# Chapter 4

## 4.1 Early Theories of Matter

The Greek Philosopher Democritus was the first person to propose the existence of atoms. In 1808, Dalton proposed his atomic theory, which was based on numerous scientific experiments. All matter is composed of atoms. An atom is the smallest particle of an element. Atoms of one element are different from atoms of other elements. The vocabulary:

Daltons Atomic theory- based on the promise that the atoms of different elements could be distinguished by differences in their weight.

Atom- a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons.

## 4.2 Subatomic Particles and the Nuclear Atom

Atoms are composed of negatively charged electrons, neutral neutrons, and positively charged protons. Electrons have a 1-charge, protons have a 1+ charge, and neutrons have no charge. Both protons and electrons have masses approximately 1840 times that of an electron. The nucleus of an atom contains all of its positive charge and nearly all of its mass. The nucleus occupies an extremely small volume of space at the center of an atom. Most of an atom consists of empty space surrounding the nucleus through which the electrons move. Vocabulary:

Cathode- a electrode through which electric current flows out of a polarized electrical device.

electron- a subatomic particle with a negative elementary electric charge.

nucleus- a membrane enclosed oganelle found in eukaryotic cells

proton- a subatomic particle with the p or p+ symbol and a positive electric charge of 1 charge.

neutron- a subatomic hadron particle which has the symbol n or n0, no net electric charge and mass slightly larger than protons.

## 4.3 How Atoms Differ

The number of protons in an atoms uniquely identifies an atom. This number of protons is the atomic number of the atom. Atoms have equal numbers of protons and electrons, and thus, no overall electrical charge. An atom's mass number is equal to its total number of protons and neutrons. Atoms of the same element with different numbers of neutrons and different masses are called isotopes. The atomic mass of an element is a weighted average of the masses of all the naturally occurring isotopes of that element. Vocabulary:

Atomic Number- is the number of protons found in the nucleus of an atom.

isotope- Variants of a particular chemical element.

mass number- the total number of protons and neutrons.

atomic mass unit- the standard unit that is used for indicating mass on an atomic or molecular scale.

atomic mass- the mass of a specific isotope.