The Periodic Table and Atomic Structure

The Periodic Table contains all known elements, arranged in order of increasing atomic number (Z). The shape is no accident — elements line up by their chemical properties.

Periods are the rows (1 to 7). The period number tells you how many electron shells the element's atoms have. Period 2 elements have 2 shells (e.g. Li, Be, B, C, N, O, F, Ne); Period 3 have 3 shells.

Groups are the columns (1 to 18, or 1, 2, 3-12, 13-18 with old notation). Elements in the same group have similar chemical properties because they have the same number of outer-shell electrons. For main-group elements:

• Group 1 = 1 outer electron (alkali metals: Li, Na, K ...).
• Group 2 = 2 outer electrons (alkaline earth metals).
• Group 7/17 = 7 outer electrons (halogens: F, Cl, Br ...).
• Group 0/18 = 8 (or 2 for He) — full outer shell, NOBLE GASES.

In 1869, Russian chemist Dmitri Mendeleev noticed that elements showed REPEATING (periodic) patterns of properties when arranged by atomic mass. He drew up a table — but with a stroke of genius, he LEFT GAPS where the pattern said an element should be but none was known.

He predicted the properties of these missing elements. When gallium (1875) and germanium (1886) were discovered, their properties matched Mendeleev's predictions almost exactly — incredible confirmation that the Periodic Table was capturing something real about nature.

Today's table differs from Mendeleev's in two main ways:

1. Elements ordered by atomic number (not atomic mass) — fixed a few anomalies.
2. Includes the noble gases (Group 0/18), which Mendeleev didn't know existed because they're so unreactive they were hard to detect.

MYP criterion D often uses Mendeleev's story to test how science progresses: prediction → experiment → confirmation.

Two main trend directions to know:

Down a group:

• Atomic radius INCREASES (more shells).
• For alkali metals (Group 1), reactivity INCREASES down the group (outer electron is easier to lose because it's farther from the nucleus).
• For halogens (Group 7/17), reactivity DECREASES down the group (it's harder to attract an extra electron from farther away).

Across a period (left to right):

• Atomic radius generally DECREASES (more protons pull electrons inwards).
• Metallic character DECREASES (left = metallic; right = non-metallic).
• The element on the FAR RIGHT is always a noble gas — full outer shell, unreactive.

These trends fall straight out of electron configuration — which is the deep reason the Periodic Table looks the way it does.