NOTES 4-1 Intro to Atoms

WHAT’S AN ATOM?

Smallest particle of an element

ATOMIC THEORIES

There have been many theories of proposed models of the atom.

Some were completely wrong!

And some were partially correct or close to correct.

We will look at some of the most known theories and models.

DALTON’S ATOMIC THEORY Dalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces.

He was credited with coming up with the earliest model of the atom

Most of this theory is still accepted today

THOMSON’S MODEL

Thomson suggested that atoms had negatively charged electrons embedded in a positive sphere.

This is the first model that included smaller particles of the atom

His model is similar to blueberries in a muffin

RUTHERFORD’S MODEL

Developed the first model that included a nucleus

Similar to a peach with a pit, the pit = the heavy mass of protons in the nucleus, the fruit = the empty space made up of electrons

RUTHERFORD’S GOLD FOIL EXPERIMENT Rutherford was surprised that a few particles were deflected strongly. This led him to propose an atomic model with a positively charged nucleus.

BOHR’S MODEL Said electrons could only have a certain amount of energy, so they must move in certain orbits

Similar to planets orbiting the sun (sun = nucleus), or the rings/layers of an onion

CLOUD MODEL Says Bohr’s model is wrong, electrons do not orbit the nucleus like planets do the sun

Instead, electrons can be found anywhere in a cloud-like region around the nucleus, rapidly orbiting in every direction

LATER ATOMIC MODELS Through the first part of the twentieth century, atomic models continued to change. REMEMBER: Electrons are the subatomic particles that are lost, gained or shared in chemical reactions.

MODERN ATOMIC MODEL Accepted since the 1930s

At the center of the atom is a massive nucleus, containing positively charged protons and neutrally charged neutrons (no charge)

Surrounding the nucleus is a cloud-like region of moving negatively charged electrons

# protons in atom = # electrons in atom

# neutrons can change

PARTICLES IN AN ATOM

An atom is composed of positively charged protons, neutral neutrons, and negatively charged electrons. Protons and neutrons are about equal in mass. An electron has about 1/2,000 the mass of a proton or neutron.

ATOMIC NUMBER Every atom of a given element has the same number of protons in its nucleus

This is known as the element’s atomic number

For example, Helium’s atomic number is 2, so every atom of helium has 2 protons in its nucleus. If the element has more than 2 protons, it is NOT helium!

Carbon’s atomic number is 6. How many protons does an atom of carbon have?

ISOTOPES: ATOMS OF ELEMENTS HAVE SAME NUMBER OF PROTONS, BUT DIFFERENT NUMBERS OF NEUTRONS

Atoms of all isotopes of carbon contain six protons and six electrons, but they differ in their number of neutrons. Carbon-12 is the most common isotope.

MASS NUMBER

An isotope is identified by its mass number

Mass number = #protons + #neutrons

Example: Carbon-12, the mass number is 12. We know carbon has 6 protons by looking at its atomic number on the periodic table. How many neutrons does carbon-12 have? 6

How do you know?Because if the mass number is 12, 12 – 6 protons = 6 neutrons

PROBLEM:

The atomic number of nitrogen is 7.

How many protons, neutrons, and electrons make up Nitrogen-15?

Protons = 7

Neutrons = 8

Electrons = 7

4-2, 4-3 AND 4-4 NOTES

Organizing the Elements

MENDELEEV’S PERIODIC TABLE 1869 Russian scientist who discovered a pattern of the elements

Arranged the elements in order of increasing atomic mass (average mass of all the isotopes of an element)

Then he lined them up in groups of similar properties

Developed the first periodic table of elements

Contained 63 elements

THE MODERN PERIODIC TABLE Changed as new elements were discovered

Contains over 100 elements

Arranged in order of increasing atomic number (not atomic mass like Mendeleev’s)

Atomic number = number of protons in an atom of that element

The properties of an element can be predicted from its location on the periodic table

THE PERIODIC TABLE Elements are organized into rows and columns based on their atomic number.

THE PERIODIC TABLE Elements are organized into rows and columns based on their atomic number.

THE PERIODIC TABLE Elements are organized into rows and columns based on their atomic number.

CLASSES OF ELEMENTS

Open your books to pages 134-135

Each colored region corresponds to a different class of elements

Metals = blue

Semimetals = yellow

Nonmetals = light green

Inert gases = dark green

PERIODS

Horizontal rows

There are 7 periods on the table, look how they are labeled on the left side of the table

Properties within each period change in a pattern

Notice the lanthanides and actinides have been taken out of the table and placed underneath it

This is to save space and make the table easier to read

Look at the expanded table on page 136

GROUPS

Vertical columns

Aka family

Elements within the same groups have similar characteristics

INVESTIGATION: DEFINE THE FOLLOWING WORDS AND CLASSIFY EACH (METAL, SEMIMETAL, NONMETAL OR INERT GAS)

Ductile

Malleable

Magnetic

Reactive

Noble gas

Lanthanides*

Actinides*

Brittle Good conductor Alloy Semimetal Alkaline earth metal(Group 2) Alkali metals(Group 1) Diatomic molecule Tracers (radioactive) Halogen

FINDING DATA ON ELEMENTS REVIEW: Each square in the periodic table lists four pieces of information: an element’s atomic number, chemical symbol, name, and atomic mass.

GROUPS (FAMILIES) HAVE SIMILAR CHARACTERISTICS

NOTES 4-5 Radioactive Elements

REVIEWING ISOTOPES

Atoms with same number of protons but different numbers of neutrons

Example:

Carbon-12 (normal carbon) versus Carbon-14

Some isotopes are unstable

Nuclei break down in a process of radioactive decay, releasing particles and energy

RADIOACTIVITY

Unstable nuclei of an element spontaneously emit radiation

Example: Uranium, Polonium, Radium

There are 3 types of radioactive decay

TYPES OF RADIOACTIVE DECAY During alpha decay, a nucleus loses an alpha particle, which consists of two protons and two neutrons.

* Decreases atomic # by 2, and mass # by 4

TYPES OF RADIOACTIVE DECAY During beta decay, a neutron inside an unstable nucleus changes into a negatively charged beta particle and a proton.

* Atomic # increases by 1, mass # remains the same

TYPES OF RADIOACTIVE DECAY Gamma radiation has no charge and does not cause a change in either the atomic mass or the atomic number.

TYPES OF RADIOACTIVE DECAY The three types of nuclear radiation were named based on how easily each one could be blocked. Alpha, beta, and gamma are the first three letters of the Greek alphabet.

USING RADIOACTIVE ISOTOPES•Cancer treatment

•Chemical reactions

•Industrial processes

•Electricity

•Smoke detectors

•Pest control

•Rock/ fossil dating

•Food treatment

•Nuclear weapons

RISKS OF RADIOACTIVE ISOTOPES Burn causing

Cancer causing

Genetic mutations

death

REVIEW Use your periodic table to predict the element that forms in each case of radioactive decay:

Isotope Type of Decay Element Formed

Uranium-238 Alpha

Nickel-63 Beta

Iodine-131 Beta

Radium-226 Alpha

Thorium-234Copper-63Xenon-131

Radon-222