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2.4 Chemistry Fundamentals: Atomic Structure, Elements & Chemical Reactions
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1 Chapter 1: Guide Scope, AFQT & Service-Specific Policy
1.1 Editorial Scope and Service-Specific AFCT Policy 1.2 AFQT Meaning and Study Priorities 1.3 Service Composites and Targeted Skill Planning 1.4 Format-Agnostic Test Preparation and Pacing
2 Chapter 2: General Science (GS)
2.1 Life Science: Cell Biology, Human Anatomy & Genetics 2.2 Physical Science: Matter, Energy, Forces & Motion 2.3 Earth & Space Science: Geology, Meteorology & Astronomy 2.4 Chemistry Fundamentals: Atomic Structure, Elements & Chemical Reactions
3 Chapter 3: Arithmetic Reasoning (AR)
3.1 Word Problems: Rates, Ratios, Proportions & Percentages 3.2 Financial & Distance Math: Interest, Profit/Loss, Speed & Time 3.3 Work & Mixture Problems: Joint Rates & Percentage Concentrations 3.4 Multi-Step Problem Solving & Algebraic Word Problems
4 Chapter 4: Word Knowledge (WK)
4.1 General Academic & Technical Vocabulary 4.2 Context Clues, Synonyms & Antonyms 4.3 Word Roots, Prefixes & Suffixes (Etymology) 4.4 Advanced Technical & Operational Terminology
5 Chapter 5: Paragraph Comprehension (PC)
5.1 Identifying Main Ideas, Central Themes & Topic Sentences 5.2 Extracting Supporting Details & Factual Information 5.3 Drawing Logical Inferences & Author's Purpose 5.4 Analyzing Text Structure & Drawing Conclusions
6 Chapter 6: Mathematics Knowledge (MK)
6.1 Pre-Algebra & Number Operations: Exponents, Radicals & Order of Operations 6.2 High School Algebra: Linear Equations, Inequalities & Polynomials 6.3 Geometry Fundamentals: Angles, Triangles, Quadrilaterals & Circles 6.4 Coordinate Geometry, Perimeter, Area & Volume Formulas
7 Chapter 7: Electronics Information (EI)
7.1 Electrical Principles: Voltage, Current, Resistance & Ohm's Law (V=IR, P=VI) 7.2 Electric Circuits: Series, Parallel & Combination Circuit Analysis 7.3 Electrical Components: Resistors, Capacitors, Inductors & Transformers 7.4 Electronic Semiconductors, Transistors, Diodes & Logic Gates
8 Chapter 8: Auto & Shop Information (AS)
8.1 Automotive Systems: Internal Combustion Engines, Fuel & Ignition Systems 8.2 Automotive Mechanical Systems: Transmission, Suspension, Brakes & Cooling 8.3 Hand & Power Tools: Identification, Uses & Fasteners 8.4 Shop Practice, Woodworking, Metalworking & Safety Rules
9 Chapter 9: Mechanical Comprehension (MC)
9.1 Simple Machines: Levers, Pulleys, Inclined Planes & Mechanical Advantage 9.2 Force, Motion, Work & Power: Newton's Laws & Energy Transformations 9.3 Fluid Dynamics & Pressure: Pascal's Principle, Hydraulics & Pneumatics 9.4 Mechanical Components: Gears, Belts, Springs & Friction
10 Chapter 10: Spatial Assembly Practice (AO Skill)
10.1 Spatial Connections and Assembly Patterns
2.4 Chemistry Fundamentals: Atomic Structure, Elements & Chemical Reactions
Key Takeaways Explain This Topic AI
Atoms consist of a dense central nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons in energy shells.
The periodic table organizes elements by atomic number ( Z Z Z , proton count); elements within the same vertical group share valence electron configurations and chemical reactivity patterns.
Chemical bonds form when atoms achieve stable valence electron octets: ionic bonds involve electron transfer between metals and nonmetals, while covalent bonds involve electron sharing between nonmetals.
Chemical reactions follow the Law of Conservation of Mass; balanced chemical equations maintain equal numbers of each element's atoms across reactants and products.
The pH scale quantifies acidity/alkalinity from 0 to 14: solutions with pH < 7 are acidic, pH = 7 is neutral, and pH > 7 are basic (alkaline).
Last updated: July 2026
Chemistry Fundamentals: Atomic Structure, Elements & Chemical Reactions
Chemistry is the central science investigating matter, its properties, how substances combine or separate to form other substances, and how substances interact with energy.
1. Atomic Structure & Subatomic Particles
All matter is composed of atoms . An atom is the smallest unit of an element that retains the chemical identity of that element.
Subatomic Particle Properties
- Particle Symbol Charge Mass (amu) Location in Atom
- Proton p + p^+ p + + 1 +1 + 1 ≈ 1.0 amu \approx 1.0\text{ amu} ≈ 1.0 amu Nucleus
- Neutron n 0 n^0 n 0 0 0 0 (Neutral) ≈ 1.0 amu \approx 1.0\text{ amu} ≈ 1.0 amu Nucleus
Electron e − e^- e − − 1 -1 − 1 ≈ 0.0005 amu \approx 0.0005\text{ amu} ≈ 0.0005 amu (Negligible) Electron Shells / Orbitals
Fundamental Atomic Metrics
Atomic Number ( Z Z Z ) : The number of protons in the nucleus. Defines the identity of an element (e.g., Carbon always has Z = 6 Z = 6 Z = 6 ). In a neutral atom, Protons = Electrons.
Mass Number ( A A A ) : The total count of protons + neutrons in the nucleus: A = Z + N A = Z + N A = Z + N .
Isotopes : Atoms of the same element (same proton count Z Z Z ) that contain different numbers of neutrons ( N N N ), resulting in different mass numbers (e.g., Carbon-12 with 6 neutrons vs. Carbon-14 with 8 neutrons).
Ions : Charged atoms formed when an atom gains or loses electrons.
Cation : Positively charged ion formed by losing electrons (e.g., N a + Na^+ N a + ).
Anion : Negatively charged ion formed by gaining electrons (e.g., C l − Cl^- C l − ).
2. The Periodic Table of Elements
The periodic table arranges chemical elements by increasing atomic number ( Z Z Z ) in horizontal rows ( Periods ) and vertical columns ( Groups or Families).
Major Element Groups
Group 1 (Alkali Metals) : Highly reactive metals with 1 valence electron (e.g., L i , N a , K Li, Na, K L i , N a , K ). React violently with water.
Group 2 (Alkaline Earth Metals) : Reactive metals with 2 valence electrons (e.g., M g , C a Mg, Ca M g , C a ).
Groups 3–12 (Transition Metals) : Hard, dense metals with variable valence states (e.g., F e , C u , A u , A g Fe, Cu, Au, Ag F e , C u , A u , A g ).
Group 17 (Halogens) : Highly reactive nonmetals with 7 valence electrons (e.g., F , C l , B r , I F, Cl, Br, I F , Cl , B r , I ). Form salts when combined with metals.
Group 18 (Noble Gases) : Inert, unreactive gases with a full outer electron shell (8 valence electrons, octet rule; e.g., H e , N e , A r He, Ne, Ar He , N e , A r ).
Key Periodic Trends
Atomic Radius : Increases moving down a group (adding electron shells); decreases moving left-to-right across a period.
Electronegativity : An atom's ability to attract shared electrons. Increases moving left-to-right across a period and up a group (Fluorine is the most electronegative element).
Ionization Energy : Energy required to remove an electron from an atom. Increases moving left-to-right across a period and up a group.
3. Chemical Bonding & Molecular Structure
Atoms form chemical bonds to achieve lower potential energy and stable outer electron configurations (typically 8 valence electrons, fulfilling the Octet Rule ).
Bond Type Mechanism Characteristics Example
Ionic Bond Complete transfer of electrons from a metal to a nonmetal High melting points, soluble in water, conducts electricity when dissolved N a C l NaCl N a Cl (Table Salt)
Covalent Bond Sharing of electron pairs between two nonmetal atoms Lower melting points, non-conductive, forms discrete molecules H 2 O , C O 2 , C H 4 H_2O, CO_2, CH_4 H 2 O , C O 2 , C H 4
Metallic Bond Sea of delocalized electrons shared among positive metal cations Malleable, ductile, excellent conductors of heat and electricity C u , F e , A l Cu, Fe, Al C u , F e , A l
Nonpolar Covalent Bond : Equal sharing of electrons between atoms with similar electronegativity (e.g., O 2 , H 2 O_2, H_2 O 2 , H 2 ).
Polar Covalent Bond : Unequal sharing of electrons due to electronegativity differences, creating partial positive ( δ + \delta+ δ + ) and negative ( δ − \delta- δ − ) poles (e.g., H 2 O H_2O H 2 O ).
4. Chemical Reactions & Balancing Equations
A chemical reaction rearranges atoms to transform reactants into products, breaking and forming chemical bonds.
Law of Conservation of Mass : Mass is neither created nor destroyed during a chemical reaction. The total mass of reactants must equal the total mass of products, requiring chemical equations to be balanced.
Five Primary Types of Chemical Reactions
Synthesis (Combination) : Two or more reactants combine to form a single product. A + B → A B ( e . g . , 2 H 2 + O 2 → 2 H 2 O ) A + B \rightarrow AB \quad (e.g., 2H_2 + O_2 \rightarrow 2H_2O) A + B → A B ( e . g . , 2 H 2 + O 2 → 2 H 2 O )
Decomposition : A single compound breaks down into two or more simpler substances. A B → A + B ( e . g . , 2 H 2 O → 2 H 2 + O 2 ) AB \rightarrow A + B \quad (e.g., 2H_2O \rightarrow 2H_2 + O_2) A B → A + B ( e . g . , 2 H 2 O → 2 H 2 + O 2 )
Single Replacement (Displacement) : One uncombined element replaces another element in a compound. A + B C → A C + B ( e . g . , Z n + 2 H C l → Z n C l 2 + H 2 ) A + BC \rightarrow AC + B \quad (e.g., Zn + 2HCl \rightarrow ZnCl_2 + H_2) A + BC → A C + B ( e . g . , Z n + 2 H Cl → Z n C l 2 + H 2 )
Double Replacement : Ions from two reacting compounds swap partners to form two new compounds. A B + C D → A D + C B ( e . g . , A g N O 3 + N a C l → A g C l ↓ + N a N O 3 ) AB + CD \rightarrow AD + CB \quad (e.g., AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3) A B + C D → A D + CB ( e . g . , A g N O 3 + N a Cl → A g Cl ↓ + N a N O 3 )
Combustion : A hydrocarbon reacts rapidly with oxygen gas ( O 2 O_2 O 2 ) to release heat, carbon dioxide, and water. Hydrocarbon + O 2 → C O 2 + H 2 O + Heat \text{Hydrocarbon} + O_2 \rightarrow CO_2 + H_2O + \text{Heat} Hydrocarbon + O 2 → C O 2 + H 2 O + Heat
- 5. Acids, Bases, and the pH Scale
- Acid-Base Definitions
Arrhenius Definition : Acids produce hydrogen ions ( H + H^+ H + ) in aqueous solution; Bases produce hydroxide ions ( O H − OH^- O H − ).
Brønsted-Lowry Definition : Acids are proton ( H + H^+ H + ) donors ; Bases are proton ( H + H^+ H + ) acceptors .
The pH Scale
The pH scale measures the logarithmic concentration of hydrogen ions in a solution: pH = − log [ H + ] \text{pH} = -\log[H^+] pH = − lo g [ H + ] . ACIDIC ( pH 0 – 6.9 ) ⟵ NEUTRAL ( pH 7.0 ) ⟶ BASIC / ALKALINE ( pH 7.1 – 14 ) \text{ACIDIC } (\text{pH } 0\text{--}6.9) \longleftarrow \mathbf{\text{NEUTRAL } (\text{pH } 7.0)} \longrightarrow \text{BASIC / ALKALINE } (\text{pH } 7.1\text{--}14) ACIDIC ( pH 0 – 6.9 ) ⟵ NEUTRAL ( pH 7.0 ) ⟶ BASIC / ALKALINE ( pH 7.1 – 14 )
Category pH Range Characteristics Examples
Strong Acid 0 to 3 0 \text{ to } 3 0 to 3 Tastes sour, turns blue litmus paper red, reacts with metals Stomach acid ( H C l HCl H Cl ), Battery acid ( H 2 S O 4 H_2SO_4 H 2 S O 4 )
Weak Acid 4 to 6 4 \text{ to } 6 4 to 6 Mildly acidic Vinegar (acetic acid), Citrus juice (citric acid)
Neutral 7.0 7.0 7.0 Pure water at 25 ∘ C 25^\circ\text{C} 2 5 ∘ C ( [ H + ] = [ O H − ] [H^+] = [OH^-] [ H + ] = [ O H − ] ) Distilled water, blood plasma ( ∼ 7.4 \sim 7.4 ∼ 7.4 )
Weak Base 8 to 10 8 \text{ to } 10 8 to 10 Slippery feel, turns red litmus paper blue Baking soda solution ( N a H C O 3 NaHCO_3 N a H C O 3 )
Strong Base 11 to 14 11 \text{ to } 14 11 to 14 Tastes bitter, highly caustic/corrosive Household bleach, Lye ( N a O H NaOH N a O H ), Drain cleaner
Neutralization Reaction : Combining an acid and a base yields a salt and water: Acid + Base → Salt + Water \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} Acid + Base → Salt + Water (e.g., H C l + N a O H → N a C l + H 2 O HCl + NaOH \rightarrow NaCl + H_2O H Cl + N a O H → N a Cl + H 2 O ).
6. Solutions & Solubility
Solution : A homogeneous mixture of two or more substances.
Solute : The substance being dissolved (e.g., salt, sugar).
Solvent : The dissolving medium (e.g., water, known as the "universal solvent").
Molarity ( M M M ) : A measure of solution concentration expressed as moles of solute per liter of solution ( M = moles of solute liters of solution M = \frac{\text{moles of solute}}{\text{liters of solution}} M = liters of solution moles of solute ).
Factors Increasing Dissolution Rate : Increasing temperature (for solid solutes), stirring/agitation, and increasing solute surface area (crushing).