Atoms & The Periodic Table

Chapters 4 & 5

• John Dalton developed an atomic theory in 1808. Dalton’s was the first atomic theory with a scientific basis.

• Atom - the smallest part of an element that still has the element’s properties.

• Atoms are the building blocks of molecules.

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

• nucleus - small, dense with a positive electric charge located at the center of each atom.• made of protons - a subatomic

particle that has a positive charge • neutrons - a subatomic particle that

has no charge• Electrons - subatomic particles with

negative charges.• move around outside the nucleus

• Unreacted atoms have no overall charge.– Although atoms are made of charged

particles, they do not have an overall charge because they have an equal number of protons and electrons whose charges exactly cancel.

• Bohr’s model compares electrons to planets.– In 1913, Niels Bohr suggested that electrons

in an atom move in set paths around the nucleus much like the planets orbit the sun in our solar system.

• In Bohr’s model, electrons can only be in certain energy levels.

Bohr’s Model

• Electrons are found in orbitals within energy levels.

• Orbital - a region in an atom where there is a high probability of finding electrons

• An s orbital is shaped like a sphere• A p orbital is dumbbell shaped and

can be oriented three different ways in space:

• Valence electrons - found in the outermost shell of an atom and determine the atom’s chemical properties.• The electrons in an atom that participate in

chemical bonding • Every atom has between one and eight

Periodic Table

• The periodic table groups similar elements together.– Makes it easier to predict the properties of an

element based on where it is in the periodic table.

• Elements are listed in order of number of protons

• Periodic law - states that when elements are arranged this way, similarities in their properties will occur in a regular pattern.

• The periodic table helps determine electron arrangement.

• periods - horizontal rows in the periodic table– As the number of protons an atom has

increases as you move from left to right across a period, so does its number of electrons.

• Elements in the same group have similar properties.

• Group - a vertical column of elements in the periodic table. • Atoms of elements in the same group have

the same number of valence electrons, so these elements have similar properties.

• Ion - an atom or group of atoms that has lost or gained one electrons and has a negative or positive charge• A lithium atom loses one electron to form a 1+

charged ion:

• Ion - an atom or group of atoms that has lost or gained one electrons and has a negative or positive charge• A lithium atom loses one electron to form a 1+

charged ion:

•A fluorine atom gains one electron to form a 1 charged ion:

• atomic number - Z, of an atom equals the number of protons in the nucleus.

• mass number - A, of an atom equals the number of protons plus the number of neutrons in the nucleus.

Atomic number

Mass Number

• isotope - an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons.– Example: Hydrogen has three isotopes,

• If you know the atomic number and mass number of an atom, you can calculate the number of neutrons it has.

• Example: uranium-235 has a mass number of 235. Like all uranium atoms, it has an atomic number of 92. The number of neutrons it has is:

Mass number (A): 235Atomic number (Z): –92Number of neutrons: 143

Isotopes

•average atomic mass - is a weighted average of the masses of all naturally-occurring isotopes of an element.

• The elements are classified into three groups.

• Metals - elements that are shiny and conduct heat and electricity well • most elements

• Nonmetals, all except hydrogen of which are found on the right side of the periodic table, may be solids, liquids, or gases at room temperature.

• Metalloids / Semiconductors - elements that can conduct electricity under certain conditions• between metals and nonmetals

• alkali metals- in Group 1 are very reactive.

• alkaline-earth metals- found in Group 2 of the are somewhat less reactive than the alkali metals.

• The transition metals, found in Groups3–12

Nonmetals • Carbon is found in three

different forms and can form many compounds

• Nonmetals and their compounds are plentiful on Earth.

• Halogens - located in Group 17

• Noble gases, found in Group 18 – they are unreactive.

• Metalloids / Semiconductors - intermediate conductors of heat and electricity

• Silicon is the most familiar semiconductor.– Silicon is an important part of

computer chips, as well as other semiconductor devices such as transistors, LED display screens, and solar cells

A mole (mol) is the number of particles that is the same as the number of atoms of carbon in 12 g of carbon-12. useful for counting small particles

• Avogadro’s constant - the number of particles per mole of a substance: 6.022 × 1023

• Moles and grams are related.

• molar mass - mass in grams of 1 mol of a substance– For example, 1 mol of carbon-12 atoms has a molar mass of

12.00 g.– The molar mass of an element = average atomic mass,

which is listed in the periodic table.

• To convert between moles and grams and vice versa, you can use a conversion factor: a ratio that is derived from the equality of two different units.

• Let’s say that a shopkeeper knows that exactly10 gumballs have a total mass of 21.4 g. This relationship can be written as either one of two equivalent conversion factors:

10 gumballs21.4 g

21.4 g10 gumballs

Conversion Factors What is the mass of exactly 50 gumballs?

1. List the given and unknown values. Given: mass of 10 gumballs = 21.4 gUnknown: mass of 50 gumballs = ? g

Math Skills

2. Write down the conversion factor that converts number of gumballs to mass.The conversion factor you choose should have the unit you are solving for (g) in the numerator and the unit you want to cancel (number of gumballs) in the denominator.

21.4 g10 gumballs

3. Multiply the number of gumballs by this conversion factor, and solve.

21.4 g50 gumballs10 gumballs

107g

Converting Mass to Amount Determine the amount of iron present in 352 g of iron.

1. List the given and unknown values. Given: mass of iron = 352 g Fe molar mass of iron = 55.85 g/mol FeUnknown:amount of iron = ? mol Fe

• 2. Write down the conversion factor that converts grams to moles.

• The conversion factor you choose should have what you are trying to find (moles of Fe) in the numerator and what you want to cancel (grams of Fe) in the denominator.

1 mol Fe55.85 g Fe

3. Multiply the mass of iron by this conversion factor, and solve.

1 mol Fe352 g Fe55.85 g Fe

6.30 mol Fe