Atoms, Elements and the Periodic Table: Mechanism

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to Atoms, Elements and the Periodic Table.

An atom is the smallest particle of an element that still behaves like that element. An element is a pure substance made of only one kind of atom — it cannot be broken down into simpler substances by chemical means. About 118 elements are known, and roughly 90 of them occur naturally.

Every element has a symbol: either one capital letter ($\text{H}$, $\text{C}$, $\text{O}$) or a capital followed by a small letter ($\text{Na}$ for sodium, $\text{Fe}$ for iron, $\text{Cu}$ for copper, $\text{Au}$ for gold). Several symbols come from Latin names — natrium, ferrum, cuprum, aurum — which is why they look nothing like the English word.

The periodic table arranges all the elements in order of increasing atomic number, which is the number of protons in one atom of that element.

• Periods are the horizontal rows.
• Groups are the vertical columns. Elements in the same group behave in similar ways because they have the same number of electrons in their outer shell.
• Group 1 (the alkali metals: $\text{Li}$, $\text{Na}$, $\text{K}$) are soft, very reactive metals stored under oil.
• Group 7 / 17 (the halogens: $\text{F}$, $\text{Cl}$, $\text{Br}$) are reactive non-metals.
• Group 8 / 18 (the noble gases: $\text{He}$, $\text{Ne}$, $\text{Ar}$) are almost completely unreactive.
• Metals occupy the left and middle: shiny, malleable, good conductors of heat and electricity. Non-metals sit on the right: mostly dull, brittle when solid, and poor conductors.

Worked example — reading sodium off the table. Sodium's symbol is $\text{Na}$ and its atomic number is 11, so one sodium atom has 11 protons. It sits in group 1, so it is a soft, highly reactive metal — which is exactly why a laboratory keeps it under oil, away from the water in the air.