The Periodic Table and Electron Arrangement
≈ 30 minThe periodic table arranges elements by increasing atomic number into periods (rows) and groups (columns).
- Elements in the same group have the same number of outer (valence) electrons and so behave chemically alike. Group 1 (e.g. Na) are reactive metals; Group 18 (e.g. Ne) are unreactive noble gases.
- For the first 20 elements, electrons fill shells in the pattern 2, 8, 8: up to 2 in the first shell, then up to 8, then up to 8.
Example: sodium has 11 electrons → arrangement 2, 8, 1. That single outer electron is why sodium is so reactive.
Write the electron arrangement of an atom with 12 electrons (magnesium), using commas, e.g. 2,8,1.
Why do all the elements in Group 1 (e.g. lithium, sodium, potassium) behave chemically alike?
In the periodic table, the horizontal rows are called periods and the vertical columns are called groups. How many outer-shell electrons does an atom with arrangement 2,8,7 have?