Ch02 lecture

Catherine E. MacGowanArmstrong State University

Lecture Presentation

Chapter 2

Atoms and Elements

Imaging and Moving Atoms• Gerd Binnig and Heinrich Rohrer were experimenting with an electrical

current passing over a flat metal surface and discovered that this “tunneling” current allowed them to scan the surface on an atomic scale.

• They were essentially taking pictures of atoms on the surface.

• Their discovery led to the development of electron scanning tunneling microscopy (STM).

Atomic Theory of Matter and Its Laws• Law of the conservation

of matter• In a chemical reaction

matter is neither created nor destroyed. Total mass of used

reactants = total mass of products produced.

Total number of reactant atoms = total number of product atoms.

Atomic Theory of Matter and Its Laws• Dalton’s atomic theory of matter

– Atoms are small, discreet, indivisible pieces of matter.– All elements are made up particles called atoms.– An element’s atoms are identical in size, mass, and

chemical properties.• Scientists did not know about isotopes.

– Isotopes are elemental atoms that differ in their mass due to different number of neutrons..

– Molecules (compounds) are formed when two or more elements are combined.

– Molecules are simple whole-number ratios of the combined elements.

Atomic Theory of Matter and Its Laws• Dalton’s Atomic Theory notes that

– “Atoms of different elements can combine to form molecules in simple whole-number combination of atoms.”

• Law of definite proportions

• Law of multiple proportions

Law of Definite Proportions• For a given compound, the elements always combine in the same

proportion.

– For example• A sodium chloride molecule (NaCl) is always a 1:1 ratio of

one sodium atom to one chlorine atom.

– A 100.0 g sample of NaCl contains 39.3 g Na and 60.7 g Cl.

Mass Cl = 60.7 g = 1.54Mass Na 39.3 g

– A 58.44.0 g sample of NaCl contains 22.99 g Na and 35.44 g Cl.

Mass Cl = 35.44 g = 1.54Mass Na 22.99 g

Law of Multiple Proportions• Two elements, A and X, can form different compounds by combining in

different proportions. • These combinations can be represented as a ratio.

– For example• A molecule of carbon dioxide (CO2) has a ratio of one C atom

to every two atoms of oxygen or 1:2.

• A molecule of hydrogen peroxide (CO) has a ratio of one C atom to one atom of oxygen or 1:1

Discovery of the Electron and Its Properties• Discovery of electron

– Thomson’s charge to mass experiment used cathode rays.• Thomson believed that these particles were the ultimate

building blocks of matter.• These cathode ray particles became known as electrons.

– The Millikan oil drop experiment• showed that the particle had the same amount of charge as

the hydrogen ion as Thomson observed in his experiments.

Discovery of the Atom’s Electron• Thomson’s charge to mass experiment investigated the effect on a

cathode ray when placing an electric field around a tube.1. Charged matter is attracted to an electric field.2. Light’s path is not deflected by an electric field.

• Cathode rays are made of tiny particles.– These particles had a negative charge because the beam always

deflected toward the (+) plate.

• The amount of deflection was related to the charge and mass of the particles.– The charge–mass ratio of these particles was

(−)1.76 × 108 C/g.

Charge to Mass Ratio of Electron Experiment

Millikan’ Oil Drop Experiment• Investigation led to determining

the charge of the electron.– Charge on an electron

(–)1.60 × 10–19 C

charge × (mass/charge) = mass

–1.60 × 10–19 C x(1 gram/1.76 × 108 C) = 9.10 × 10–28 grams

• Thomson’s experiments determined that the mass of one electron would be 2000 times lighter than a hydrogen atom.

Atomic Structure of the Electron

• Electrons are particles found in all atoms.– One of the fundamental pieces of matter

• The electron has a charge of −1.60 × 10–19 C.

• The electron has a mass of 9.1 × 10−28 g.

J.J. Thomas: The Plum Pudding Model of the Atom

• J.J. Thomas (plum pudding model) – The atom is composed of a positive cloud of matter

in which electrons are embedded.• Explains the positive (+) and negative (–)

charged behavior of matter

Rutherford’s Gold Foil Experiment• Gold foil experiment

– could not explain Thomson’s plum pudding atom model.

– led to the discovery of the atom’s nucleus.

Rutherford’s Gold Foil Experiment: Discovery of the Nucleus• From the gold foil experiment the following conclusions

were proposed:– The atom contains a tiny dense center called the

nucleus.– The nucleus has essentially the entire mass of the atom.

• The electrons weigh so little that they give practically no mass to the atom.

– The nucleus is positively charged.• The amount of positive charge balances the negative charge of

the electrons.• The electrons are dispersed in the empty space of the atom

surrounding the nucleus.

Atomic Structure: Historical Perspective• Rutherford’s model (solar system)

– stated that the atom is mostly empty space with a dense center of mass (nucleus) and circling electrons.

– proposed that the nucleus had a particle that had the same amount of charge as an electron but the opposite sign. • These particles are called protons.

– charge = +1.60 × 1019 C – mass = 1.67262 × 10−24 g

– Since protons and electrons have the same amount of charge, for the atom to be neutral, there must be equal numbers of protons and electrons.

Number of Protons in an Atom Determines the Element’s Identity• The number of protons located in an atom’s nucleus determines the

element’s identity.– The number of protons in the nucleus of an atom is called the

atomic number.

• Each element has a unique chemical name and symbol.– The symbol can either be one or two letters.

• O (oxygen) or Fe (iron)

• The elements are arrangedon the periodic table in orderof their atomic numbers.

Isotopes: Same Element but with Different Masses• Isotopes are elements whose atoms differ in mass due to

varying numbers of neutrons.– They are the same element because they have the

same number of protons (atomic number).– They are chemically identical.

• Isotopes are identified by their mass numbers.– Protons + neutrons = mass number

• Isotopic symbol13Al26.981

Atomic number, Z

Atom symbol

Atomic weight

How many protons, electrons, and neutrons are in the following atoms?

Protons Electrons Neutrons

1. 32 S 16

2. 65 Cu

3. U-240

How many protons, electrons, and neutrons are in the following atoms?

Protons Electrons Neutrons

1. 32 S 16 16 16

2. 65 Cu 29 29 36

3. U – 240 92 92 148

Note: Neutral atoms will have the same number of protons as electrons.

Problem:Complete the following table:

Protons Neutrons ElectronsAtomicNumber

MassNumber

AtomicSymbol

55 133

Answer:

Protons Neutrons ElectronsAtomicNumber

MassNumber

AtomicSymbol

6 7 6 6 13

42 54 42 42 96

13 14 13 13 27

55 78 55 55 133

Isotopes and Atomic Mass (weight)• What is the connection between an element’s isotopes and its

atomic mass?– The observed mass of an element is a weighted average of

the weights of all the naturally occurring atoms.• Average mass = atomic weight (amu)

• What information is needed to determine the atomic mass ofan element?– The natural abundance of each of the element’s isotopes

• The percentage of an element that is one isotope is called the isotope’s natural abundance.

– Example problem • Boron has two isotopes.

– Boron is 19.9% 10B and 80.1% 11B.

• Boron atomic weight = 0.199 (10.0 amu) + 0.801 (11.0 amu) = 10.8 amu

Problem: Calculating an element’s atomic massThe element silver (Ag) has an atomic mass of 107.868 amu. It has two isotopes—Ag-109 (108.905 amu) with an abundance of 48.1600% and Ag-107. Determine the amu for Ag-107.

Answer: Calculating an element’s atomic massThe element silver (Ag) has an atomic mass of 107.868 amu. It has two isotopes—Ag-109 (108.905 amu) with an abundance of 48.1600% and Ag-107. Determine the amu for Ag-107.

Problem Strategy:1. Set up an algebraic equation.2. Solve for x, which is the amu for Ag-107.

Answer: 1. Set up an algebraic equation.

(0.481600) 108.905 amu + x (0.518400) = 107.868 amu

2. Solve for x, which is the amu for Ag-107.

52.4490 amu + 0.518400x = 107.868 amu0.518400 x = 55.4180 amux = 106.905 amu

Ions are Charged Atoms• IONS are atoms or groups of atoms with a positive (+) or negative (–)

charge.

• Taking away an electron from an atom gives a cation with a positive charge.– More protons in nucleus versus electrons surrounding nucleus

– Metals tend to form cations to achieve the “full shell” look.

• Adding an electron to an atom gives an anion with a negative charge.– Fewer protons in the nucleus versus electrons surrounding

nucleus

– Nonmetals tend to form anions to achieve the “full shell” look.

Cations• A cation forms when an atom

loses one or more electrons from its outer (valence) shell (energy level).

• Cations are positively charged because the atom has more protons (+) than electrons (–).– Mg atom has 12 protons and

12 electrons.– Mg+2 ion has 12 protons and

10 electrons.

• Metal elements tend to form cations.

• Example – Mg Mg2+ + 2 e–

Anions• An anion forms when an atom

gains one or more electrons into its outer (valence) shell (energy level).

• Anions are negatively charged because the atom has less protons (+) than electrons (–).– F atom has 9 protons and

9 electrons.– F– ion has 9 protons and 10

electrons.

• Nonmetal elements tend to form anions.

• Example– F + e– F–

Periodic Table: Historic perspective• Dmitri Mendeleev (1834–1907)

developed the modern periodic table.– Argued that element properties

are periodic functions of their atomic weights

• Periodic law – When the elements are arranged

in order of increasing atomic mass, certain sets of properties recur periodically.

– Elements having similar physical and chemical properties fall within a column.

• We now know that element properties are periodic functions of their atomic numbers.

Periodic Table: Organization• The elements are arranged from left to right in increasing

atomic number (number of protons an element has).

• Rows in the periodic table are referred to as periods.

• Columns within the periodic table are sometimes referred to as families.– Because the elements within the column have similar

physical and chemical properties

• The elements, their names, and symbols are given on the periodic table.

Areas of the Periodic Table

• Major Areas– Metals

• Main group metals• Transition metals• Inner transition metals

– Lanthanides– Actinides

– Metalloids

– Non metals

Periodic Table: Major Divisions

Periodic Table: Metals• Characteristics

– Solid at room temperature (except Hg)

– Reflective surface• Shiny

– Conduct heat and electrical current

– Malleable• Can be shaped

– Ductile

– Lose electrons and form cations

– About 75% of the elements in the periodic table are metals.

Periodic Table: Metalloids

• Characteristics

– Can exhibit the properties of metals and/or nonmetals

– Known as semiconductors• Poor conductors of heat

– Solids at room temperature

Periodic Table: Nonmetals

• Characteristics

– Can be found in all three states (gas, liquid, and solid) of matter

– Poor conductors of heat and electricity

– Solids are brittle.

– Gain electrons to become anions

– Except for H, found mostly in the upper right on the periodic table

Periodic Table: Families

Families: Grouping of the Periodic Table

• Elements within a column are considered “families.”– They have similar chemical and physical properties.– They all have the same number of electrons in the

outermost shell (energy level).

• Families of the periodic table– Alkali metals (column 1/Group number 1A)– Alkaline earth metals (column 2/Group number 2A)– Halogens (column 17/Group number 7A)– Noble/inert gases (column 18/Group number 8A)

Ions and the Periodic Table• Metals tend to form cations.

– Lose electrons in their outermost energy level to “obtain the nearest noble gas electron configuration”

• Nonmetals tend to form anions.– Gain electrons in their outermost energy level to

“obtain the nearest noble gas electron configuration”

• Metalloids tend to form cations but can also form anions.

Periodic Table: Ion Formation

What is a Mole?

• Chemistry is quantitative in nature—its unit is the mole.

• The mole as a unit versus “dozen” as a unit.– The unit “dozen” is associated with twelve units.– The unit mole is associated with 6.02 × 1023 units.

• There is Avogadro’s number of units in every mole.– 6.02 × 1023 units is known as Avogadro’s number.– 1 mole = 6.02 × 1023 units

• One mole of Cu atoms has– an atomic mass of 63.55 grams, which is– 6.022 × 1023 Cu atoms, which is – approximately 22 pennies.

Problem: How large is a mole?You have inherited ¼ mole of pennies. If you spent 1 × 1012 dollars every second how many decades would it take for you to spend your inheritance?

Answer:How large is a mole?You have inherited ¼ mole of pennies. If you spent 1 × 1012 dollars every second how many decades would it take for you to spend your inheritance?

¼ mol × ( 6.02 x 1023 cents/1 mol) × ($1/100 cents) = 1.51 × 1021 dollars

1.51 × 1021 dollars × (1 sec/1 × 1012 dollars) × (1 min/60 sec) × (1 hr /60 min) × (1 day/24 hrs) × (1 yr/365 days) × (1 decade/10 yrs) = 4.79 decades.

Every mole contains 6.02 × 1023 units. That is a lot of pieces!

Mass to Mole Conversions• To go from mass to mole

Mass(g) × (1 mol/atomic mass of element) = mole of element

• Example– 24.00 grams C × (1 mole carbon/12.011 grams) = 2.00 moles of

carbon atoms

Mass of element (g) Mole of element

Mole to Mass Conversions• To go from mole to mass

Mole of element × (atomic mass (element)/1 mole) = mass (g)

• Example– 8.00 mole He × (4.00 grams/1 mole He)

= 32.0 grams He atoms

Mass of element (g)Mole of element

Mass to Number of Atoms Conversions• To go from mass to number of atoms, you must go through the mole.

Mass (g) × (1 mol/atomic mass) × (6.02 × 1023 atoms/1 mole) = # atoms

• Example:

– 45.99 g Na × (1 mol Na/22.99 g) × (6.02 × 1023 Na atoms/1 mole)

= 1.204 × 1024 Na atoms

Mole of element

Mass of element (g)

Number of atoms of the element

Atoms to Mass Conversions• To go from atoms to mass you must go through the mole.

(# of atoms) × (1 mole/6.02 × 1023 atoms) × (atomic mass/1 mole) = mass (g)

• Example:

– 8 atoms Am × (1 mole/6.02 × 1023 Am atoms) × (243 g/1 mol Am) = 3.23

× 10–21 grams of Am

Mole of element

Mass of element (g)

Number of atoms of the element

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