Atomic Structure

Atoms are the tiny building blocks of everything. Each atom has a small, heavy nucleus in the centre with electrons moving around it in shells (energy levels).

Parts of an atom

• Proton: in the nucleus, charge $+1$, relative mass $1$
• Neutron: in the nucleus, charge $0$, relative mass $1$
• Electron: in shells, charge $-1$, relative mass $1/1836$ (very small)

Most of an atom’s mass is in the nucleus. In a neutral atom, number of electrons = number of protons.

Proton number and mass number

Atomic symbols can be written as $^{A}_{Z}\text{X}$.

• Proton (atomic) number, $Z$: number of protons. This identifies the element.
• Mass (nucleon) number, $A$: total protons + neutrons.
• Neutrons = $A - Z$

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons. Example: carbon-12 and carbon-14.

Electron shells (for proton numbers 1–20)

Electrons fill the lowest shell first. Maximums for the first 20 elements: first shell 2, second 8, third 8, fourth 2. Write configurations like “2,8,8,2”.

• H ($Z=1$) → 1
• O ($Z=8$) → 2,6
• Na ($Z=11$) → 2,8,1
• Ca ($Z=20$) → 2,8,8,2

Noble gases are unreactive because their outer shells are full.

Common mistakes

• Mass number is not the mass in grams; it counts protons + neutrons.
• Changing electrons makes ions; changing protons makes a different element.
• Electrons are not in the nucleus; they occupy shells around it.