# Topic II: Microscopic World (I) Revision

F.4 Chemistry · Topic II Microscopic World (I) · Lesson 4

Connect atomic particles, electronic arrangements and bonding to chemical formulae and properties.

## Key points

- **Particles**: atomic number $Z=p$; mass number $A=p+n$; $p$, $n$, $e$ count protons, neutrons, electrons. Neutral atoms have $e=p$; ion formation changes **electrons only**.
- **Isotopes (同位素)**: same element and proton number, different neutron numbers. **Relative atomic mass (相對原子質量)** is an abundance-weighted average of relative isotopic masses, with no unit.
- **Main-group electronic arrangement**: use the **neutral atom** to find period from occupied shells; for **Group I–VII**, group number equals outermost electron count. Reconstruct the atom when given an ion.
- **Group II**: alkaline earth metals have two outermost electrons; reactivity increases down the group. Calcium is more reactive than magnesium.
- **Ionic compounds**: opposite ions attract; formulae give the **simplest charge-balanced ratio**. Cations may be NH~4~^+^ rather than metal ions. Solid ions are fixed; molten/dissolved compounds conduct through **mobile ions**.
- **Names and colours**: FeSO~4~ is **iron(II) sulphate**. Aqueous Cu^2+^ is **blue**; colour can come from a cation or an anion.
- **Covalent bonds** share electron pairs; a **dative covalent bond (配位共價鍵)** has both electrons supplied by one atom. A **lone pair (孤電子對)** is not shared. NH~3~: three bond pairs and one lone pair on N.
- **Bonding dot-and-cross diagrams** show outermost electrons, including lone pairs. **Total electron count** also includes inner shells; C~2~H~4~ has 16 electrons in total.
- **Property explanations**: name the structure, particles and attractions first. Simple molecular substances have strong covalent bonds **within** molecules and weaker attractions **between** them.
- **Giant structures**: metallic attraction persists as ion layers slide. Graphite layers slide over weak attractions; **delocalized electrons (離域電子)** move along layers. Quartz's strong covalent network restricts atomic motion, making it hard.

## Must know

A nickel sample contains the following three isotopes; each neutral atom has $Z=28$.

| Isotope | Protons | Neutrons | Electrons | Abundance |
|---|---|---|---|---|
| Ni-58 | 28 | 30 | 28 | 68.2% |
| Ni-60 | 28 | 32 | 28 | 26.2% |
| Ni-62 | 28 | 34 | 28 | 5.6% |

Using mass numbers as approximate isotope masses:

$A_\mathrm{r}=\dfrac{58\times68.2+60\times26.2+62\times5.6}{100}=58.748\approx58.7$

**Charge check**: Mg^2+^ and N^3−^ form Mg~3~N~2~; $3(+2)+2(-3)=0$.

## Common mistakes

- ✗ "Isotopes have different proton numbers." ✓ Same element and **same protons**, but different neutrons.
- ✗ "Relative atomic mass is a simple average, in g." ✓ Weight by **relative abundance**; relative atomic mass has **no unit**.
- ✗ Ca~2~Cl or Mg~2~N~3~. ✓ Balance the ion charges: **CaCl~2~** and **Mg~3~N~2~**.
- ✗ "An outer-shell diagram shows every electron." ✓ It omits inner shells. Count **all electrons** only when the question asks for the total.
- ✗ "Graphite mainly conducts between layers." ✓ Delocalized electrons move mainly **along layers**; weak attractions **between layers** allow sliding.

## Quick check

1. Ni-62 has $Z=28$. How many protons, neutrons and electrons are in Ni^2+^?
2. A neutral atom has electronic arrangement 2,8,8,2. State its period and group.
3. Write the formula of magnesium nitride and show that its total ionic charge is zero.
4. How many bond pairs and lone pairs surround nitrogen in NH~3~? How does a dative bond differ in electron donation?
5. Explain why nickel is malleable and graphite conducts electricity, using their particles and bonding.

## Answers

1. 28 protons, 34 neutrons, 26 electrons.
2. Period 4, Group II.
3. Mg~3~N~2~: $3(+2)+2(-3)=0$.
4. Three bond pairs, one lone pair. In a dative bond, one atom supplies both shared electrons.
5. Nickel: ion layers slide while attraction to delocalized electrons remains. Graphite: mobile delocalized electrons move along the layers and carry charge.

## Did you know?

Graphene is an individual single layer of graphite.