The Periodic Table: Arrangement and Trends

Elements are arranged in order of proton number. Vertical columns are groups: elements in a group have the same number of outer electrons and similar reactions. Horizontal rows are periods: elements in a period have the same number of electron shells. Metals are on the left, non-metals on the right; across a period, elements change from metallic to non-metallic.

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Section 1 of 14

Learning objectives

Describe the arrangement of the Periodic Table.
Relate group and period to electronic configuration.
Describe the change from metal to non-metal across a period.
Predict properties from position.

Pause and think

Mendeleev left gaps in his table in 1869. Why was that a brilliant move?

Section 2 of 14

Core idea

Start hereA map of all matter

Every substance ever made, from your phone screen to the stars, is built from about 90 natural elements. The Periodic Table arranges them so well that you can predict an element's chemistry from its address alone.

The big idea

Elements are ordered by proton number. Their position follows from their electron arrangement: group = outer electrons, period = shells.

1. Reading the table

Worked exampleFind the address

Phosphorus has proton number 15.

Goal: Group, period and type.
Strategy: Write the configuration.

$2,8,5$
Fill shells 2, 8, then 5.
$\text{group V, period 3}$
5 outer electrons, 3 shells.

AnswerGroup V, period 3, a non-metal.

✓ CheckIt's right of the metal–non-metal staircase ✓.

2. Trends

Trends at a glance

Drawing diagram…

Metals left, non-metals right.

Key takeawayDown a group Atoms get bigger. Metals lose electrons more easily (more reactive). Non-metals attract electrons less strongly (less reactive).

Edge caseArgon and potassium Argon (

A_r

40) comes before potassium (

A_r

39). Ordering by mass would put a noble gas among the alkali metals.

Pause and think

Germanium sits below silicon. Predict one property.

Summary

Order by proton number.
Group = outer electrons; period = shells.
Metal → non-metal across.

Group: A vertical column of elements with the same number of outer electrons.
Period: A horizontal row of elements with the same number of shells.
Metalloid: Element with properties between metals and non-metals.

HistoryMendeleev's cards. Dmitri Mendeleev is said to have written each element on a card and arranged them like a game of patience, until the pattern appeared to him, reportedly in a dream.

Fun factNamed after a village. Four elements, yttrium, ytterbium, terbium and erbium, are all named after Ytterby, a small village in Sweden whose quarry yielded the minerals.

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