The Periodic Table

The Periodic Table

The periodic table is a systematic arrangement of all known chemical elements in order of increasing atomic number (number of protons). It was originally devised by Dmitri Mendeleev in 1869, who arranged elements by atomic mass and left gaps for undiscovered elements — many of which were subsequently found with exactly the predicted properties.

The periodic table has approximately 118 confirmed elements, ranging from hydrogen (atomic number 1) to oganesson (118).

Structure — Groups and Periods

Periods (horizontal rows):

• Elements in the same period have the same number of electron shells
• As you move across a period (left to right), atomic number increases by 1
• Chemical properties change significantly across a period (from metallic to non-metallic)

Groups (vertical columns):

• Elements in the same group have the same number of outer shell electrons
• This gives them similar chemical properties
• Groups are numbered 1–7 (and 0 for noble gases) in the traditional system, or 1–18 in the IUPAC system

Key principle: the number of outer electrons determines how an element reacts chemically.

Metals and Non-Metals

Metals are found on the left and centre of the periodic table; non-metals are on the right. A diagonal "staircase" line separates them. Elements along this line are called metalloids or semi-metals (e.g., silicon, germanium) and have intermediate properties.

Property	Metals	Non-metals
Electrical conductivity	Good	Poor (except graphite)
Thermal conductivity	Good	Poor
Melting/boiling point	Usually high	Usually low
Physical state (at room temp)	Solid (except mercury)	Solid, liquid, or gas
Appearance	Shiny, lustrous	Dull (except iodine)
Malleability	Malleable and ductile	Brittle (if solid)
Oxide formed	Basic oxide	Acidic oxide

Group 1 — Alkali Metals

Members: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), Francium (Fr)

Properties:

• Soft, low-density metals (can be cut with a knife)
• Shiny when freshly cut, quickly oxidise and turn dull
• Stored under oil to prevent reaction with oxygen and water in air
• React vigorously with water to produce a metal hydroxide and hydrogen gas

Reaction with water: Metal + Water → Metal hydroxide + Hydrogen

For sodium: 2Na + 2H2O → 2NaOH + H2

Reactivity trend: increases DOWN the group

• Outer electron is in a higher shell → further from nucleus → less strongly attracted → more easily lost
• Li reacts gently; Na fizzes vigorously; K ignites the hydrogen; Rb and Cs react explosively

Group 7 — Halogens

Members: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At)

Properties:

• Reactive non-metals; exist as diatomic molecules (F2, Cl2, Br2, I2)
• At room temperature: F2 (pale yellow gas), Cl2 (green gas), Br2 (brown liquid), I2 (grey solid)
• React with metals to form ionic salts called halides (e.g., NaCl — sodium chloride)
• React with hydrogen to form hydrogen halides (e.g., HCl — hydrochloric acid)

Reactivity trend: decreases DOWN the group

• Outer shell gets further from nucleus as you go down → harder to gain an extra electron
• Fluorine is the most reactive element in the entire periodic table

Displacement reactions: a more reactive halogen displaces a less reactive one from its salt: Chlorine + Potassium bromide → Potassium chloride + Bromine (Cl2 + 2KBr → 2KCl + Br2)

Group 0 — Noble Gases

Members: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn)

Properties:

• All are colourless, odourless, monatomic gases at room temperature
• Extremely unreactive — have a full outer electron shell (stable configuration)
• Do not normally form compounds
• Low boiling points; low density

Uses: He in balloons and airships; Ne in illuminated signs; Ar in light bulbs (prevents filament oxidation); Kr and Xe in specialist lighting.

Transition Metals

The transition metals occupy the large central block (periods 4–7, groups 3–12). Examples: iron (Fe), copper (Cu), gold (Au), silver (Ag), zinc (Zn), titanium (Ti).

Properties:

• High melting and boiling points (generally higher than Group 1)
• Hard and dense
• Good conductors of heat and electricity
• Can form coloured compounds (e.g., copper sulphate is blue)
• Can act as catalysts (iron in Haber process; platinum in catalytic converters)
• Often have variable oxidation states (e.g., iron can be Fe2+ or Fe3+)

Periodic Trends

Property	Trend across a period (left to right)	Trend down a group
Atomic number	Increases	Increases
Atomic radius	Decreases (more protons pull electrons closer)	Increases
Metallic character	Decreases	Increases
Reactivity (metals)	Decreases	Increases
Reactivity (non-metals)	Increases	Decreases

Worked Example — Predicting Properties

Rubidium (Rb) is in Group 1, Period 5. Predict its properties:

• It has 1 outer electron (Group 1) → highly reactive
• It has 5 electron shells (Period 5) → larger atom than Na or K
• It is more reactive than potassium (further down Group 1)
• It reacts violently with water, producing RbOH and H2 gas
• It is stored under oil

Key Terms

• Atomic number — the number of protons in the nucleus; defines the element
• Period — a horizontal row of the periodic table; elements have the same number of electron shells
• Group — a vertical column; elements have the same number of outer electrons
• Alkali metals — Group 1 reactive metals; reactivity increases down the group
• Halogens — Group 7 reactive non-metals; reactivity decreases down the group
• Noble gases — Group 0/18; unreactive due to full outer electron shells
• Transition metals — central block; hard, dense, coloured compounds, variable oxidation states

Common Mistakes

• Saying period number = number of outer electrons — period number = number of electron shells
• Saying Group 7 reactivity increases down the group — it decreases (opposite to Group 1)
• Confusing noble gases being unreactive because they have no electrons — they have electrons, but a full outer shell
• Thinking all metals have high melting points — Group 1 metals have low melting points

Tips and Tricks

• Group number = number of outer electrons (for Groups 1–7)
• Period number = number of electron shells
• Group 1 reactivity: goes up going DOWN the group (more reactive at bottom)
• Group 7 reactivity: goes DOWN going DOWN the group (less reactive at bottom)
• Noble gases = Group 0 = full outer shell = unreactive — the only group where inactivity is the defining property