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Structure 3. Classification of matter - The periodic table: Classification of elements

Grade 12IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Elements in the periodic table are arranged in order of increasing atomic number ZZ.

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Periods are the horizontal rows. The period number corresponds to the principal quantum number nn of the highest occupied energy level.

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Groups are the vertical columns. Elements in the same group have the same number of valence electrons and exhibit similar chemical properties.

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The periodic table is divided into blocks (s,p,d,fs, p, d, f) based on which subshell is being filled. For example, ss-block elements have their valence electrons in an ss orbital.

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Metals are located on the left and center of the table, non-metals are on the upper right, and metalloids (e.g., Si,Ge,AsSi, Ge, As) lie along the staircase dividing metals and non-metals.

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Effective Nuclear Charge (ZeffZ_{eff}): This is the net positive charge experienced by valence electrons. It is calculated as Zeff=Z−SZ_{eff} = Z - S, where SS is the shielding (screening) constant from inner-shell electrons.

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Across a period, ZeffZ_{eff} increases because the atomic number increases while the number of inner shielding electrons remains constant.

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Down a group, ZeffZ_{eff} remains approximately constant as the increase in nuclear charge is offset by the increase in the number of shielding electron shells.

📐Formulae

Zeff=Z−SZ_{eff} = Z - S

Group Number (for p-block)=Valence electrons+10\text{Group Number (for } p\text{-block)} = \text{Valence electrons} + 10

Max electrons in shell n=2n2\text{Max electrons in shell } n = 2n^2

💡Examples

Problem 1:

An atom has the electronic configuration 1s22s22p63s23p51s^2 2s^2 2p^6 3s^2 3p^5. Identify its period, group, and block.

Solution:

Period: 33, Group: 1717, Block: pp-block.

Explanation:

The highest principal quantum number is n=3n=3, indicating Period 33. The valence shell is the 3rd3^{rd} shell with 2+5=72 + 5 = 7 valence electrons; for pp-block elements, the group is 7+10=177 + 10 = 17. Because the last electron occupies a pp orbital, it is in the pp-block.

Problem 2:

Compare the ZeffZ_{eff} experienced by the valence electrons of Magnesium (Z=12Z=12) and Sulfur (Z=16Z=16).

Solution:

Zeff(Mg)≈12−10=+2Z_{eff}(Mg) \approx 12 - 10 = +2 Zeff(S)≈16−10=+6Z_{eff}(S) \approx 16 - 10 = +6

Explanation:

Both elements are in Period 3 and have 1010 core electrons (1s22s22p61s^2 2s^2 2p^6). Sulfur has a higher atomic number (Z=16Z=16) compared to Magnesium (Z=12Z=12), resulting in a much higher effective nuclear charge, which pulls the valence electrons closer to the nucleus.

Problem 3:

Which element is a metalloid in Period 3?

Solution:

Silicon (SiSi)

Explanation:

In Period 3, Na,Mg,AlNa, Mg, Al are metals, P,S,Cl,ArP, S, Cl, Ar are non-metals. SiSi (Atomic number 14) has properties intermediate between metals and non-metals and is classified as a metalloid.