The Structure of an Atom Explained for Students

What Is an Atom?

An atom is the smallest unit of matter that retains the properties of an element. Everything around you — water, air, your body, the screen you're reading this on — is made of atoms.

Atoms are incredibly small. About 10 million atoms side by side would span 1 millimeter.

Despite their tiny size, atoms have an internal structure with even smaller particles inside.

The Three Subatomic Particles

Atoms are made of three types of subatomic particles:

| Particle | Charge | Location | Relative Mass | |----------|--------|----------|---------------| | Proton | Positive (+1) | Nucleus | 1 | | Neutron | Neutral (0) | Nucleus | 1 | | Electron | Negative (-1) | Electron shells | ~1/1836 |

Key relationships:

• The number of protons determines what element the atom is
• In a neutral atom, the number of electrons equals the number of protons
• Neutrons add mass but don't affect the charge

The Nucleus

The nucleus is the tiny, dense center of the atom. It contains all the protons and neutrons, which means it holds almost all of the atom's mass (over 99.9%).

Despite containing most of the mass, the nucleus is incredibly small — about 100,000 times smaller than the atom itself.

Analogy: If an atom were the size of a football stadium, the nucleus would be a marble at the center. The rest is mostly empty space where electrons move.

The protons in the nucleus are all positively charged, so they repel each other. They're held together by the strong nuclear force, which is the strongest force in nature but only works at very short distances.

Electron Shells and Energy Levels

Electrons orbit the nucleus in regions called electron shells (or energy levels). Each shell can hold a limited number of electrons:

| Shell | Maximum Electrons | |-------|------------------| | 1st (closest) | 2 | | 2nd | 8 | | 3rd | 18 (usually 8 at this level in basic chemistry) | | 4th | 32 |

Electrons fill from the inside out. The innermost shell fills first, then the next, and so on.

Example — Sodium (Na), 11 electrons:

• Shell 1: 2 electrons
• Shell 2: 8 electrons
• Shell 3: 1 electron

The electrons in the outermost shell are called valence electrons. They determine how an atom bonds with other atoms. Learn more about this in our guide to chemical bonds.

Atomic Number and Mass Number

Atomic number (Z) = number of protons

This is what defines an element. Every atom of carbon has 6 protons. Every atom of oxygen has 8 protons. You can find the atomic number on the periodic table.

Mass number (A) = protons + neutrons

Example: Carbon-12 has 6 protons + 6 neutrons = mass number of 12.

Finding neutrons: Neutrons = Mass number - Atomic number

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.

Example — Carbon isotopes:

| Isotope | Protons | Neutrons | Mass Number | |---------|---------|----------|-------------| | Carbon-12 | 6 | 6 | 12 | | Carbon-13 | 6 | 7 | 13 | | Carbon-14 | 6 | 8 | 14 |

All three are carbon (6 protons), but they have different masses. Carbon-14 is radioactive and is used in carbon dating to determine the age of ancient objects.

Ions – Charged Atoms

An ion is an atom that has gained or lost electrons, giving it an electrical charge.

• Lose electrons → more protons than electrons → positive ion (cation)
  • Na → Na⁺ (sodium loses 1 electron)
• Gain electrons → more electrons than protons → negative ion (anion)
  • Cl → Cl⁻ (chlorine gains 1 electron)

Ions are crucial for chemical bonds — ionic bonds form when one atom gives electrons to another.

Learn with Mr Jarven

Atomic structure is the foundation of chemistry. Mr Jarven AI Tutor can help you understand protons, neutrons, electron configurations, and more at your own pace.

You can:

• Ask questions about specific elements and their structure
• Practice drawing electron shell diagrams
• Generate quizzes on atomic structure
• Move on to the periodic table or chemical bonds

Ready to learn chemistry? Ask Mr Jarven AI Tutor →