electrons (not needed for Silicon)
- Valence Electrons: 4 (from energy level 3)
- Core Electrons: 2 + 8 = 10 (from energy levels 1 and 2)