Atomic Structure

Everything around you -- water, air, your phone -- is made of atoms. Atoms are the building blocks of all matter. Understanding their structure is the foundation of chemistry.

Subatomic Particles

| Particle | Charge | Mass | Location | |----------|--------|------|----------| | Proton | Positive (+1) | 1 amu | Nucleus | | Neutron | Neutral (0) | 1 amu | Nucleus | | Electron | Negative (-1) | Nearly zero | Shells around nucleus |

Atomic Number and Mass Number

Atomic Number (Z) = Number of protons (defines the element)

Mass Number (A) = Protons + Neutrons. So Neutrons = A - Z.

Example: Carbon has Z = 6, A = 12. Protons = 6, Neutrons = 6, Electrons = 6.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

Example: Carbon-12 (6p, 6n), Carbon-13 (6p, 7n), Carbon-14 (6p, 8n -- radioactive, used for dating).

Isotopes have the same chemical properties but different physical properties.

Electron Configuration

Electrons fill shells in order:

• 1st shell (K): max 2 electrons
• 2nd shell (L): max 8 electrons
• 3rd shell (M): max 8 electrons (for lighter elements)

Examples:

• Hydrogen (Z=1): 1
• Carbon (Z=6): 2, 4
• Sodium (Z=11): 2, 8, 1
• Chlorine (Z=17): 2, 8, 7
• Calcium (Z=20): 2, 8, 8, 2

The outermost shell electrons are valence electrons -- they determine bonding behavior.

Worked Example: Aluminum (Z=13, A=27): Protons=13, Neutrons=14, Electrons=13. Configuration: 2, 8, 3. Valence electrons: 3.

Did You Know? Scientists around the world continue to discover new elements. The periodic table now has 118 confirmed elements, with element 117 (Tennessine) being one of the most recent additions.

Key Takeaways

• Atoms have protons and neutrons in the nucleus, electrons in shells
• Atomic number = protons; mass number = protons + neutrons
• Isotopes: same atomic number, different mass numbers
• Electrons fill shells: 2, 8, 8 for lighter elements