Science Basics: Atoms & Molecules

Everything in the universe is made of atoms. Here is what they are and how they work.

What Is an Atom?

An atom is the smallest unit of a chemical element that still has the properties of that element. Everything you can see, touch, and breathe is made of atoms. A single grain of sand contains more atoms than there are stars in the Milky Way galaxy.

Atoms are incredibly small. You cannot see one with a regular microscope.

Parts of an Atom

An atom has three main parts. Two of them live in the center (called the nucleus), and one orbits the outside.

             Electrons (orbit outside)
               *   *
           *           *
         *               *
        *    [NUCLEUS]    *
         *               *
           *           *
               *   *

  NUCLEUS contains:
  (+) Protons   -- positively charged
  (o) Neutrons  -- no charge (neutral)

  OUTSIDE nucleus:
  (-) Electrons -- negatively charged, move in shells

Protons

Protons carry a positive electric charge. The number of protons in an atom determines what element it is. This number is called the atomic number.

Hydrogen has 1 proton. Carbon has 6 protons. Gold has 79 protons.

Neutrons

Neutrons have no electric charge. They sit alongside protons in the nucleus and add to the atom's mass. Different versions of the same element (called isotopes) have different numbers of neutrons.

Electrons

Electrons carry a negative charge. They orbit the nucleus in layers called electron shells. In a neutral atom, the number of electrons equals the number of protons.

Memory Trick ProNucleus: Protons and Neutrons are in the Nucleus. Electrons Escape to the Exterior.

Electron Shells

Electrons do not float randomly around the nucleus. They occupy specific energy levels called shells. Each shell has a maximum number of electrons it can hold.

Shell Max Electrons Notes
1st shell (K)2Closest to nucleus
2nd shell (L)8Most common for simple elements
3rd shell (M)18Holds up to 18 in full

Example: Carbon Atom (atomic number 6)

Carbon has 6 protons, 6 neutrons, and 6 electrons.

Shell 1: 2 electrons | Shell 2: 4 electrons

   Shell 1:  2 electrons  (full)
   Shell 2:  4 electrons  (has room for 4 more)

Those 4 empty spots in the outer shell are why carbon bonds so easily with other atoms.

Elements and the Periodic Table

An element is a substance made of only one type of atom. Gold is made only of gold atoms. Oxygen is made only of oxygen atoms.

The periodic table organizes all known elements by their atomic number (number of protons). As of today, 118 elements have been confirmed.

  Common Elements:
  H  - Hydrogen  (1 proton)
  C  - Carbon    (6 protons)
  N  - Nitrogen  (7 protons)
  O  - Oxygen    (8 protons)
  Na - Sodium    (11 protons)
  Fe - Iron      (26 protons)
  Au - Gold      (79 protons)

What Is a Molecule?

When two or more atoms join together, they form a molecule. Most substances you encounter are not single atoms - they are molecules.

Familiar Molecules

  Water Molecule (H2O):

       H   H
        \ /
         O

  Carbon Dioxide (CO2):

   O = C = O

How Atoms Bond

Atoms bond because they want a full outer electron shell - it makes them more stable. There are three main ways they do this.

Ionic Bonds

One atom gives an electron to another. This creates two charged particles (ions) that attract each other. Salt (NaCl) is the classic example: sodium gives an electron to chlorine.

Covalent Bonds

Two atoms share electrons rather than transfer them. Water (H2O) uses covalent bonds. This type is very common in biological molecules.

Metallic Bonds

In metals, electrons float freely between many atoms. This is why metals conduct electricity so well.

Why This Matters

Understanding atoms and molecules explains a huge amount about the world:

Summary

Frequently Asked Questions

Is an atom the smallest thing that exists?
No. Atoms themselves are made of smaller particles: protons, neutrons, and electrons. Protons and neutrons are made of even smaller particles called quarks. At the level of everyday chemistry, however, the atom is the smallest meaningful unit.
What is the difference between an atom and a molecule?
An atom is a single particle of one element. A molecule is two or more atoms bonded together. A molecule can be made of the same type of atom (like O2) or different types (like H2O).
Why do atoms bond with each other?
Atoms are more stable when their outer electron shell is full. Bonding with other atoms lets them share or transfer electrons to fill that outer shell. It is purely a matter of reaching a lower energy, more stable state.
What is an isotope?
An isotope is a version of an element that has the same number of protons but a different number of neutrons. For example, carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Both are carbon, but carbon-14 is radioactive and is used in dating ancient materials.
How do scientists study atoms if they are invisible?
Scientists use techniques like X-ray crystallography, electron microscopy, and particle accelerators. These tools detect how atoms interact with energy or particles, letting scientists build detailed models of atomic structure without ever seeing an atom directly.

Quick Quiz

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