Introduction to Atoms

Chapter 4: Section 1

The Atom is the smallest particle of an element

Atomic Models

The Atomic theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised.

All elements are composed of atoms that cannot be divided

Dalton’s Theory:

All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses

An atom of one element cannot be changed into an atom of another element.

Atoms cannot be created or destroyed in any chemical change, only rearranged

◦Law of ____________ __ ____

Every compound is composed of atoms of different elements, combined in a specific ratio

 

Discovered that atoms had negatively charged particles and reasoned that the atom had to contain positively charged particles because the atom is electrically neutral

 

Thomson:

Inferred that an atom’s positive charge must be clustered in a tiny region in its center called a nucleus

Rutherford:

Electrons moved around the nucleus

He also named protons

Theory came from Gold foil experiment. (see figure 4 & 5, pages 104 & 105)

some of the alpha particles had run into massive concentrations of positive charge and, since like charges repel, had been hurled straight back by them

Showed that electrons could only have specific amounts of energy, leading them to move in certain orbits around the nucleus

Bohr:

Electrons do not orbit the nucleus like planets, but can be anywhere in a cloud like region around the nucleus

Electron Cloud

An electron’s movement is related to its energy level. The specific amount of energy the electron

has

Electrons of different energy levels are found in different places.

James Chadwick discovered the neutrons, which are found in the nucleus, they have no charge and almost as much mass as the protons

Modern Atomic Model

The model consists of an atom made up of a nucleus which contains the protons and neutrons surrounded by a cloudlike region of moving electrons contained in energy levels

Protons are shown by a plus sign(1+), AMU = 1

Particle Charges

Electrons are shown by a minus sign(1-), AMU = 1/ 1, 836

◦Neutrons are shown by a zero sign(0), ◦ AMU = 1

 

Every atom of an element has the same number of protons

Atomic Number

The atomic number identifies the element

Since an element is electrically neutral, the atomic number also equals the number of electrons.

The number of neutrons in an atom can vary.

Isotopes and Mass Number

◦Atoms with the same number of protons but different numbers of neutrons are called isotopes.

◦ Figure 10, page 108

An isotope is identified by its mass number

The sum of protons and neutrons in the nucleus

Mass Number

◦ Number of neutrons can be found by subtracting the atomic number FROM the Mass number.

  Mass number minus Atomic number = Neutron

number