Atom and Elements

8/3/2019 Atom and Elements

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Atoms and Elements

Atomic Number and Mass Number

Copyright 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings


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The atomic number

is specific for each element.

is the same for all atoms of an element.

is equal to the number of protons in an atom.

appears above the symbol of an element.

Atomic Number

11Na

Atomic Number

Symbol


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An atom of

an element is electrically neutral; the net charge ofan atom is zero.

has an equal number of protons and electrons.number of protons = number of electrons

Aluminum has 13 protons and 13 electrons. The net

charge is zero.13 protons (13+) + 13 electrons (13 -) = 0

Electrons in An Atom


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Mass Number

The mass number

represents the number of particles in the nucleus.

is equal to the number of protons + the numberof neutrons.


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Atomic Models


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Atoms and Elements

The Atom



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Daltons Atomic Theory

In Daltons Atomic Theory, atoms

are tiny particles of matter.

of an element are similar anddifferent from other elements.

of two or more different elementscombine to form compounds.

are rearranged to form newcombinations in a chemicalreaction.



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Subatomic Particles

Atoms contains subatomic particles,

protons have a positive (+) charge.

electrons have a negative (-) charge. like charges repeland unlike chargesattract.

neutrons are neutral.



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Rutherfords Gold-Foil Experiment

In Rutherfords gold-foil experiment, positively

charged particles

were aimed at atoms of gold.

mostly went straight through the atoms. were deflected only occasionally.

Conclusion:

There must be a small, dense, positively

charged nucleus in the atom that deflects

positive particles that come close.


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Rutherfords Gold-Foil Experiment



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Structure of the Atom

An atom consists

of a nucleus thatcontains protonsand neutrons.

of electrons in alarge emptyspace around thenucleus.



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Atomic Mass Scale

On the atomic mass scale for subatomic particles,

1 atomic mass unit (amu) has a mass equal to 1/12 of

the mass of the carbon-12 atom. a proton has a mass of about 1 (1.007) amu.

a neutron has a mass of about 1 (1.008) amu.

an electron has a very small mass, 0.000549 amu.


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Particles in the Atom


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Atoms and Elements

Isotopes and Atomic Mass

24Mg 25Mg 26Mg12 12 12



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Isotopes

are atoms of the same element that have differentmass numbers.

have the same number of protons, but differentnumbers of neutrons.

Isotopes


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A nuclear symbol

represents a particular atom of an element.

gives the mass number in the upper left corner andthe atomic number in the lower left corner.

Example: An atom of sodium with atomic number11 and a mass number 23 has the following atomicsymbol:

mass number 23

Naatomic number 11

Nuclear Symbol


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From the nuclear symbol, we can determine the numberof protons (p+), neutrons, (n), and electrons (e-) in aparticular atom.

16 31 65

O P Zn8 15 30

8 p+ 15 p+ 30 p+

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

Information from Nuclear Symbols


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Isotopes of Magnesium

In naturally occurring magnesium,there are three isotopes.

24Mg 25Mg 26Mg12 12 12

Isotopes of Mg


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Atomic Mass

The atomic mass of an element

is listed below the symbol of each elementon the periodic table.

gives the mass of an average atom ofeach element compared to 12C.

is not the same as the mass number.

Na

22.99


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Isotopes of Some Elements andTheir Atomic Mass

Most elements have two or more isotopes thatcontribute to the atomic mass of that element.



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Atomic Mass for Cl

The atomic mass of chlorine is

due to all the Cl isotopes. not a whole number.

the average of two isotopes:35Cl and 37Cl.



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Atoms and Elements

Electron Energy Levels



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Energy Levels

Energy levels

are assigned numbers n=1, 2, 3, 4 and so on.

increase in energy as thevalue of nincreases.

are like the rungs of aladder with the lowerenergy levels nearer theground.

n= 4

n= 3

n= 2

n= 1


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Energy Levels

Energy levels have a maximum number of electronsequal to 2n2.

Energy level Maximum number of electrons

n= 1 2(1)2 = 2(1) = 2

n= 2 2(2)2 = 2(4) = 8

n= 3 2(3)2 = 2(9) = 18


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Orbitals

An orbital

is a three-dimensional space around a nucleuswhere an electron is most likely to be found.

has a shape that represents electron density (nota path the electron follows).

can hold up to 2 electrons.


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sOrbitals

An s orbital

has a spherical shapearound the nucleus.

is found in each energylevel.


n= 1

n= 2

n= 3


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pOrbitals

A p orbital

has a two-lobed shape.

is one of three porbitals in each energy level from n = 2.



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Electrons in Energy Levels n= 1- 4

Energy Orbitals Maximum TotalLevel No. of Electrons Electrons1 1s 2 2

2 2s 2 8

2p 63 3s 2 18

3p 6

3d 10

4 4s 2 324p 6

4d 10

4f 14


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Atoms and Elements

Periodic Trends



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Valence Electrons

The valence electrons

determine the chemical properties of the elements.

are the electrons in the highest energy level.

are related to the group number of the element.

Example: Phosphorus has 5 valence electrons.

5 valence electrons

P Group 5A(15) 2, 8, 5


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All the elements in a group have the same number of

valence electrons.

Example: Elements in group 2A(2) have two (2)valence electrons.

Be 2, 2

Mg 2, 8, 2

Ca 2, 8, 8, 2

Sr 2, 8, 18, 8, 2

Groups and Valence Electrons

P i di T bl d V l


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Periodic Table and ValenceElectrons

Representative Elements Group Numbers

1 2 3 4 5 6 7 8

H He

1 2

Li Be Al C N O F Ne

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

Li Mg Ge Si P S Cl Ar

2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8


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Electron-Dot Symbols

An electron-dot symbol

shows the valence electronsaround the symbol of theelement.

for Mg has two valenceelectrons as single dots on thesides of the symbol Mg.

. .

Mg or Mg or Mg or Mg



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Writing Electron-Dot Symbols

Electron-dot symbols for

groups 1A(1) to 4A(14) use single dots.

Na Mg Al C

groups 5A(15) to 7A(17) use pairs and single dots.

P : O


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Groups and Electron-Dot Symbols

In a group, all the electron-dot symbols have thesame number of valence electrons (dots).

Example: Atoms of elements in Group 2A(2) eachhave 2 valence electrons.

Be

Mg

Ca Sr

Ba


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A. X is the electron-dot symbol for

1) Na 2) K 3) Al

B. X is the electron-dot symbol of

1) B 2) N 3) P

Learning Check


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A. X is the electron-dot symbol for

1) Na 2) K

B. X

is the electron-dot symbol of

2) N 3) P

Solution


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Atomic Size

Atomic size is described using the atomic radius; thedistance from the nucleus to the valence electrons.



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Atomic Radius Within A Group

Atomic radius increases

going down each groupof representativeelements.



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Atomic Radius Across a Period

Going across a period left to right, an increase in number of protons increases attraction

for valence electrons.

atomic radius decreases.



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Ionization Energy

Ionization energy is the energy it takes to remove avalence electron.

Na(g) + Energy (ionization) Na+(g) + e-


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Ionization Energy In a Group

Going up a group ofrepresentative elements,

the distance decreasesbetween nucleus and

valence electrons.

the ionization energyincreases.



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Ionization Energy

Metals havelowerionizationenergies.

Nonmetalshave higherionization

energies.



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Summary of Properties

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Atom and Elements