The Periodic Table is not a random list. Elements are placed in order of proton number, and their positions follow their electron arrangements: the group gives the outer electrons, the period gives the shells. That is why elements in the same column behave alike.
The Periodic Table arranges elements in order of increasing proton number. Vertical columns are groups; horizontal rows are periods. For Groups I to VII the group number equals the number of outer-shell electrons, and the period number equals the number of occupied shells. Elements change from metallic to non-metallic across a period. Extended: elements in the same group have similar chemical properties because they have the same number of outer electrons; the group number predicts the charge on the ion: I +1, II +2, III +3, V −3, VI −2, VII −1.
Step across Period 3 in the simulator, from sodium to argon. Watch the outer-shell electrons increase by one each time and the character change from metal to non-metal.
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Elements in the Periodic Table are arranged in order of increasing:
An element has the electronic configuration 2,8,3. Which group is it in?
What is a horizontal row of the Periodic Table called?
How does the character of the elements change across a period from left to right?
Extended. Because the outer electrons decide how an element reacts, its position lets you predict its chemistry.
| Group | Outer electrons | Usual ion | Example |
|---|---|---|---|
| I | 1 | +1 | Na+ |
| II | 2 | +2 | Mg2+ |
| III | 3 | +3 | Al3+ |
| V | 5 | −3 | N3− |
| VI | 6 | −2 | O2− |
| VII | 7 | −1 | Cl− |
The Periodic Table is ordered by proton number, not by relative atomic mass. Argon (Ar 40) comes before potassium (Ar 39) because argon has 18 protons and potassium 19. And Group VIII (noble gases) has 8 outer electrons, except helium, which has 2.
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Element X is in Period 2 and Group VI. What is its electronic configuration?
(Extended) Why do lithium, sodium and potassium have similar chemical properties?
(Extended) What is the charge on an ion formed by an element in Group VI?
Why is argon placed before potassium although its relative atomic mass is higher?
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