Chapter Three

Atoms and the Periodic Table

Chapter 3

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Dalton’s Atomic Theory (1808)

1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size,and chemical properties. The atoms of one element are different from the atoms of all other elements.

2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same.

3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions. 2.1

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What are atoms composed of

?

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The Structure of Atoms

Cathode-Ray Tube (Thomson, 1856–1940):

Cathode rays

consist of tiny

negatively

charged particles,

now called

electrons.

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The Structure of Atoms Oil Drop Experiment (Millikan, 1868–1953): Applied a

voltage to oppose the downward fall of charged drops and suspend them.

Voltage on plates place 1.602176 x 10-19 C of charge on each oil drop.

Millikan calculated the electron’s mass as 9.109382 x 10-28 grams.

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Discovery of Nucleus (Rutherford, 1871 – 1937):

Rutherford irradiatedgold foil with a beamof alpha () particlesto search for positivecharged particles.

The Structure of Atoms

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Discovery of Nucleus (Rutherford, 1871–1937):Rutherford irradiatedgold foil with a beamof alpha () particlesto search for positivecharged particles.

Atom must be mostly empty space except for a central positive mass concentration.

The Structure of Atoms

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The Structure of Atoms Structure of the Atom:

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The atomic number is equal to the number of protons in an atom.

On the periodic chart, the atomic number appears above the symbol of an element.

Atomic Number

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Na

Atomic Number

Symbol

11

Atomic Numbers and Protons for Lithium and Carbon Atoms

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Subatomic Particles

Chemists are interested in three particles in atoms: protons, electrons, and neutrons.

Protons have a positive (+) charge; electrons have a negative (-) charge.

Like charges repel and unlike charges attract.

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Summary of Subatomic Particles

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An atom consists of a nucleus and a cloud of electrons.

The nucleus contains all of the protons and neutrons.

The rest of the atom, which is mostly empty space, is occupied by the electrons.

Structure of the Atom

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State the number of protons in each. A. Nitrogen

1) 5 protons 2) 7 protons 3) 14 protons

B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons

C. Barium 1) 137 protons 2) 81 protons 3) 56 protons

Learning Check

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State the number of protons in each. A. Nitrogen 2) Atomic number 7; 7 protons

B. Sulfur 2) Atomic number 16; 16 protons

C. Barium 3) Atomic number 56; 56 protons

Solution

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An atom is electrically neutral; the net charge is zero.

In an atom, the number of protons is equal to the number of electrons.

# protons = # electrons Therefore, the atomic number is also equal to the

number of electrons in a neutral atom.

Electrons in An Atom

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The mass number gives the total number of protons and neutrons in the nucleus. Mass number = # protons + # neutrons

Mass Number

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The atomic symbol represents a particular atom of an element.

The atomic symbol shows the mass number in the upper left corner and the atomic number in the lower left corner.

For example, an atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol:

mass number 23 Na atomic number 11

Atomic Symbol

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When we know the atomic symbol of an atom, we can determine the number of protons (p+), neutrons, (n), and electrons (e-).16 31 65

O P Zn 8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Information from Atomic Symbols

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Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.

12C 13C 14C 6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

Learning Check

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12C 13C 14C 6 6 6

Protons 6 6 6

Neutrons 6 7 8

Electrons 6 6 6

Solution

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Write the atomic symbols for atoms with the following subatomic particles:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Learning Check

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Write the atomic symbols for atoms with the following subatomic particles:

16OA. 8 p+, 8 n, 8 e- 8

B. 17p+, 20n, 17e- 37Cl 17

C. 47p+, 60 n, 47 e- 107Ag 47

Solution

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An atom of zinc has a mass number of 65.A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom? 1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that has 37 neutrons?

1) 37 2) 65 3) 67

Learning Check

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An atom of zinc has a mass number of 65.A. How many protons are in this zinc atom?

1) 30 B. How many neutrons are in the zinc atom?

2) 35

C. What is the mass number of a zinc atom that has 37 neutrons?

3) 67

Solution

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An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is 1) 14 2) 16 3) 34

C. The element is 1) Si 2) Ca 3) Se

Learning Check

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An atom has 14 protons and 20 neutrons.A. It has atomic number

1) 14

B. It has a mass number of 3) 34

C. The element is 1) Si

Solution

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2.5 Isotopes and Atomic Mass

24Mg 25Mg 26Mg 12 12 12

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Isotopes are atoms of the same element that have different mass numbers.

Isotopes have the same number of protons, but different numbers of neutrons.

All elements have two or more isotopes.

Isotopes

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Isotopes of Magnesium

In naturally occurring atoms of magnesium, there are three isotopes.

24Mg makes up 78.9% of magnesium atoms.

25Mg makes up 10.0% and 26Mg makes up 11.1%.

24Mg 25Mg 26Mg 12 12 12

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Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check

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Using the periodic table, specify the atomic mass of each element (round to the tenths place):

A. calcium 40.1 amu

B. aluminum 27.0 amu

C. lead 207.2 amu

D. barium 137.3 amu

E. iron 55.8 amu

Solution

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3. 6 Electronic Structure of Atoms

Quantum mechanical model of atomic structure:

Electrons are not perfectly free to move about in an atom.

Each electron is restricted to moving about only in a certain region of space within the atom, depending on the amount of energy the electron has.

Probability distribution (Probability distribution ( 22) for an ) for an electron in a 1electron in a 1ss orbital. orbital.

Boundary surfaces of a 1Boundary surfaces of a 1s s orbitalorbitaland a 2and a 2s s orbital.orbital.

1s1s 2s2s

A boundary surface encloses the regionA boundary surface encloses the regionwhere the probability of finding an electronwhere the probability of finding an electronis high—on the order of 90-95%is high—on the order of 90-95%

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Different orbitals have different shapes. Different orbitals have different shapes. Orbitals in Orbitals in s s subshells are spherical (a), while subshells are spherical (a), while orbitals in orbitals in p p subshells are roughly dumbbell subshells are roughly dumbbell shaped (b).shaped (b).

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s and p Orbitals

In an atom, all the orbitals are centered around the nucleus. For example, the illustration of the combination of s and p orbitals is

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Subshells and Orbitals

Each subshell has a specific number of orbitals. Each s subshell contains one s orbital. Each p subshell contains three p orbitals. Each d subshell contains five d orbitals. Each f subshell contains seven f orbitals.

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d-Orbitals

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f-Orbitals

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The first shell has only an s subshell The second shell has an s and a p

subshell The third shell has an s, a p, and a d

subshell. The fourth shell has an s, a p, a d, and

an f subshell.

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The number of subshells in a given shell is equal to the shell number. For example, shell number 3 has 3 subshells.

Within each subshell, electrons are further grouped into orbitals, regions of space within an atom where the specific electrons are more likely to be found.

There are different number of orbitals within the diff erent kinds of subshells.

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Different electrons have different amounts of energy and thus occupy different regions within the atom.

The energies of electrons are quantized, or restricted to having only certain values.

The electrons in an atom are grouped around the nucleus into shells.

Within the shells, electrons are further grouped into sub shells of four different types, identified as s, p, d, and f in order of increasing energy.

Energy of Electrons

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Electrons fill orbitals in ascending order, from the lowest-energy orbitals upward. Generally, this means that lower-numbered shells fill before higher-numbered shells. However, some overlap in energy levels occurs starting in the third energy level.

Fig 3.5 order of orbital Fig 3.5 order of orbital

energy levelsenergy levels

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1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

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First PeriodH 1s1

He 1s2

Second PeriodLi 1s2 2s1

C 1s2 2s2 2p2

Third PeriodNa 1s2 2s2 2p6 3s1

Al 1s2 2s2 2p6 3s2 3p1

S 1s2 2s2 2p6 3s2 3p4

Some Electron Configurations

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

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Electronic configuration of a few elements are shown below:

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Electron Configurations for Elements

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Noble Gas Notation A noble gas notation is used to abbreviate the

completed subshells as [noble gas]. For example, the complete configuration of

bromine is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

Ar configuration The noble gas notation for bromine is [Ar] 4s2 3d10 4p5

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

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Write the complete electronic configuration and noble gas notation for each of the following elements:

A. Cl

B. Sr

C. I

Learning Check

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

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A. Cl 1s2 2s2 2p6 3s2 3p5

[Ne] 3s23p5

B. Sr 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2

[Kr] 5s2

C. I 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5

[Kr] 5s2 4d10 5p5

Solution

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

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Group Numbers and Subshells The Group Number is the sum of the s and p

electrons in the outermost shell.

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Fig 3.2 The periodic table

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The table has 113 boxes, each of which tells the symbol, atomic number, and atomic weight of an element. Last update: 117 elements known to date

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Main Groups: The two large groups on the far left and the six on the far right are called the main groups.Lanthanides: 14 elements following lanthanum in group 6.Actinides: 14 elements following actinium in group 7.Transition Metal Groups: Elements in the groups numbered 3 through 12 (Sc-Zn). Inner Transition Metal Groups: The 14 groups shown separately at the bottom of the table and are not numbered.

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3.8 Electron Configuration and the Periodic table

The periodic table can be divided into four regions or blocks, of elements according to the shells and subshells as shown in Fig 3.6.Valence Shell : Outermost shell of an atom.Valence electrons: An electron in an outermost shell of an atom. These electrons are loosely held, they are most important in determining an element’s properties.

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Group Numbers and Subshells The Group Number is the sum of the s and p

electrons in the outermost shell.

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ns1

ns2

ns2

np1

ns2

np2

ns2

np3

ns2

np4

ns2

np5

ns2

np6

d1

d5 d10

4f

5f

Electron Configuration and the Periodic Table

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Electron Configuration of Atoms06

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a)S [Ne]3s23p4

Using periodic table write Noble gas notation for the following elements:

b)Fe[Ar] 4s23d6

c)Se [Ar] 4s23d104p4

d)Gd [Xe]6s2 5d1 4f7

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Fig 3.6 electron configurations and the periodic table

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Fig 3.2 The periodic table

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Chapter Summary All matter is composed of atoms. An atom is the smallest and simplest unit into which an element can be divided while maintaining the properties of the element. Atoms are made up of subatomic particles called

protons, neutrons and electrons. Protons have positive, electrons have negative, and neutrons

have zero electrical charges. Elements differ from each other according to the

number of protons their atoms contain (Atomic number, Z)

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Chapter Summary Contd.

Mass number (A): Total number of protons and neutrons in an atom.

Isotopes: Atoms with identical number of protons but different numbers of neutrons.

Periodic table: Tabular arrangement of elements according to their valence shells.

The electrons surrounding an atom are grouped into shells. Within each shell, electrons are grouped

into subshells, and within subshells into orbitals – regions of space where electrons are more

likely to be found.

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Chapter Summary Contd. s orbitals are spherical and p orbitals are bumbbell shaped. Each orbital and each shell can hold a specific number of electrons. - The first shell can hold only two electrons. 2 electrons in an s orbital (1S2). - The second shell can hold 8 electrons.

2 electrons in an s orbital and 6 electrons in 3 p orbitals. (2S2 2p6).

- The third shell can hold 18 electrons. 2 electrons in an s orbital, 6 electrons in 3 p orbitals, and 10 electrons in 5 d orbitals (3S2 3p6 3d10); and so on.

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End of Chapter 3