Atoms and the Periodic Table

Introduction

In the Junior School CBC (KICD), Integrated Science studies the structure of matter. Everything is made of atoms, and the periodic table arranges the elements in an organised way.

Structure of an atom

In a neutral atom, protons equal electrons.

Atomic number and mass number

• Atomic number (Z) = number of protons.
• Mass number (A) = protons + neutrons.
• Neutrons = A - Z.

Example: Sodium has Z = 11 and A = 23, so it has 11 protons, 11 electrons and 12 neutrons.

Electron arrangement

Shells hold up to 2, then 8, then 8 electrons for the first 20 elements.

• Carbon (6): 2, 4
• Sodium (11): 2, 8, 1
• Chlorine (17): 2, 8, 7

The periodic table

Elements are arranged by increasing atomic number.

• Groups (vertical columns): same number of outer electrons, similar properties.
• Periods (horizontal rows): the number of occupied shells. Group I: reactive metals (lithium, sodium, potassium). Group VII: halogens. Group VIII (0): noble gases, unreactive.

Worked example

Where is chlorine in the table? Electron arrangement 2, 8, 7: three shells so Period 3; seven outer electrons so Group VII.

Importance

The table helps predict how elements react and which ones are metals or non-metals. Mendeleev proposed the first widely accepted table in 1869.

Try it

• Draw the electron arrangement of oxygen (atomic number 8).
• Find protons, electrons and neutrons for an atom with Z = 12 and A = 24.
• State the group and period of an element with arrangement 2, 8, 2.