What are Atoms? Atoms are tiny subunits that determine the properties of all matter.

In the 4th century B.C., the Greek philosopher Democritus suggested that the universe was made of invisible units called atoms. The word atom is derived from the Greek word meaning “unable to be divided”

Many people did not believe Democritus because there was not enough proof. Over the next several hundred years, many theories were created but none were accepted because of the lack of scientific evidence.

As the science of chemistry was developing, a new theory was developed by an English school teacher named John Dalton.

This theory was widely accepted because there was scientific evidence that supported it.

Dalton’s Theory: Every element is made of tiny, unique particles called atoms that cannot be divided. Atoms of the same element are exactly alike Atoms of the different elements can join to form molecules

Less than 100 years after Dalton proposed his theory, scientists determined that atoms could be split, or broken down into smaller parts

Nucleus: the center of an atom; made up of protons and neutrons

Protons: a positively charged subatomic particle in the nucleus of an atom

Neutron: a neutral subatomic particle in the nucleus of an atom

Electron: a tiny negatively charged subatomic particle moving around the outside of the nucleus of an atom

Atoms have no charge. Even though atoms are made up of charged protons and electrons, they do not have an overall charge. This is because the charge from the protons cancels the charge from the electrons.

Helium Atom: Charge of two protons: +2 Charge of two neutrons: Charge of two electrons: -2 Total charge of a helium atom:

Electron Energy Levels Orbital path of electrons around nucleus First level can hold two electrons Second and third levels can hold eight electrons

Valence electrons: electrons that are found in the outer most energy level.