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Chemistry
Unit 2 Atomic Structures
INSTRUCTOR: Mrs. Richards
October 16th, 2025
Unit 2: Atomic Structure
Section 3.1 — Defining the Atom
Democritus’s Ideas
● Democritus (460–370 B.C.) , a Greek philosopher, was one of the first to suggest atoms exist. ● The word atom comes from the Greek word “atomos,” meaning indivisible. ● He believed: ○ Atoms are indivisible and indestructible. ○ Matter is made of tiny particles called atoms. ● His theory agreed with later science in some ways but: ○ Didn’t explain chemical behavior. ○ Wasn’t based on experiments —only philosophy and reasoning.
Dalton’s Atomic Theory (1800s, Experiment-Based)
● John Dalton (1766–1844) developed the first scientific theory about atoms.
● His key postulates:
- All elements are made of tiny indivisible particles called atoms.
- Atoms of the same element are identical. ■ Atoms of different elements are different.
- Atoms of different elements combine in simple whole-number ratios to form compounds.
- Chemical reactions involve rearranging atoms — they are combined, separated, or rearranged — but never changed into atoms of another element.
Sizing Up the Atom
● Atoms are extremely small but still have the properties of the element. ● Example: ○ 100,000,000 copper atoms lined up = 1 cm long. ● Even though they’re tiny, individual atoms can be observed with: ○ Scanning Tunneling Microscopes (STM) or Electron Microscopes.
Section 3.2 — Structure of the Nuclear Atom
Discovery of Subatomic Particles
● Dalton thought atoms were indivisible, but we now know they contain: ○ Electrons (e ⁻ ) ○ Protons (p⁺) ○ Neutrons (n⁰)
○ Mass ≈ 1.67 × 10 ⁻ ²⁴ g , nearly equal to a proton.
Subatomic Particle Summary
Particl e Symbol Charge Mass (g) Locati on
Electron e⁻ -1 9.11 × 10 ⁻²⁸
Electro n cloud
Proton p⁺ +1 1.67 × 10 ⁻²⁴
Nucleu s
Neutron n⁰ 0 1.67 × 10 ⁻²⁴
Nucleu s
Thomson’s “Plum Pudding” Model
● Thomson pictured the atom as: ○ Electrons (negative “plums”) scattered within a positively charged “pudding.” ● Later proven incorrect.
Rutherford’s Gold Foil Experiment (1911)
● Ernest Rutherford shot alpha particles (helium nuclei) at thin gold foil. ● Expected most to pass through evenly, but: ○ Most passed straight through. ○ Some deflected slightly. ○ A few deflected greatly (bounced back).
Conclusions:
- The atom is mostly empty space.
- All positive charge and most of the mass are concentrated in a small, dense center called the nucleus.
- The nucleus contains protons and neutrons.
- Electrons orbit around the nucleus.
→ Model known as the Nuclear Model.
Section 3.3 — Distinguishing Among Atoms
Atomic Number (Z)
● Each element has a unique number of protons. ● Atomic Number = # of protons = # of electrons (in a neutral atom).
Element # of Protons Atomic # (Z)
Carbon 6 6
Phosphor us
Hydrogen- (Tritium)
³H 1 1 2
Naming Isotopes
● Written as: Element–Mass Number ○ Examples: carbon-12, carbon-14, uranium-
Atomic Mass
● Actual atomic masses vary due to isotope mixtures. ● Atomic mass = weighted average of all isotopes.
Measured in atomic mass units (amu)
● 1 amu = 1/12 the mass of carbon-12 atom.
Formula:
(mass of isotope × % abundance) + (mass of isotope × % abundance) + …
Example – Carbon:
Isotope Symbol Protons Neutrons % in Nature
Carbon- 12
¹²C 6 6 98.89%
Carbon- 13
¹³C 6 7 1.11%
Carbon- 14
¹⁴C 6 8 <0.01%
Average atomic mass = 12.011 amu
The Periodic Table — A Preview
● Periodic Table: organizes elements by repeating (periodic) properties. ● Helps compare chemical and physical properties.
Organization
● Rows = Periods (7 total) ● Columns = Groups/Families ○ Elements in the same group have similar properties. ○ Groups are labeled with numbers and A or B.
Key Takeaways
● Atoms are made of protons, neutrons, and electrons. ● Protons define the element (atomic number). ● Neutrons change the isotope (mass number). ● Electrons determine chemical behavior. ● Rutherford’s model introduced the nucleus. ● Atomic mass depends on the isotope’s abundance.
