Sci8 Q2W7D3: Electron Configuration & Valence — Reading the Outermost Shell

Day 3: Electron Configuration & Valence — Reading the Outermost Shell

Today you’ll turn the periodic table into a reading tool for electron configurations and valence electrons. You’ll practice Grade-8 patterns (2–8–8 for early periods), connect “address” to period and group, and write compact configuration strings for common elements. You will also explain how outer electrons predict bonding behavior and quick formulas. By the end, you will produce correct configurations for Periods 1–3, identify valence counts fast, and justify a simple compound using clear charge balance—ready for quizzes and short exams that favor concise, evidence-based answers.

• Subject: Science 8
• Grade: 8
• Day: 3 of 4

By the end of the lesson, you will be able to:

1. Write Grade-8 electron configurations for Periods 1–3 using a clear pattern in under 60 seconds per atom.
2. Identify the valence level and count from a configuration and link them to period and group behavior.
3. Use valence to predict a likely ion and a correct binary formula in 2 lines of working.

• Electron Configuration — a compact way to show how electrons occupy levels/orbitals.
• Period — row number; often equals the main valence level for main-group elements.
• Group (Family) — column; similar outer electron patterns and reactivity.
• Valence Electrons — outer level electrons responsible for bonding.
• Octet Idea — many atoms are most stable with 8 valence electrons (basic model).
• Ion — atom with net charge after losing ($+$) or gaining ($-$) electrons.

Answer first, then open to check.

1. What equals the number of electrons in a neutral atom?
Show Answer
Atomic number $Z$ (also the proton count).
2. Which table feature tells you the main valence level for many main-group elements?
Show Answer
The period number.
3. What do we call electrons that take part in bonding?
Show Answer
Valence electrons.

How to use this section: Work through each checkpoint. Each one gives a mini-goal, guided discussion, real-life tie-in, a mini-summary, and three guiding questions with hidden answers.

Checkpoint 1 — From Table to Configuration: The 2–8–8 Pattern

Mini-goal: Convert atomic number into a Grade-8 configuration string for Periods 1–3.

Guided discussion: For early periods, a quick distribution rule works well: fill $n=1$ with up to 2 electrons, then $n=2$ with up to 8, then $n=3$ with up to 8 for main-group counting. Start from $Z$ and place electrons inside-out. Example: Sodium, $Z=11$, becomes 2–8–1. Chlorine, $Z=17$, becomes 2–8–7. Neon, $Z=10$, is 2–8. This pattern aligns with the idea that Period 2 elements have valence electrons chiefly in level 2, and Period 3 in level 3. While high-school chemistry adds sublevel detail (like 3s, 3p), your job today is speed and correctness for table reading and bonding predictions in common quiz items.

Real-life tie-in: Firework colors and neon signs reflect electrons moving between levels; your 2–8–8 strings help you predict which level changes are involved.

Mini-summary: Use 2–8–8 to distribute electrons quickly and reveal the valence count for Periods 1–3.

1. Write the configuration (Grade-8 pattern) for Mg.
Show Answer
2–8–2 (12 electrons).
2. Give the configuration for P ($Z$=15).
Show Answer
2–8–5.
3. Which level holds the valence electrons of Si?
Show Answer
$n=3$ (Period 3).

Checkpoint 2 — Reading Valence from the Configuration

Mini-goal: Identify the outer level and count, then link to period and group behavior.

Guided discussion: In a string like 2–8–x, the last number is the valence count and the position (third slot) signals the main level ($n=3$). That count predicts behaviors: metals with 1–3 valence electrons often lose electrons; nonmetals with 5–7 often gain electrons; 8 suggests stability (noble gases). Link this to the table: Period 3 → valence level 3; Group 1 → valence 1; Group 17 → valence 7. Practice stating the logic in one neat sentence: “P is 2–8–5, so 5 valence electrons at $n=3$, predicting nonmetal behavior that gains 3 or shares to reach 8.” This sentence earns reasoning points and helps when multiple-choice answers look similar.

Real-life tie-in: Corrosion resistance (Al forms a protective oxide) and salt formation (NaCl) are outcomes of valence behavior you can read at a glance.

Mini-summary: The last number in the configuration string = valence count; its position = valence level; both connect to group/period patterns.

1. How many valence electrons does Cl have?
Show Answer
7 (2–8–7).
2. Complete the claim: “Al has valence 3, so it tends to …”
Show Answer
lose 3 electrons → Al^$+3$.
3. Which set is likely stable without reacting much: Ne or Na? Why?
Show Answer
Ne (2–8) has a complete outer level; Na (2–8–1) tends to lose one.

Checkpoint 3 — From Valence to Ions and Quick Formulas

Mini-goal: Use valence counts to predict common ions and write correct binary formulas.

Guided discussion: Main-group metals on the left often lose their few outer electrons, forming positive ions: Group 1 → $+1$, Group 2 → $+2$. Nonmetals on the right often gain to reach 8: Group 17 → $-1$, Group 16 → $-2$. To build a formula, balance charges to zero by cross-multiplying absolute values or using the least common multiple. Example: Mg (2–8–2) tends to $+2$, Cl (2–8–7) tends to $-1$; the formula is MgCl₂. For Al (2–8–3) and O (2–6), match $+$3 with $-$2 → Al₂O₃. Always show two steps: (1) expected charges, (2) balanced subscripts. This short working wins method marks even if a slip happens later.

Real-life tie-in: Table salt, plaster, and ceramics are all predictable from valence logic—an everyday reason these patterns matter.

Mini-summary: Read valence → infer charge → balance to zero → write the formula neatly with subscripts.

1. Predict ions and a formula for Ca and F.
Show Answer
Ca^$+2$, F^$-1$ → CaF₂.
2. Give the formula from K and O.
Show Answer
K₂O.
3. Finish: “Sulfur often forms S^__ in salts.”
Show Answer
S^$-2$.

Checkpoint 4 — Writing Simple Configuration Notation (s/p Level)

Mini-goal: Translate 2–8–x into a basic s/p configuration for Periods 1–3 (Grade-8 depth).

Guided discussion: A compact notation uses level + sublevel + superscript electrons. For our scope, remember: 1s holds up to 2; 2s up to 2; 2p up to 6; 3s up to 2; 3p up to 6. Sodium: 1s² 2s² 2p⁶ 3s¹ (which matches 2–8–1). Chlorine: 1s² 2s² 2p⁶ 3s² 3p⁵ (2–8–7). This notation adds detail used in upper grades but is still manageable when limited to s and p sublevels. When writing fast, stack the terms in order of increasing level: 1s → 2s → 2p → 3s → 3p. Underline or circle the highest level terms to show the valence pieces (e.g., the 3s and 3p parts for Period-3 nonmetals).

Real-life tie-in: The shapes of s (spherical) and p (two-lobed) orbitals affect molecule shapes; this notation hints at where bonds will point.

Mini-summary: Use 1s, 2s, 2p, 3s, 3p up to their caps; highlight the highest-level terms to read valence quickly.

1. Write the s/p configuration for Mg.
Show Answer
1s² 2s² 2p⁶ 3s².
2. Write the s/p configuration for Al.
Show Answer
1s² 2s² 2p⁶ 3s² 3p¹.
3. Which parts are “valence” for sulfur’s configuration?
Show Answer
3s and 3p terms (3s² 3p⁴ → 6 valence e⁻).

Checkpoint 5 — Patterns Across a Period and Down a Group

Mini-goal: Use configuration and valence to explain pattern changes (metallic character, typical ions).

Guided discussion: Across a period (left → right), the valence level stays the same but the count increases. In Period 3: Na (3s¹) → Mg (3s²) → Al (3s²3p¹) … → Cl (3s²3p⁵) → Ar (3s²3p⁶). As the outer count grows, metallic character generally decreases and nonmetallic character increases. Down a group, the main valence level increases (e.g., Li → Na → K), which often makes the outer electron easier to lose for metals (more reactive). Linking this to quick ion predictions gives fast test answers: Group 1 metals form $+1$; Group 17 nonmetals form $-1$; noble gases remain largely unreactive because their outer level is “full.”

Real-life tie-in: Battery chemistry, corrosion, and disinfectants (halogens) all exploit these repeating patterns.

Mini-summary: Across = same level, rising valence count; down = higher level, often easier loss (metals) or changing gain strength (nonmetals).

1. Which is more metallic: Na or Cl? Why?
Show Answer
Na—fewer valence electrons (1), tends to lose; Cl (7) tends to gain.
2. Which Group 1 element (Li or K) is usually more reactive?
Show Answer
K (valence level farther from nucleus).
3. Why is Ar generally unreactive?
Show Answer
Outer level is complete (3s²3p⁶ → 8 valence e⁻).

Checkpoint 6 — Configuration to Explanation: Scoring Full Marks

Mini-goal: Write short, high-yield explanations that examiners reward.

Guided discussion: Use a 4-part micro-paragraph: Claim (classification or prediction), Evidence (configuration + valence), Map Link (period/group), Conclusion (ion or formula). Example: “Claim: Magnesium forms Mg²⁺. Evidence: 1s²2s²2p⁶3s² → 2 valence electrons. Map: Period 3, Group 2. Conclusion: It loses 2 to reach a stable outer level, so MgCl₂ with chlorine.” Keep sentences brief and focused; show a single line of arithmetic for charge balance. This structure improves both short-answer and multiple-choice confidence.

Real-life tie-in: Clear notes and lab write-ups rely on the same structure—others can see your reasoning fast.

Mini-summary: Claim → Evidence (configuration/valence) → Map → Conclusion. Two lines, full logic.

1. Complete the pattern for Al with O using the 4-part method.
Show Answer
Al: 3 valence (Group 13) → Al³⁺; O: 6 valence (Group 16) → O²⁻; balance → Al₂O₃.
2. Write the evidence line that proves Cl tends to gain one.
Show Answer
Cl has 3s²3p⁵ → 7 valence → needs 1 to reach 8.
3. Finish the conclusion: “Therefore Na with O makes …”
Show Answer
Na₂O (2×Na⁺ balances O²⁻).

1. Configuration → valence: Phosphorus (P).
   Show Answer

   2–8–5; 1s²2s²2p⁶3s²3p³ → 5 valence (nonmetal).
2. Predict an ion: Calcium (Ca) from its configuration.
   Show Answer

   …3s²3p⁶4s² at higher level in full chart; Grade-8 view: Period 4 metal with 2 valence → Ca²⁺.
3. Write a formula: Al with Cl.
   Show Answer

   AlCl₃ (Al³⁺ balances 3×Cl⁻).
4. Explain stability: Why is Ne less reactive than F?
   Show Answer

   Ne has a complete outer level (2–8); F has 7 and tends to gain 1.
5. Spot valence quickly: Which part of S’s configuration is the “business end”?
   Show Answer

   3s and 3p terms—together 6 valence electrons.

1. Write 2–8–x configurations for: Na, Si, Cl.
   Show Answer

   Na 2–8–1; Si 2–8–4; Cl 2–8–7.
2. State valence level and count for Mg and P.
   Show Answer

   Mg: level 3, 2e⁻; P: level 3, 5e⁻.
3. Write s/p configuration for Al (Grade-8 depth).
Show Answer
1s² 2s² 2p⁶ 3s² 3p¹.
4. Predict likely ions for K and O; then write the formula.
   Show Answer

   K⁺, O²⁻ → K₂O.
5. Explain in one sentence why Ar is unreactive.
Show Answer
It has a full outer level (octet), so it neither needs to gain nor lose electrons.
6. Give the 4-step explanation (Claim→Evidence→Map→Conclusion) for Mg + Cl.
Show Answer
Mg has 2 valence (P3, G2) → Mg²⁺; Cl has 7 (P3, G17) → Cl⁻; MgCl₂.
7. Choose which element from Period 3 is most likely to form a $-2$ ion.
Show Answer
S (2–8–6) or O in Period 2; from Period 3 specifically, S.
8. Write a one-line rule to spot valence from s/p notation.
Show Answer
“Look at the highest level terms (ns, np); add their superscripts.”
9. From “2–8–3,” predict metal/nonmetal and typical ion.
Show Answer
Metal (e.g., Al), typically $+3$.
10. Write the formula for Ca and N and show charge logic.
Show Answer
Ca²⁺ and N³⁻ → Ca₃N₂.

1. Multiple choice: Which string matches Si?
   A) 2–8–4 B) 2–8–6 C) 2–8–7 D) 2–8–2
   Show Answer

   A.
2. True/False: The last number in 2–8–x is the valence count.
Show Answer
True.
3. Fill-in: Group 17 elements often form ______ ions.
Show Answer
$-1$.
4. Short answer: Why does Na form Na⁺?
Show Answer
It has one valence electron (2–8–1) that it readily loses to reach a stable outer level.
5. Multiple choice: Which is a correct s/p configuration for Mg?
   A) 1s² 2s² 2p⁶ 3s² B) 1s² 2s² 2p⁵ 3s³ C) 1s² 2p⁶ 3s⁴ D) 1s² 2s² 3p⁶
   Show Answer

   A.
6. True/False: Period number usually tells you the main valence level for main-group elements.
Show Answer
True.
7. Fill-in: The “business” sublevels for Period-3 nonmetals are ______ and ______.
Show Answer
3s and 3p.
8. Short answer: Write the balanced formula for Al with O.
Show Answer
Al₂O₃.
9. Multiple choice: Which set is least reactive?
   A) Group 1 B) Group 2 C) Group 17 D) Group 18
   Show Answer

   D (noble gases).
10. True/False: Chlorine’s valence electrons are in level 2.
Show Answer
False—level 3 (Period 3).
11. Fill-in: To build a binary formula, total positive and negative ______ must balance to zero.
Show Answer
charges.
12. Short answer: Give one reason metallic character decreases across a period.
Show Answer
Valence count increases within the same level, so atoms more often gain/share rather than lose.
13. Multiple choice: Which element is most likely 2–8–2?
    A) S B) Ne C) Mg D) Cl
    Show Answer

    C (Mg).
14. True/False: The notation 3p⁵ tells you five electrons at level 3, p sublevel.
Show Answer
True.
15. Fill-in: Claim → Evidence → Map → ______ is our explanation pattern.
Show Answer
Conclusion.

1. Poster: “Spot the Valence” — rules for reading 2–8–x and s/p notation; include 3 worked samples.
2. Period-3 Strip: Fill a mini table from Na→Ar with configurations and predicted common ions/behaviors.
3. Compare Models: In 100–120 words, explain when to prefer s/p notation vs. 2–8–x strings.
4. Formula Sprint: Ten quick pairs (e.g., K–S, Ca–N). Record charges and balanced formulas.
5. Safety Snapshot: Write guidelines for handling salts and metals in class (labels, goggles, no tasting, handwashing, tidy bench).

Notebook Task: Choose any two Period-3 elements from different groups. For each, write (a) 2–8–x configuration, (b) s/p notation (up to 3p), (c) valence level and count, and (d) a likely ion or bonding style with one everyday use. End with a two-sentence comparison: Which reading—2–8–x or s/p—felt faster and why?

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