# T.1(a).3–T.1(a).4 Elements and Atoms

F.3 Chemistry · Topic I(a) · Lesson 2

Metals, non-metals and semi-metals; what an atom is made of; isotopes and relative atomic mass.

## Figure

Full atomic symbol: mass number A on top, atomic number Z below.

## Key points

- The **zig-zag line** of the periodic table separates **metals** (left; solids except **mercury**) from **non-metals** (right; solids or gases except **bromine**). **Semi-metals ／ 半金屬** (metalloids, e.g. **boron, silicon, germanium**) sit along the line: brittle shiny solids that are **semi-conductors**. The **transition elements** lie between the main groups.
- Metals: shiny, good conductors of electricity and heat, malleable and ductile, usually high m.p. and b.p., strong, dense. Non-metals: dull, poor conductors (except **graphite**), brittle, usually low m.p. and b.p. (except **diamond and graphite**), low density.
- **Allotropes ／ 同素異形體** are different forms of the same element with different structures and properties: carbon as **graphite**, **diamond**, **fullerenes and nanotubes**.
- An **atom** is the smallest part of an element which has the chemical properties of that element; an element is made up of **only one kind of atom**. An atom's volume is about $10^{-24}$ cm^3^ and its mass about $10^{-23}$ g.
- The **nucleus** at the centre holds the **protons** and **neutrons**; the **electrons** move around it in mostly **empty space**. Proton: relative mass 1, charge +1. Neutron: relative mass 1, charge 0. Electron: relative mass $\frac{1}{1837}$ (negligible), charge −1.
- Protons and electrons are **exactly equal** in number, so an atom has **no overall charge**. Electrons are not counted in the mass number: their mass is negligible.
- **Isotopes ／ 同位素** have the same number of electrons, so the **same chemical properties**; different numbers of neutrons give different masses and **slightly different physical properties**. Chlorine: Cl-35 (75 %) and Cl-37 (25 %); hydrogen: protium, deuterium, tritium.
- **Relative isotopic mass** = mass of one atom of the isotope on the ^12^C = 12.00 scale ≈ its **mass number**; no unit. **Relative atomic mass ／ 相對原子質量** (Type 1): chlorine $= 35 \times 75\% + 37 \times 25\% = 35.5$. Reverse (Type 2): neon is 20.2 from ^20^Ne and ^22^Ne; let ^20^Ne be $y\%$: $20y + 22(100 - y) = 2020$, so $y = 90$.

## Must know

| Quantity | Meaning | Remember |
|-----|-------------|----------|
| **Atomic number (Z)** | number of **protons** (= number of **electrons** in a neutral atom) | fixed for each element |
| **Mass number (A)** | number of **protons** + number of **neutrons** (electrons not counted) | number of neutrons = A − Z |
| **Isotopes** | atoms of the **same** element (same Z) with different numbers of **neutrons** (different A) | **same** chemical properties; slightly different physical properties (mass) |
| **Relative atomic mass** | **weighted average** of the relative isotopic masses of all the natural isotopes, on the ^12^C = 12.00 scale | chlorine: $(35 \times 75 + 37 \times 25) \div 100 = 35.5$; no unit; usually not a whole number |

## Common mistakes

- ✗ "Chlorine is 35.5, so every chlorine atom has 17 protons and 18.5 neutrons." ✓ No atom has half a neutron. 35.5 is the **weighted average** of Cl-35 (75 %, 18 neutrons) and Cl-37 (25 %, 20 neutrons): each atom has 18 **or** 20.
- ✗ "A neutral atom has equal numbers of protons and neutrons." ✓ Equal numbers of **protons and electrons**. An ordinary hydrogen atom (protium) has one proton and **no neutron**.
- ✗ "Isotopes have different chemical properties." ✓ Same number of **electrons**, so the **same chemical properties**; only mass-related physical properties differ.
- ✗ "The relative atomic mass of strontium is 87.6 g." ✓ Relative isotopic mass and relative atomic mass are **ratios** on the ^12^C = 12.00 scale: **no unit**.
- ✗ "Carbon and beryllium are metalloids." ✓ Semi-metals include **boron, silicon and germanium**. Carbon is a non-metal and beryllium a metal.

## Quick check

1. Give the numbers of protons, neutrons and electrons in $^{39}_{19}\text{K}$.
2. Boron: ^10^B 19.7 %, ^11^B 80.3 %. Calculate the relative atomic mass of boron.
3. Neon (relative atomic mass 20.2) has the isotopes ^20^Ne and ^22^Ne. Find the relative abundance of ^20^Ne.
4. Which of silicon, carbon, beryllium and boron are semi-metals, and what electrical property makes them useful?
5. ^10^BCl~3~ gives steamy fumes with water. What does ^11^BCl~3~ do with water, and why?

## Answers

1. 19 protons, 20 neutrons (39 − 19), 19 electrons.
2. $10 \times 19.7\% + 11 \times 80.3\% = 10.8$ (no unit).
3. Let ^20^Ne be $y\%$: $20y + 22(100 - y) = 2020$, $y = 90$. ^20^Ne 90 %, ^22^Ne 10 %.
4. Silicon and boron. They are semi-conductors.
5. Also steamy fumes: isotopes have the same number of electrons, so the same chemical properties.

## Did you know?

If an atom were the size of a football stadium, its nucleus would be about the size of a pea at the centre — the rest is the space of the electrons.