The Periodic Table
Describe the organisation of the periodic table into periods and groups, explain the contribution of Mendeleev, and use the table to identify metals, non-metals, and predict patterns in reactivity
What a learner can do afterwards
- Explains why the periodic table is arranged into periods (rows) and groups (columns)
- States that elements in the same group have similar chemical properties
- Locates metals, non-metals, and metalloids in the periodic table
- Explains how Mendeleev ordered elements by atomic weight and predicted missing elements
1 · Read
You already know that everything is built from atoms and elements. Scientists took every known element and put it on one chart called the periodic table, so you can guess how a new element might behave just from where it sits. The chart has rows called periods and columns called groups.
In 1869, a chemist named Dmitri Mendeleev lined up all the known elements in order of their atomic weight. A pattern of properties kept repeating. When an element seemed to be missing from the pattern, he left a blank space instead of forcing a bad fit, and he predicted what that missing element would be like. Years later, chemists found those exact elements, and his predictions matched.
Use where an element sits to guess what it is. Most of the table, the left and the middle, is metal. Non-metals crowd the top right corner. A jagged staircase line runs between them, and elements touching that line are metalloids, part metal and part non-metal.
Periods are rows, groups are columns, elements in the same group behave alike, and that pattern is exactly what let Mendeleev predict elements before anyone found them.
More to read
The periodic table
The periodic table organizes all known elements by atomic number. Each element's square shows its symbol, atomic number, and atomic mass. Elements are arranged in rows called periods and columns called groups. Elements in the same group share similar properties because they have the same number of valence electrons. The table divides into metals on the left, nonmetals on the right, and metalloids along a stair-step line. Group 1 are the alkali metals, very reactive. Group 18 are the noble gases, inert and unreactive.
2.5 The Periodic Table
The periodic law states that properties of elements repeat periodically when arranged by atomic number. Mendeleev organized elements by atomic weight and left gaps for undiscovered elements, predicting their properties accurately. The modern table uses atomic number. Periods show repeating patterns of properties. Groups share similar chemical behaviour. Metals lose electrons to form cations. Nonmetals gain electrons to form anions. Metalloids show intermediate behaviour.
18.1 Periodicity
Periodicity is the repeating pattern of element properties across the table. Across a period, atomic radius decreases as more protons pull electrons closer. Ionization energy increases for the same reason. Down a group, atomic radius increases as electrons occupy higher energy levels, and ionization energy decreases. Electron affinity and electronegativity follow similar trends. These patterns let you predict an element's reactivity and bonding behaviour just from its position.
The periodic table
The periodic table is organized by atomic number, which is the number of protons. This reveals patterns in physical and chemical properties. Elements are categorized as metals, nonmetals, or metalloids. Vertical groups share similar chemical properties based on valence electrons. The octet rule explains how atoms achieve stability by gaining, losing, or sharing electrons to fill their outer shell. Alkali metals have one valence electron and form +1 ions. Halogens have seven and form -1 ions. Noble gases have full outer shells and rarely react.
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2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.