3.2 What is an Atom?

📚 Key Concepts

The Building Blocks of Matter: Atoms are the fundamental units of all matter in the universe. They are incredibly small - smaller than anything we can imagine or compare with in our everyday experience.

Atomic Radius: The size of atoms is measured in nanometres (nm). One nanometre is 1/10⁹ meters - that's one billionth of a meter!

Atomic Mass: Each element has a characteristic atomic mass, measured in atomic mass units (u), where 1 u equals 1/12 the mass of a carbon-12 atom.

Table 3.1: Symbols for some elements

Element	Symbol	Element	Symbol	Element	Symbol
Aluminium	Al	Copper	Cu	Nitrogen	N
Argon	Ar	Fluorine	F	Oxygen	O
Barium	Ba	Gold	Au	Potassium	K
Boron	B	Hydrogen	H	Silicon	Si
Bromine	Br	Iodine	I	Silver	Ag
Calcium	Ca	Iron	Fe	Sodium	Na
Carbon	C	Lead	Pb	Sulphur	S
Chlorine	Cl	Magnesium	Mg	Uranium	U
Cobalt	Co	Neon	Ne	Zinc	Zn

Table 3.2: Atomic masses of a few elements

Element	Atomic Mass (u)
Hydrogen	1
Carbon	12
Nitrogen	14
Oxygen	16
Sodium	23
Magnesium	24
Sulphur	32
Chlorine	35.5
Calcium	40

Test Your Understanding

1. Define the atomic mass unit.

An atomic mass unit (u) is a mass unit equal to exactly one-twelfth (1/12) the mass of one atom of carbon-12. This standard was chosen in 1961 as a universal reference for measuring atomic masses.

2. Why is it not possible to see an atom with naked eyes?

Atoms are extremely small - typically around 10⁻¹⁰ meters in diameter. This is far smaller than the wavelength of visible light (400-700 nm), which means light cannot resolve individual atoms. Our eyes can only see objects that are at least about 0.1 mm in size, which is about a million times larger than an atom!

Welcome to the Atomic World

Relative Sizes (in meters)

H

📚 Key Concepts

Table 3.1: Symbols for some elements

Table 3.2: Atomic masses of a few elements

Test Your Understanding