What is 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 is one of the most powerful tools in science because it allows us to predict the properties of elements simply by knowing their position in the table.
Key idea: Elements in the same column (group) have similar chemical properties because they have the same number of electrons in their outermost shell.
History: Mendeleev's great idea
1864
John Newlands — Law of Octaves
Newlands noticed that when elements were arranged by mass, every eighth element had similar properties — like notes in a musical scale. Dismissed at first, his idea laid the groundwork.
1869
Dmitri Mendeleev — The Periodic Table
Russian chemist Mendeleev arranged 63 known elements by atomic mass and noticed repeating (periodic) patterns in their properties. He left gaps for undiscovered elements and predicted their properties — and was proved right when gallium, scandium, and germanium were later found.
1913
Henry Moseley — Atomic Number
Moseley used X-rays to determine that elements should be ordered by atomic number (protons), not mass. This fixed some inconsistencies in Mendeleev's table and gave us the modern periodic table.
Periods and Groups
The periodic table is organised into rows and columns:
- Periods (rows): There are 7 periods. Each period corresponds to a new electron shell being filled. Period 1 has 2 elements; Period 2 has 8; and so on. As you move across a period from left to right, the atomic number increases by 1 each time.
- Groups (columns): There are 18 groups. Elements in the same group have the same number of valence electrons (outermost electrons) and therefore similar chemical properties. Group 1 elements all have 1 valence electron; Group 17 all have 7.
Period number = number of electron shells. Group number (for main-group elements) = number of valence electrons.
Metals, Metalloids, and Non-metals
The periodic table can be broadly divided into three regions:
- Metals (left and centre): shiny, good conductors of heat and electricity, malleable (can be hammered into shape), ductile (can be drawn into wire), and solid at room temperature (except mercury). Examples: iron (Fe), copper (Cu), gold (Au).
- Non-metals (right side): poor conductors, often dull and brittle as solids, may be gases at room temperature. Examples: oxygen (O), chlorine (Cl), sulfur (S).
- Metalloids / Semi-metals (staircase boundary): have properties of both metals and non-metals. Examples: silicon (Si), germanium (Ge), arsenic (As). Silicon is a semiconductor used in computer chips.
Important Groups
- Group 1 — Alkali metals (Li, Na, K, Rb, Cs, Fr): Very reactive metals that react vigorously with water, producing hydrogen gas and a metal hydroxide. Reactivity increases down the group.
- Group 2 — Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra): Less reactive than Group 1, but still react with water. Calcium is essential for bones and teeth.
- Groups 3–12 — Transition metals: Less reactive, harder, have higher melting points. Include iron, copper, gold, and silver. Many form coloured compounds.
- Group 17 — Halogens (F, Cl, Br, I, At): Very reactive non-metals with 7 valence electrons. Reactivity decreases down the group. Fluorine is the most reactive element of all.
- Group 18 — Noble gases (He, Ne, Ar, Kr, Xe, Rn): Completely unreactive — full outer shells mean no desire to gain or lose electrons. Used in lighting and as inert atmospheres.
Periodic Trends
Properties change in predictable ways across and down the table:
- Atomic radius: Increases down a group (more shells) and decreases across a period left to right (more protons pull electrons closer).
- Reactivity of metals: Increases down a group (valence electron is further from nucleus and easier to remove).
- Reactivity of non-metals: Increases up a group and across a period to the right (more "eagerness" to gain electrons to complete outer shell).
- Metallic character: Decreases across a period from left to right.
Remember: The periodic table is described as "periodic" because similar properties recur at regular intervals — every time a new period begins, the pattern repeats.