# Chemical periodicity

Physics I · Chemistry · https://tryals.app/learn/physics-i/chemical-periodicity

The **Periodic Table** organises elements by increasing atomic number $Z$ and groups them by chemical behaviour. Trends stem from one variable: **effective nuclear charge**, defined as $Z_{\text{eff}} = Z - S$, where $S$ is the **shielding constant** from inner electrons.

| Property | Across Period ($\to$) | Down Group ($\downarrow$) | Dominant Driver |
| --- | --- | --- | --- |
| **Atomic radius** | **Decreases** | **Increases** | Charge ($\to$) vs Distance ($\downarrow$) |
| **Ionisation energy** | **Increases** | **Decreases** | Tightly bound electrons |
| **Electronegativity** | **Increases** | **Decreases** | Greater bonding attraction |

**Common pitfall:** Trends feature competing drivers. Always state which driver dominates (charge vs distance/shielding), rather than just the trend.

## Blocks and Calculations

The table divides elements into specific **blocks** based on their valence orbitals:

| Block | Groups | Characteristics |
| --- | --- | --- |
| **s-block** | 1–2 | Highly reactive metals, low ionisation energy |
| **p-block** | 13–18 | Diverse: metals, metalloids, non-metals, noble gases |
| **d-block** | 3–12 | Transition metals with variable oxidation states |

**Mole-mass bridge**: Use the molar mass $M$ from the periodic table to convert grams to moles with the formula $n = m / M$.

## Practice questions

4 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. Moving **down** the alkali metals (Li → Na → K → Rb), the atoms get bigger, yet they give up their outer electron **more** easily. Why?

A. Lower rows have more outer electrons to spare
B. Heavier atoms are hotter, shaking electrons loose
C. Bigger atoms have more protons, which push electrons away
D. The outer electron is farther out and more screened

**Answer:** D. The outer electron is farther out and more screened

**Why:** Distance plus shielding beat the extra protons: the valence electron of Rb feels roughly the same effective charge as Li’s but from much farther away. That is why reactivity of alkali metals **increases** downward, cesium ignites on water while lithium merely fizzes.

Page: https://tryals.app/practice/physics-i/chemical-periodicity/moving-down-the-alkali-metals-li-na-k-rb-the-atoms-get

### 2. Atomic radius decreases across a period even though electrons are being added. Why does adding electrons make the atom smaller?

A. Because atomic radius is measured differently for each element
B. Because the nuclear charge rises while the shell stays
C. Because the nucleus loses neutrons across a period
D. Because the added electrons pair up and occupy less space

**Answer:** B. Because the nuclear charge rises while the shell stays

**Why:** Nuclear charge grows faster than shielding within a period. Down a group the trend reverses, because a genuinely new shell is added and distance wins.

Page: https://tryals.app/practice/physics-i/chemical-periodicity/atomic-radius-decreases-across-a-period-even-though-electrons-are

### 3. Order the key events in the historical development of the periodic table.

**Answer:**

1. Döbereiner groups elements into triads sharing similar properties (1817)
2. Newlands proposes the Law of Octaves, periodicity every 8 elements (1865)
3. Mendeleev publishes table ordered by atomic mass; predicts missing elements (1869)
4. Moseley shows atomic number (not mass) is the true basis of periodicity (1913)

**Why:** The modern periodic law (Moseley, 1913) replaced Mendeleev's mass-based ordering with atomic number ordering.

Page: https://tryals.app/practice/physics-i/chemical-periodicity/order-the-key-events-in-the-historical-development-of-the-periodic

### 4. Rank these ions and atom from **smallest to largest** radius: they all have exactly 10 electrons.

**Answer:**

1. $Mg^{2+}$ (12 protons)
2. $Na^{+}$ (11 protons)
3. Ne (10 protons)
4. $F^{-}$ (9 protons)
5. $O^{2-}$ (8 protons)

**Why:** An isoelectronic series is a controlled experiment: the cloud is fixed at 10 electrons, so size is set purely by nuclear charge. $Mg^{2+}$ (12 p⁺) crushes the cloud smallest; $O^{2-}$ (8 p⁺) barely contains it. Size is a tug-of-war, not a fixed property.

Page: https://tryals.app/practice/physics-i/chemical-periodicity/rank-these-ions-and-atom-from-smallest-to-largest-radius-they-all