The Periodic Table
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Organisation of the Periodic Table
- The Periodic Table is a chart that organises all known chemical elements in a systematic way
- Elements are arranged to show patterns in their properties and behaviour
- Periodic Table: an arrangement of elements in order of increasing atomic number, showing patterns in their properties
How Elements are Arranged
- Elements are arranged in order of increasing atomic number (number of protons)
- Elements are organised into rows (periods) and columns (groups)
- The arrangement reveals patterns in chemical and physical properties
Periods and Groups
| Name | Arrangement | Significance |
|---|---|---|
| Periods | Horizontal rows (numbered 1-7) | Period number equals number of electron shells |
| Groups | Vertical columns (numbered 1-18) | Elements in same group have similar chemical properties |
- Period 1 elements have 1 electron shell
- Period 2 elements have 2 electron shells
- Period 3 elements have 3 electron shells
The Periodic Table, showing horizontal periods and vertical groups
Predicting Atomic Structure from Position
- For elements with proton numbers 1 to 20, an element's position in the Periodic Table can be used to predict its atomic structure and properties:
- the atomic (proton) number gives the number of protons and electrons
- the period number gives the number of electron shells
- the group number gives the number of electrons in the outer shell (for Groups 1, 2, and 13-18)
- Edge case: an atom with an electron configuration of just "2" (a single, full shell and no others) is not Group 2 - a full outer shell means the element belongs to Group 8 (Group 0), the noble gases; this applies to helium, whose electron configuration is 2
Worked Example: Predicting Structure from Position
Muchindu is told that an element is in Period 3, Group 2 of the Periodic Table, and wants to work out its electron configuration.
- Step 1: Period 3 means the atom has 3 electron shells
- Step 2: Group 2 means the atom has 2 electrons in its outer shell
- Step 3: filling from the innermost shell outward: 2, 8, 2 - this is magnesium (proton number 12)
Metals and Non-metals
- The Periodic Table is divided into two main regions: metals and non-metals
- A diagonal stair-step line separates these two categories
Metals
- Metals are found on the left and centre of the Periodic Table and share similar physical and chemical properties:
- good conductors of heat and electricity
- shiny when polished (metallic lustre)
- malleable - can be hammered into shapes
- ductile - can be drawn into wires
- usually have high melting and boiling points
- solid at room temperature (except mercury)
- Examples: sodium (Na), iron (Fe), copper (Cu), magnesium (Mg)
Non-metals
- Non-metals are found on the right side of the Periodic Table and their properties contrast sharply with metals:
- poor conductors of heat and electricity (insulators)
- dull appearance (not shiny)
- brittle when solid - break easily
- usually have lower melting and boiling points than metals
- can be gases, liquids, or solids at room temperature
- Examples: oxygen (O), chlorine (Cl), carbon (C), nitrogen (N)
- Elements along the stair-step line are called metalloids or semi-metals (like silicon) - they have properties intermediate between metals and non-metals
The Periodic Table colour-coded to show metals, non-metals and metalloids
Key Groups in the Periodic Table
- Certain groups have special names and distinctive properties that make them important to learn
- Elements in these groups share similar chemical behaviour due to having the same number of outer shell electrons
Group 1: Alkali Metals
- Elements: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), francium (Fr)
- Very reactive metals that react vigorously with water
- Reactivity increases down the group
- Soft metals that can be cut with a knife
- Low melting points (for metals)
- All have 1 electron in their outer shell
Physical Property Trends Down Group 1
| Element | Melting Point (°C) | Density (g/cm³) |
|---|---|---|
| Lithium | 181 | 0.53 |
| Sodium | 98 | 0.97 |
| Potassium | 64 | 0.89 |
| Rubidium | 39 | 1.53 |
| Caesium | 29 | 1.87 |
- Going down Group 1: melting point decreases (e.g. 181°C for lithium down to 29°C for caesium)
- Boiling point also decreases down the group (e.g. lithium 1342°C, sodium 883°C, potassium 759°C)
- Density generally increases down the group, although (as shown below) this trend is not perfectly regular
Worked Example: Extrapolating a Value from a Trend
Aiko is given the boiling points of lithium (1342°C), sodium (883°C) and potassium (759°C), and asked to predict the boiling point of rubidium, which is not given.
- Step 1: identify the trend - boiling point decreases down Group 1
- Step 2: look at the size of the decrease between known values: 1342 → 883 is a decrease of 459°C; 883 → 759 is a decrease of 124°C (the decrease gets smaller each time)
- Step 3: continue the pattern beyond the last known value to extrapolate a sensible prediction, e.g. roughly 759 − 70 ≈ 685-690°C
- Any value that reasonably continues the decreasing trend is accepted (the accepted range for rubidium's boiling point was 635-720°C; its actual boiling point is about 688°C)
Worked Example: Spotting a Non-Monotonic (Anomalous) Trend
Malaika looks at the density of Group 1 elements: lithium 0.53, sodium 0.97, potassium 0.89, rubidium 1.53, caesium 1.87 g/cm³.
- Step 1: check whether the values rise or fall consistently down the group
- Step 2: density rises from lithium to sodium (0.53 → 0.97), but then falls from sodium to potassium (0.97 → 0.89) - this breaks the expected increasing pattern
- Because the trend is not perfectly consistent, it is not reliable to extrapolate or predict a precise unknown value from it - the correct answer is that "there is no consistent trend" or "the trend goes up then down"
Group 7: Halogens
- Elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I)
- Very reactive non-metals
- Reactivity decreases down the group
- Exist as diatomic molecules (F₂, Cl₂, Br₂, I₂)
- All have 7 electrons in their outer shell
- Form salts when they react with metals (e.g., sodium chloride)
Group 18 (or 0): Noble Gases
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