The Basics of Atomic Structure

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The location and charge of protons, neutrons, and electrons
An atom consists of a nucleus and the electrons surrounding it. Inside the nucleus, positively (+) charged protons and uncharged neutrons are packed together, while negatively (−) charged electrons move rapidly around the outside of the nucleus. The nucleus takes up only a tiny fraction of the atom's total volume but accounts for most of its mass.
The meaning of atomic number and mass number
The atomic number indicates the number of protons in the nucleus, and this is the unique value that determines which element it is. The mass number is the sum of the number of protons and neutrons, giving an approximate figure for the mass of the nucleus. For example, carbon has an atomic number of 6, meaning it has 6 protons, and its most common isotope, carbon-12, has a mass number of 12 (6 protons plus 6 neutrons).
The concept of electron shells (energy levels)
Electrons don't orbit the nucleus just anywhere — they fill a set number of positions within fixed energy-level orbits (electron shells), such as the K, L, M, and N shells. The maximum number of electrons a given shell can hold follows the formula 2n² (where n is the shell number), and the outermost electrons in the outermost shell determine chemical bonding and reactivity.
The concept of isotopes
Isotopes are atoms that have the same number of protons (atomic number) but a different number of neutrons, giving them different mass numbers. Their chemical properties are nearly identical, but their mass and stability can differ, and some isotopes are unstable, decaying while emitting radiation as radioactive isotopes. For example, carbon-12 and carbon-14 are isotopes of the same element, carbon, but with different numbers of neutrons.
The difference between an atom and a molecule
An atom is the smallest basic unit that makes up matter, while a molecule is a particle formed when two or more atoms are joined by chemical bonds, giving it independent properties. For example, a single oxygen atom (O) is highly unstable, but an oxygen molecule (O₂), formed by two oxygen atoms bonding together, exists as the stable gas we breathe. When atoms of different elements bond together, as in a water molecule (H₂O), the result is called a compound.
The historical development of atomic models
In 1911, Rutherford discovered that a small, heavy nucleus exists at the center of an atom through an experiment in which he fired alpha particles at thin gold foil. Afterward, in 1913, Bohr proposed a model in which electrons exist only in specific orbits (energy levels), and modern quantum mechanics uses an electron cloud (orbital) model that represents the probability of finding an electron rather than its exact location.
The principle behind how ions form
When an atom loses or gains electrons, the number of protons and electrons no longer match, and the atom becomes a charged ion. Losing an electron creates a positively (+) charged cation, while gaining an electron creates a negatively (−) charged anion. When a sodium atom loses one electron to become Na⁺ and a chlorine atom gains one electron to become Cl⁻, and the two bond together, sodium chloride (table salt) is formed.

Why do we need to understand atoms?

Every substance around us is made up of extremely small atoms invisible to the eye. Understanding the internal structure and properties of atoms allows us to logically explain how substances bond to form molecules and why different elements have different chemical properties.

It's easier alongside the periodic table

Once you've mastered the concepts of atomic number and outermost electrons, take a look at our guide to reading the periodic table as well. Your knowledge of atomic structure carries directly over into understanding the periodic table's layout.

Frequently Asked Questions

How small a portion of an atom does the nucleus take up?

The nucleus is extremely small, only about one ten-thousandth of the atom's total diameter, but it accounts for most of the atom's mass (over 99.9%). Most of an atom's space is effectively the near-empty region where electrons are active.

Are there atoms without any neutrons?

Yes — the most common hydrogen atom (hydrogen-1) has just one proton and one electron, with no neutrons. Deuterium, an isotope of hydrogen, has one neutron, and tritium has two.