Atomic Structure and Electronic Configuration

Manual: General · Subject: Chemistry

Build a deeper model of atoms, isotopes, and electron arrangement to explain chemical behavior.

From Particles to Atoms

Atoms in chemistry

Atoms are composed of a dense nucleus containing protons and neutrons, surrounded by electrons in quantized energy levels. The number of protons, $Z$ , defines the element, while the mass number is $A = Z + N$ , where $N$ is the number of neutrons.

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Why isotopes matter

Isotopes have the same atomic number but different neutron counts, so they behave almost identically in chemistry but can differ in mass, stability, and nuclear properties.

Quantum model

Electrons do not orbit in fixed planetary paths; instead, they occupy orbitals described by wavefunctions. Each orbital is a region of high probability, and the allowed states are constrained by quantum numbers $n$ , $l$ , $m_l$ , and $m_s$ .

Quantum numbers

Quantum number	Meaning	Allowed values
$n$	Principal level	$1,2,3,\dots$
$l$	Subshell shape	$0$ to $n-1$
$m_l$	Orbital orientation	$-l$ to $+l$
$m_s$	Spin	$+\tfrac12$ or $-\tfrac12$

Filling orbitals

1

Step 1: Fill lower-energy orbitals first using the Aufbau principle.
2

Step 2: Place one electron in each degenerate orbital before pairing, following Hund's rule.
3

Step 3: Never put two electrons in the same orbital with the same spin, due to the Pauli exclusion principle.

Which quantum number primarily determines the size and energy of an orbital?

n

l

m_l

m_s

How many electrons can occupy a single orbital?

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Key takeaway

Electron configuration explains periodic trends, bonding behavior, and the typical charges atoms form in compounds.

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Atomic Structure and Electronic Configuration

From Particles to Atoms

Atoms in chemistry

Quantum model

Filling orbitals