Atom Structure

The smallest particle of an element that retains the properties of the element is called

 

an atom. 

An instrument called the scanning tunneling microscope (STM) allows

 

individual atoms to be seen. 

The Cathode-Ray Tube - When an electric charge is applied, a ray of radiation, called a cathode ray, travels from the 

cathode to the anode. It is called a cathode ray. 

Cathode rays are a stream of particles carrying a

negative charge.

 electrons. 

The particles carrying a negative charge

Charge-to-Mass Ratio - Cathode-ray experiments were used to

 determine the ratio of an electron’s charge to its mass.  

Results showed that the mass of the charged particle was much less than

that of a hydrogen atom, the lightest known atom.  

Results showed that Dalton had been incorrect; atoms were

 divisible into smaller subatomic particles. 

The Oil-Drop Experiment and the Charge of an Electron 

The charge of an electron was determined using an oil-drop apparatus. 

The Plum Pudding Model 

J. J. Thomson’s proposed model of the atom had:  

A spherical shape  

Uniformly distributed positive charge in which the

individual negatively charged electrons resided 

Rutherford studied how

positively charged alpha particles interacted with solid matter. 

By aiming the particles at a thin sheet of gold foil, he expected the

paths of the alpha particles to be only slightly altered by a collision with an electron.  

Although most of the alpha particles went through the gold foil,

a few of them bounced back, some at large angles. 

Rutherford concluded that the plum pudding model was incorrect because it

could not explain the results of the gold foil experiment. 

In Rutherford’s nuclear model, the atom is composed of a

dense, positively charged nucleus surrounded by negative electrons. 

proton

a subatomic particle carrying a charge equal to but opposite that of an electron.  

neutron

a subatomic particle that has a mass nearly equal to that of a proton, but which carries no electric charge. 

• Atoms of each element contain

 a unique positive charge in the nuclei.

The number of protons in an atom is called the

 atomic number.  

In the periodic table, each element is represented by its

chemical name, atomic number, chemical symbol, and average atomic mass. 

Elements are listed in their

natural state on the periodic table

Therefore, the number of protons and electrons for an element are

equal.  

atomic number

= number of protons 

  = number of electrons 

The atomic number of an atom equals it

number of protons and its number of electrons 

 

All atoms of an element have the same number of protons and electrons, but the number of neutrons can   

 

differ

isotopes.  

 

Atoms with the same number of protons but different number of neutrons

The mass number identifies each . 

isotope of an element

mass number

It is the sum of the atomic number (or number of protons) and neutrons in the nucleus 

In nature, most elements are found as

mixtures of isotopes.  

 

Usually, the relative abundance of each isotope

is constant.

Different sources containing atoms of potassium have the same

percent composition of isotopes.  

One atomic mass unit (amu) is defined as

one-twelfth the mass of a carbon-12 atom.  

The atomic mass of an element is the

weighted average mass of the isotopes of that element. 

the atomic number is the electrons and protons when

the particle is an atom, neutal

If the atomic differs from the electron number it is

an ion

DO NOT ASSUME THE PROTONS EQUAL THE 

ELECTRON

especially if there is a charge that is

not neutral : -/+

neutral charge

atoms

postive charge

more protons

negative charge

more electrons

charge

p+ - e-

to solve the electron number

check charge, define variable and solve

use the intial p+e to check

the charge

Hyphenic notation

chemical name/symbol - mass number, not average of mass

notation of element

atomic number(z) bottom left, Mass number (A) top left, charge top right, if 0, dont write, add note

use what is given

to solve for the missing values.