ATOMIC STRUCTURE

ESSENTIAL QUESTION…..

How can you identify an element and/or isotope based on its subatomic particles?

• COMPOSED OF THREE SUBATOMIC PARTICLES:

– PROTON (POSITIVELY CHARGED)

– NEUTRON (NO CHARGE)

– ELECTRON (NEGATIVELY CHARGED)

WHAT IS AN ATOM COMPOSED OF?

BOTH IN THE NUCLEUS

ALL FOUND OUTSIDE THE NUCLEUS (ELECTRON CLOUD)

• PROTONS (p+)– Proton is nearly 2000 times more massive than the electron, but

equal in charge and opposite in sign to the electron– Number of protons in the nucleus is electrically balanced by an

equal number of electrons

• NEUTRONS (n0)– Mass almost exactly equal to that of the proton

• ELECTRONS (e-)– Mass of electron much less than the mass of a proton and

neutron– Equal number of protons and electrons = NEUTRAL ATOM

ABOUT THE PARTICLES…..

Properties of Subatomic Particles

Particle Symbol Location Charge Relative mass

Actual mass (g)

Electron e- Around nucleus 1–

1 1840

9.11×10-28

Proton p+ Nucleus 1+ 1 1.673×10-24

Neutron n0 nucleus 0 1 1.675×10-24

• REPRESENTS THE NUMBER OF PROTONS IN AN ATOM

• ELEMENTS ARRANGED BY INCREASING ATOMIC NUMBER

• UNIQUE TO A GIVEN ELEMENT• SINCE ALL ATOMS ARE

NEUTRAL, ATOMIC NUMBER ALSO TELLS US THE NUMBER OF ELECTRONS IN AN ATOM

• USUALLY FOUND AT THE TOP OF THE ELEMENT’S SYMBOL

ATOMIC NUMBER

11

Na22.990Sodium

LecturePLUS Timberlake 7

LEARNING CHECK 1

State the number of protons for atoms of each of the following:A. Nitrogen (N)1) 5 protons 2) 7 protons 3) 14 protons

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

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

• THE TOTAL NUMBER OF PROTONS AND NEUTRONS IN THE NUCLEUS OF AN ATOM

• MASS NUMBER – ATOMIC NUMBER = # NEUTRONS

• ROUND THIS NUMBER TO THE NEXT WHOLE NUMBER TO DETERMINE THE MASS NUMBER

MASS NUMBER

11

Na22.990Sodium

Example: Sodium (Na)

Mass Number = 23Atomic Number = 11

______ 12

neutrons

-

PRACTICE FROM THE PERIODIC TABLE

Complete the following chart:

Name SymbolMass

Number#

protons#

neutrons#

electrons

Lithium

Mg

26

16

• DALTON THOUGHT THAT…..– Atoms could not be divided(indivisible)

….atoms can be divided– All atoms of an element are

IDENTICAL….not really!!– Atoms of an element have same

number of p+ and e-, but the number of n0 may be different

ISOTOPES

• ISOTOPES ARE…–Atoms of the same element with

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

–Atoms of the same element (same atomic number) with different mass numbers

ISOTOPES

THE ISOTOPES OF HYDROGEN

All atoms have the same number of protons (p+=1)…….

….but different number of neutrons in the nucleus (which means these isotopes will have different mass numbers!!)

A

Z XA = mass number

(the total number of protons + neutrons)Z = atomic number

(the total number of protons)X = element symbol

ISOTOPIC NOTATION

READING ISOTOPIC NOTATION

46

21 Sc46 = mass number (the total number of protons (21) + neutrons

(25))21 = atomic number

(the total number of protons (21))Sc = element symbol

(In a neutral atom, the number of electrons (21) is equal to the number of protons)

ANOTHER WAY OF WRITING AN ISOTOPE

46

21 Sc or Scandium-46

Name of the element followed by the mass number(This format does not include the atomic number)

LecturePLUS Timberlake 16

PRACTICE

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-

LecturePLUS Timberlake 17

PRACTICE

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

1) 14 2) 163) 34

B. Its mass number is1) 14 2) 16

3) 34

C. The element is1) Si 2) Ca 3)

Se

D. Another isotope of this element is1) 34X 2) 34X 3) 36X

16 14 14

• Most elements found as a mixture of isotopes

• Relative abundance of each isotope is constant

• Example: Potassium in bananas–93.25% of K atoms have 20 n0

–6.7302% of K atoms have 22 n0

–0.0117% of K atoms have 21 n0

LecturePLUS Timberlake 18

ISOTOPES

• Isotopes of an element differ in mass

• The more neutrons in an isotope, the greater the mass

• Still, isotopes of an element have the same chemical behavior

• Why? Chemical behavior depends on the number of e-, not n0 or p+

LecturePLUS Timberlake 19

ISOTOPES

LecturePLUS Timberlake 20

CALCULATING ATOMIC MASS

Percent abundance of isotopes

Mass of each isotope of that

element

Average Atomic Mass= mass isotope1(percent abundance1) + mass isotope2(percent

abundance2) + …

100 100

LecturePLUS Timberlake 21

ATOMIC MASS OF MAGNESIUM

Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

Average atomic mass of Mg = 24.3 amu

Mg24.3

LecturePLUS Timberlake 22

LEARNING CHECK AT7

Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium?

LecturePLUS Timberlake 23

SOLUTION AT7

Ga-69 (%/100)

68.9 amu x 60.2 = 41.5 amu for Gallium-69

100Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for Gallium-71

100

Atomic mass Ga = 69.7 amu