Physical Science Notes
Chapter 4 Atoms and the Periodic Table
January 5, 2022
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4.1 Atomic Structure
What are atoms?
Our understanding of atoms required many centuries
Democritus was a Greek philosopher who started the research into the atom.
John Dalton developed an atomic theory
He established that:
All atoms of an element are alike.
Atoms could join together to make compounds.
Atoms are the building blocks of molecules
Atoms are the smallest units that keep the original properties.
What’s in an Atom?
Atoms are made of protons, neutrons, and electrons
Atoms are made up of three particles
Protons, the positively charged particle
Neutrons, the neutrally charged particle
Electrons, the negatively charged particle
Nucleus
Center of the atom
Made up of protons and neutrons
Protons and neutrons are almost identical in size and mass.
Unreacted atoms have no overall charge
An atom will have the same number of electrons as protons.
Protons and electrons have equal but opposite charges.
This makes the atom have a neutral charge.
Models of the Atom
Bohr’s model compares electrons to planets
This model established where the electrons are similar to the orbit of the planets.
The orbits they had was determined by their energy level.
More energy moved them away from the nucleus.
Loss of energy moved them in towards the nucleus.
The electron levels are still considered accurate.
Electrons act more like waves
While the levels are accurate the electrons movement between levels changed.
The electrons are now seen to behave like a vibrating string.
An electron’s exact location cannot be determined
Due to the electrons movement it is impossible to determine both exact location and speed and
direction.
Instead scientists call the area an electron is likely to be found the electron cloud.
Electrons exist in energy levels
The energy levels hold different amounts of electrons.
The closest in levels are filled first.
Electrons are found in orbitals within energy levels
The region where electrons are likely to be found are called orbitals.
Four kinds of orbitals s, p, d, and f
Every atom has between one and eight valence electrons.
Valence electrons determine the chemical properties and ability to form bonds.
4.2 A Guided Tour of the Periodic Table
Organization of the Periodic Table
The periodic table is organized to have elements with similar properties grouped together.
This is due to regular patterns in elemental properties.
The periodic table helps determine electron arrangement.
The horizontal rows on the periodic table are called periods.
The period number goes up as you move down the periodic table.
The period number tells you which energy level the valence electrons are in.
Elements in the same group have similar properties
The columns of the periodic table are called groups.
These groups have the same number of valence electrons.
Some Atoms Form Ions
When an atom gains or loses an electron (or more than one) it becomes an ion.
An atom can be positively or negatively charged depending on if it lost or gained an electron.
An ion is displayed using the symbol of the element and a + or – sign.
A positive ion is called a cation.
A negative ion is called an anion.
How Do the Structures of Atoms Differ?
Atomic number equals the number of protons
Every element has its own unique number of protons.
This number is never shared between elements.
The number of electrons is equal to the number of protons.
Mass number equals the total number of subatomic particles in the nucleus.
The subatomic particles in the nucleus are the proton and neutron.
Those two amounts are added together to make the mass number.
Normally the number of neutrons is equal to the number of protons but not always.
Isotopes of an element have different numbers of neutrons
The number of neutrons for an element can be more or less than the number of protons.
There are usually only a few isotopes for each element.
Isotopes have different mass numbers from each other.
Some isotopes are more common than others.
Calculating the number of neutrons in an atom.
37
Element are presented in the format. 17Cl
The top number is the mass number of the atom.
The bottom number is the atomic number of the atom.
To find the neutrons in that atom you subtract the mass number and the atomic number.
37-17=20
There are 20 neutrons in that atom of chlorine.
The mass of an atom
Atoms are weighed using the atomic mass unit (amu)
This is equal to one-twelfth the mass of a carbon-12 atom.
The atomic mass of an element is the weighted average of all isotopes of an element.
Families of Elements
How are Elements Classified
Elements are grouped by similar physical and chemical properties.
Elements are classified into three groups
Metals, Nonmetals, and metalloids/semiconductors
Metals
There are four different kinds of metals.
The alkali metals are very reactive
Alkali metals are soft, shiny, and react violently with water.
They are reactive due to having only one valence electron.
Alkaline-earth metals form compounds that are found in limestone and in the human body
These have two valence electrons and are not as reactive as the alkalines.
Gold, silver, and platinum are transition metals
Even less reactive than the previous two groups.
These metals are used for their ability to conduct heat and electricity.
Technetium and promethium are synthetic elements
All elements with atomic numbers greater than 92 are man-made.
They are radioactive.
They have uses in many manufacturing areas.
Nonmetals
Nonmetals and their compounds are plentiful on Earth
Carbon, oxygen, nitrogen, and sulfur are common nonmetals.
They form negative ions due to gaining electrons.
The noble gases are inert
The valence electrons for these gases are full.
They do not react under normal conditions.
Semiconductors are intermediate conductors of heat and electricity
There are only 6 elements that are semiconductors.
Boron, silicon, germanium, arsenic, antimony, and tellurium.
Silicon is the most common making up 28% of the Earth’s crust.
Using Moles to Count Atoms
Counting Things
When large amounts of things need to be counted we use a counting unit.
The movie attendant does not count individual pop corn pieces but gives you a sized bag full.
There are many different counting units
Counting units is an approximation and not meant to be exact.
Counting units can have a specific amount (500 sheets per package, 12 eggs per container)
An objects mass can also be used to count it out.
The mole is useful for counting small particles
The mole (mol) is a collection of a very large number of particles.
602 213 670 000 000 000 000 000
This is known as Avogadro’s constant or number.
One of the reasons we use scientific notation.
The mol is defined as the number of atoms in 12 grams of carbon-12
Moles and grams are related
The grams in 1 mol of a substance is called its molar mass.
This is the same as the average atomic mass (amu) of the element.