The Periodic Table

The Periodic Table is a chart that arranges all known elements in a useful order. It helps us predict how elements behave in reactions.

How elements are arranged

Elements are ordered by increasing atomic number (proton number). The atomic number is the number of protons in the nucleus: $Z = \text{number of protons}$.

• Periods (rows): going left to right, atomic number increases. All atoms in the same period have the same number of electron shells.
• Groups (columns): elements in a group have the same number of electrons in their outer shell, so they have similar chemical properties.

Think of the table like a city map: rows are streets (periods), columns are buildings with similar designs (groups).

Metals and non-metals

• Metals are mostly on the left and centre. They are shiny, conduct electricity, and usually form positive ions.
• Non-metals are on the right. They are often dull, do not conduct, and usually form negative ions or share electrons.
• Across a period, elements change from metallic to non-metallic character.

Key groups to know

• Group 1 (Alkali metals): 1 outer electron; form $+1$ ions (e.g. Na$^+$). Reactivity increases down the group.
• Group 2 (Alkaline earth metals): 2 outer electrons; form $+2$ ions (e.g. Mg$^{2+}$).
• Group 17 (Halogens): 7 outer electrons; form $-1$ ions (e.g. Cl$^-$). Reactivity decreases down the group.
• Group 18 (Noble gases) (Group 0/VIII): full outer shell; very unreactive; used in lights and balloons.
• Transition metals (middle block): good conductors, high melting points, form coloured compounds, and can have several ion charges.

Using position to predict ions and behavior

• Group number (for main groups) ≈ number of outer electrons.
• Metals (Groups 1–2) lose electrons to form $+$ ions; non-metals (Groups 16–17) gain electrons to form $-$ ions.
• Period number = number of electron shells in the atom.

Example: Magnesium is in Group 2, Period 3 → 2 outer electrons, 3 shells → forms Mg$^{2+}$ and reacts similarly to calcium.

Trends to remember

• Across a period: atoms get smaller; metallic to non-metallic; oxides change from basic to amphoteric to acidic.
• Down a group: atoms get larger (more shells). Group 1 becomes more reactive; Group 17 becomes less reactive.

Common misconceptions

• Atomic number is not the same as mass number. The table is ordered by atomic number (protons), not mass.
• Elements in the same period do not all have similar reactions; similarity is within a group.