Atoms and the Periodic Table

Chapter 4

What are Atoms?

• 4th Century B.C. Democritus suggested that the universe was made of invisible units called atoms

• Atom Derived from Greek word meaning unable to be divided– He thought that movements of atoms caused

the changes in matter that he observed

John Dalton’s Atomic Theory

• Dalton also believed that atoms could not be divided– Today, we know that atoms are made up of much

smaller particles

• Dalton’s theory:– stated that all atoms of a given element were exactly

alike– Stated that atoms of different elements could join to

form compounds– ** His theory is considered the foundation for the

modern atomic theory

Atoms: The Building Blocks of Molecules

• An atom is the smallest part of an element that still has the element’s properties (chemical and physical)

What’s in an Atom?

• Atoms are made up of protons, neutrons, and electrons.

• The center of each atom is a small, dense NUCLEUS with a positive charge.– Made of PROTONS and NEUTRONS

• Protons and neutrons are very similar in size and mass

• They differ because protons have a positive charge and neutrons have no electric charge.

What’s in an Atom?

• ELECTRONS move around the outside of the nucleus.– These subatomic particles have a negative

electric charge and have a very small mass.

What’s in an Atom?

• Unreacted atoms have no overall charge

• Atoms are not charged even though they are made up of charged subatomic particles.– Due to equal number of protons and electrons

Models of the Atom

• Bohr’s Model– He thought that electrons moved in set paths

around the nucleus.– Each electron has a certain energy that is

determined by its path around the nucleus• Electrons must gain energy to move to higher

levels or lose energy to move to lower levels.

Models of the Atom

• A new model of the atom was proposed– Stated that electrons behave like waves on a

vibrating string

• An electron’s exact location cannot be determined– Impossible to determine both the exact

location of an electron in an atom and the electron’s speed and location

Calculating Subatomic Particles

• Atomic number = 50– Tells you how many

protons are in an atom

• Mass number = 118.7– Sum of the numbers of

protons and neutrons in the nucleus of an atom

Calculating Subatomic Particles

• How do you determine the number of electrons?– The number of protons = the number of electrons

the atomic number

• How do you determine the number of neutrons in an atom?– The mass number – the atomic number = the number

of neutrons.• Remember: mass number = # of protons and neutrons

atomic number = # of protons

Electrons Exist in Energy Levels

• The number of filled energy levels an atom has depends on the number of electrons.

Electrons: Found in Orbitals Within Energy Levels

• Orbital: A region in the atom where there is a high probability of finding electrons.– Within each energy level, electrons occupy

orbitals that have the lowest energy levels.

• There are four different kinds of orbitals– Orbitals s,p,d, and f

Orbitals

• S orbital– Simplest and lowest

energy level: hold 2 electrons

– Spherical shape

• P orbital– Holds 6 electrons– Dumbbell shape– Three different

orientations

Orbitals

• D orbital– More complex– 5 possible orbitals

• F orbital– Has the greatest energy– 7 possible orbitals

Every atom has between 1 and 8 valence electrons

• A valence electron is the outermost energy level of an atom.– Determine an atom’s chemical properties and

ability to form bonds

Organization of the Periodic Table

• Periodic table groups similar elements together.

• The order of elements is based on the number of protons in the atom of elements

• Elements are arranged this way due to the periodic law– States that when elements are arranged in

this manner, similarities in their properties will occur in a regular pattern

Periodic Table Helps Determine Electron Arrangement

• Horizontal rows in the periodic table are called periods.– The number of protons and electrons increase

as you move across the periodic table from left to right

Elements In The Same Group Have Similar Properties

• Valence electrons determine the chemical properties of atoms

• Elements in the same group or vertical column (up and down) have the same number of valence electrons

Some Atoms form Ions

• Atoms that do not have a filled s or p orbital may undergo ionization.– They may gain or lose valence electrons in

order to have a full outermost s and/or p ortbital.

– Atoms that gain or lose electrons no longer have an equal number of protons and electrons the ion forms a net electric charge

IONS

• Removing an electron from an atom forms a positive ion, or a cation.

• Adding an electron to an atom forms a negative ion, or an anion.

How Do The Structure of Atoms Differ?

• Atomic number: Tells you how many protons are in an atom– Remember that atoms have a neutral charge

• What does this mean?– Protons are equal to electrons

• Mass Number: Equals the number of protons plus the number of neutrons– Atoms of an element always have the same

atomic number, but they can have different mass numbers

Isotopes

• An isotope is an atom that has the same number of protons as other atoms of the same element but has a different number of neutrons.

• Some elements can have different versions of their atom– Each atom has the same number of protons

and electrons

The Mass of an Atom

• Atomic units are extremely small. – Expressed in atomic mass units (amu).

• Atomic mass unit: A unit of mass that describes the mass of an atom or molecule. It is exactly one-twelfth of the mass of a carbon 12 atom.

The Mass of an Atom

• Average atomic mass: the weighted average of the masses of all naturally occurring isotopes of an element.– This is the number on the periodic table.– More commonly found isotopes have a

greater effect on the average than rare isotopes.

How Are Elements Classified?

• Classified into three groups:– Metals– Nonmetals– Semiconductors/Metalloids

Metals

• Characteristics– Shiny– Can be stretched and shaped– Good conductors of heat and electricity

• Similar metals are grouped together: there are different kinds of metals.

Metals

• Alkali Metals– Found in Group 1 of the periodic table– Highly reactive: only has one valence electron

• Not found in nature as elements they combine to form compounds

– Soft– Shiny– Reacts violently with water

Metals

• Alkaline-earth metals– Found in Group 2– Have two valence electrons– Calcium: form hard shells of many sea

animals, bones, and teeth– Be, Mg, Ca, Sr, Ba, and Ra

Metals

• Transition metals: one of the elements in groups 3-12 on the periodic table– Much less reactive than elements in groups 1 and 2.– Can lose electrons to form cations (positive ions)– Conduct heat and electricity– Shiny– Can be stretched– ** Mercury (Hg) is the only metal that is a liquid at

room temperature.

Synthetic Elements

• Technetium and promethium are man-made elements

• All elements with atomic numbers greater than 92 are man-made.

Nonmetals

• Except for hydrogen, nonmetals are found on the right side of the periodic table.– Include some elements in groups 13-16 and

all elements in groups 17-18.

Nonmetals

• Carbon– Found in three forms

• Graphite (pencil lead)• Diamond• Fullerenes

– Carbon is found in both living and non-living things

• Examples: Glucose, chlorophyll, gasoline, and rubber tires

Nonmetals

• Many nonmetals gain electrons in order to have a full valence shell.– Examples:

• Oxygen O2-

• Nitrogen N3-

• The most abundant gases in the air are the nonmetals nitrogen (N) and oxygen (O)

Halogens

• Halogens are located in group 17 of the periodic table.

• These elements are very reactive they need one electron to fill their valence shell.

• Just like the elements found in group one, these elements are not commonly found in nature. They are normally found in compounds.

• Elements: F, Cl, Br, I, At

Noble Gases

• Noble gases make up group 18. – Elements in this group are different because

they exist as a single atom instead of as molecules.

• These elements are inert or unreactive because the s and p orbitals are full of electrons.

• These elements do not gain or lose electrons to form ions.

• Elements: He, Ne, Ar, Kr, Xe, and Rn

Semiconductors

• Also called metalloids

• Only 6 elements:– Boron– Silicon– Germanium– Arsenic– Antimony– Tellurium

Semiconductors

• Silicon atoms account for 28% of the mass of Earth’s crust