⚗️ Chemistry introductory Lesson 1 of 5 1 min read

Atomic Structure and Periodic Trends

How protons, neutrons, and electrons build every element, and how the periodic table organizes the trends that predict chemical behavior.

Reading level

What you'll learn

Identify the three subatomic particles and describe their charge, mass, and location in the atom.
Use atomic number and mass number to distinguish elements, isotopes, and ions.
Apply the Aufbau principle, Pauli exclusion principle, and Hund's rule to write electron configurations.
Predict periodic trends in atomic radius, ionization energy, and electronegativity.

The Atom

An atom consists of:

Nucleus — protons (charge +1) and neutrons (charge 0); defines mass number (A = Z + N)
Electron cloud — electrons (charge −1) arranged in shells and subshells

Atomic number (Z) = number of protons = number of electrons (in a neutral atom).

Isotopes share the same Z but differ in neutron count (e.g., ¹²C vs. ¹⁴C).

Electron Configuration

Electrons occupy orbitals following:

Aufbau principle — fill lowest energy levels first
Pauli exclusion principle — each orbital holds ≤2 electrons with opposite spins
Hund’s rule — within a subshell, electrons occupy separate orbitals singly before pairing

Subshells in order of filling: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p …

Example — Iron (Fe, Z=26): [Ar] 3d⁶ 4s²

Periodic Trends

Property	Trend across period (→)	Trend down group (↓)
Atomic radius	Decreases (↑ nuclear charge)	Increases (more shells)
Ionization energy	Increases	Decreases
Electronegativity	Increases	Decreases
Electron affinity	Generally increases	Decreases

Valence Electrons and Reactivity

Elements in the same group share the same number of valence electrons and similar chemical properties. Group 1 (alkali metals) have 1 valence electron and are highly reactive; noble gases (Group 18) have full shells and are inert.

Medical Relevance

Understanding atomic structure explains why radioactive isotopes (e.g., ¹³¹I for thyroid imaging/therapy, ¹⁸F for PET scans) are medically useful: unstable nuclei emit detectable radiation as they decay to stable configurations.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

Orbitals, effective nuclear charge, and periodic trends

Electrons occupy quantized orbitals (s, p, d, f) that are three-dimensional probability distributions, not fixed paths; s orbitals are spherical while the three p orbitals are dumbbell-shaped along perpendicular axes. Inner electrons partially screen the nucleus, so a valence electron feels an effective nuclear charge (Zeff) smaller than Z. Zeff rising across a period while the principal quantum number stays constant is what actually drives the contraction of atomic radius and the rise in ionization energy and electronegativity.

Isotopes in nuclear medicine

Because isotopes differ only in neutron number, they are chemically nearly identical but can be nuclear-unstable, which is exploited diagnostically. Technetium-99m, a metastable isomer, emits a clean 140 keV gamma photon ideal for SPECT imaging and decays with a ~6-hour half-life. Positron emitters such as fluorine-18 in FDG annihilate with electrons to produce back-to-back 511 keV photons that PET scanners detect in coincidence.

Key terms

Atomic number (Z): The number of protons in an atom's nucleus, which defines the element's identity.
Mass number (A): The total count of protons and neutrons in a nucleus (A = Z + N).
Isotope: Atoms of the same element that share a proton number but differ in neutron number.
Valence electrons: The electrons in an atom's outermost shell, which govern its chemical reactivity and bonding.
Electron configuration: The arrangement of an atom's electrons among its orbitals and subshells.
Ionization energy: The energy required to remove an electron from a gaseous atom or ion.
Electronegativity: A measure of an atom's tendency to attract shared electrons in a chemical bond.
Ion: An atom or group of atoms carrying a net electric charge from gaining or losing electrons.

Check your understanding

5 questions · answers reveal instantly.

1.What determines the identity of a chemical element?
2.Two atoms are both carbon but have mass numbers of 12 and 14. They are best described as:
3.Moving left to right across a period, atomic radius generally:
4.Which group of elements is chemically inert because of full valence shells?
5.Which principle states that each orbital holds a maximum of two electrons with opposite spins?

Citations & References

Links open publicly available educational and peer-reviewed sources.