Chemistry

The Scale of Atoms

Atoms are tiny.  Ridiculously tiny.  In this comparison with a human hair, you'll see just how small the world of small can be.

Early Evidence of Atoms

The discovery of atoms is relatively recent — we had no real evidence of atoms until the early 1800s, and our current understanding of atomic structure has only come through experiments since the early 1900s.

Experiments that Revealed Atoms

You can't directly see atoms with your unaided eye, so how do we know they exist?  These experiments revealed the existence of atoms and helped us understand their properties.

Structure of Atoms

Atoms can be broken down into two key regions:

• Nucleus: the tiny, heavy island of protons and neutrons in the center of the atom.

• Electron Orbital Cloud: the field of mostly empty space through which the electrons travel.

In these two regions, we have three components:

• Protons provide the positive charge in the nucleus.  The total number of protons defines the identity of the atom.

• Neutrons have no charge, but each one is approximately the same mass as a proton.  They also reside in the nucleus.

• Electrons have a negative charge, but they are very, very tiny and they travel very, very fast.  They orbit the nucleus.

Writing Atomic Symbols

When writing about chemistry, atomic symbols provide a shorthand method to reveal the key information about each atom:

Atomic Symbol: the number of protons determines the identity of the atom.

Atomic Mass: the number of protons and neutrons together gives the atomic mass.  Isotopes of an element all have the name number of protons, but the mass changes depending on the number of neutrons.

Electric Charge: the number of protons minus the number of electrons gives the charge of the atom.  Depending on the element, an atom may be more likely to give or take electrons when it bonds with other elements.

Molecules

Every substance around us is composed of combinations of atoms.  A few materials like cast iron, 24-carat gold, and diamond may be composed of pure elements, but substances like water, table salt, and ammonia can only represented by formulas containing multiple elements.  These specific combinations of atoms are called molecules.

Ions

Some substances don't form discrete molecules that you can easily separate.  Instead, the individual atoms either gain or lose electrons to become charged ions — the positive ions then stick to the negative ions through the strength of that electric charge.

Emperical Formulas

To identify molecules on paper, you need to correctly write down the combinations of elements that produce the given substance.  And empirical formula is the ratio of elements necessary to create the given substance. 

Empirical formulas consist of two key parts:

• Atomic Symbols for each element present.

• Subscripts (when needed) to indicate the number of each type of atom present.

Structural Formulas

• An empirical formula only gives the ratio of atoms present — it doesn't reveal how these atoms are connected together.  A structural formula gives the positions of the atoms within each molecule.  Sometimes, two different substances might have the exact same ratios of elements, but the different arrangements of these atoms can lead to very different chemical properties.