The Periodic Table
Explore how all known elements are organised into groups and periods, understand the difference between metals and non-metals, and discover the patterns that make the periodic table one of science's greatest tools.
What Is the Periodic Table?
The periodic table is a systematic arrangement of all known chemical elements, ordered by increasing atomic number (the number of protons in the nucleus). It was largely developed by Russian chemist Dmitri Mendeleev in 1869, who noticed that elements with similar properties repeated in a pattern — a period.
Today the table contains 118 confirmed elements. Each element has a unique chemical symbol (one or two letters), an atomic number, and a relative atomic mass. The table is read left-to-right, top-to-bottom in order of increasing atomic number.
Simplified Periodic Table Layout
Key idea: The periodic table arranges elements so that elements with similar properties fall into the same vertical group. Each horizontal period represents one more electron shell being filled.
Groups and Periods
The periodic table is divided into 18 vertical columns called groups and 7 horizontal rows called periods. Understanding these helps you predict how elements behave.
Groups (Vertical Columns)
- Elements in the same group have the same number of outer electrons
- They share very similar chemical properties
- Group 1: Alkali metals (Li, Na, K) — very reactive
- Group 2: Alkaline earth metals (Mg, Ca) — reactive
- Group 17: Halogens (F, Cl, Br) — reactive non-metals
- Group 18: Noble gases (He, Ne, Ar) — unreactive
Periods (Horizontal Rows)
- Each period corresponds to one electron shell being filled
- Period 1 has 2 elements (H, He) — 1 electron shell
- Period 2 has 8 elements — 2 electron shells
- Period 3 has 8 elements — 3 electron shells
- Properties change significantly across a period
- Elements generally go from metallic to non-metallic left to right
Key Groups to Know
| Group Name | Group No. | Examples | Key Property |
|---|---|---|---|
| Alkali Metals | 1 | Li, Na, K | React vigorously with water; soft; shiny |
| Alkaline Earth Metals | 2 | Mg, Ca | Less reactive than Group 1; form 2+ ions |
| Transition Metals | 3–12 | Fe, Cu, Au, Zn | Hard, high melting points, good conductors |
| Halogens | 17 | F, Cl, Br, I | Very reactive non-metals; form -1 ions |
| Noble Gases | 18 | He, Ne, Ar | Full outer shell; extremely unreactive |
Metals vs Non-Metals
About 75% of all elements are metals. They are found on the left and centre of the periodic table. Non-metals are on the right. A narrow band of elements called metalloids (e.g. silicon, arsenic) sit along the dividing line and share properties of both.
Metals
- Good conductors of heat and electricity
- Shiny (lustrous) appearance
- Malleable (can be bent/hammered into shapes)
- Ductile (can be drawn into wires)
- High melting and boiling points (most)
- Solid at room temperature (except mercury)
- Produce metal oxides when burned in oxygen
Examples: Iron (Fe), Gold (Au), Copper (Cu), Aluminium (Al), Sodium (Na)
Non-Metals
- Poor conductors of heat and electricity
- Dull appearance (most)
- Brittle when solid
- Low melting and boiling points (most)
- Can be gases, liquids, or solids
- Produce acidic oxides when burned in oxygen
Examples: Oxygen (O), Nitrogen (N), Carbon (C), Chlorine (Cl), Sulfur (S)
Exception: Carbon in the form of graphite is a non-metal that conducts electricity — useful in pencil "leads" and electrodes. This shows that understanding properties requires knowing the form an element takes.
Trends in the Periodic Table
One of the most powerful features of the periodic table is that properties change in predictable ways — these patterns are called trends. Understanding trends lets scientists predict the behaviour of elements they have never tested.
Reactivity of Group 1 Metals (down the group)
Reactivity increases going down Group 1. Lithium reacts slowly with water, sodium reacts vigorously, and potassium reacts explosively. This is because the outer electron is further from the nucleus and more easily lost.
Reactivity of Group 17 Halogens (down the group)
Reactivity decreases going down Group 17. Fluorine is the most reactive non-metal; iodine is far less reactive. The outer electron shell is further from the nucleus, making it harder to attract an extra electron.
Atomic radius (across a period)
Atomic size generally decreases across a period from left to right, because more protons pull the electrons closer to the nucleus even though no new shells are added.
Metallic character (across a period)
Elements become less metallic (more non-metallic) moving from left to right across a period. Period 3 goes: Na (very reactive metal) → Mg → Al → Si (metalloid) → P → S → Cl → Ar (noble gas).
Key Vocabulary
Atomic Number
The number of protons in the nucleus of an atom. It uniquely identifies each element and determines its position in the periodic table.
Group
A vertical column in the periodic table. Elements in the same group have the same number of outer (valence) electrons and similar chemical properties.
Period
A horizontal row in the periodic table. All elements in a period have the same number of electron shells. Properties change significantly across a period.
Metalloid
An element with properties intermediate between metals and non-metals (e.g. silicon, boron). They are often semi-conductors, vital for electronics.
Worked Examples
An element has atomic number 17. Identify its group and period and predict its properties.
Step 1: Atomic number 17 is Chlorine (Cl).
Step 2: Chlorine is in Period 3 (3 electron shells) and Group 17 (halogens).
Step 3: As a halogen, Cl is a reactive non-metal that forms -1 ions and diatomic molecules (Cl₂).
Conclusion: Chlorine is a yellow-green gas at room temperature, toxic, and less reactive than fluorine but more reactive than bromine.
Explain why sodium (Na) is more reactive than lithium (Li).
Both are in Group 1, so both have 1 outer electron that they lose when reacting.
Li has 2 electron shells; the outer electron is relatively close to the nucleus.
Na has 3 electron shells; the outer electron is further from the nucleus.
Conclusion: The outer electron in Na experiences a weaker attraction to the nucleus and is lost more easily, making Na more reactive than Li.
Classify the following as metal, non-metal, or metalloid: Silicon (Si), Iron (Fe), Sulfur (S).
Silicon (Si): Group 14, on the staircase boundary. It is a metalloid — semi-conducts electricity, used in computer chips.
Iron (Fe): A transition metal (Group 8). It is a metal — shiny, malleable, good conductor, high melting point.
Sulfur (S): Group 16, right side of table. It is a non-metal — yellow solid, poor conductor, low melting point.
Knowledge Check
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Key Concepts Summary
- ✓ Elements are ordered by atomic number in the periodic table.
- ✓ Groups (columns) contain elements with the same number of outer electrons and similar properties.
- ✓ Periods (rows) contain elements with the same number of electron shells.
- ✓ Metals are on the left; non-metals on the right; metalloids in between.
- ✓ Reactivity of Group 1 metals increases down the group; reactivity of halogens decreases down the group.
- ✓ Noble gases (Group 18) are unreactive due to their full outer electron shells.