How do atoms differ?

Section 4.3

REVIEW: THE ATOM

The nucleus

REVIEW

Particle Location ChargeRelative Mass (amu)

Actual Mass

(g)

Electron

In the space

surrounding nucleus

1-0

(1/1840)

9.11 x 10-

28

Proton Nucleus 1+ 11.673 x

10-24

Neutron Nucleus 0 11.675 x

10-24

If all atoms are composed of protons, neutrons and

electrons, how do elements differ?

–What makes an atom of oxygen an oxygen atom?

• An element’s atomic number (Z) represents the number of protons in its nucleus

• The number of protons in an atom’s nucleus determines the identity of the atom– If an atom has 9 protons it’s fluorine, if it has

20 it’s calcium• In a neutral atom:

– number of protons = number of electrons, so:

atomic number = # protons = # electrons

ATOMIC NUMBER (Z)

THE MODERN PERIODIC TABLE

*Elements are arranged according to atomic number

HHydrogen

1

1.008

Atomic number (Z): number of protons

Element name

Symbol: one or two letters

Atomic mass: weighted average of element’s isotopes masses

– An element’s atomic number determines its position on the periodic table• The periodic table is organized left-to-

right, top-to-bottom by increasing atomic number

TAKE A LOOK AT A PERIODIC TABLE:

• Complete the following table:

ElementNumber of

protonsNumber of electrons

sodium (Na)

Copper (Cu)

Boron (B)

66

14

MASS NUMBER (A)• An atom’s mass number (A) represents the

total number of neutrons and protons in the nucleus of an atomMass number = # protons + # neutrons

orMass number = atomic number + # neutrons• How do you determine the number of

neutrons in an atom?# neutrons = mass number (A) - atomic

number (Z)

• It is always true that:– A carbon atom has 6 protons in its nucleus– A neutral carbon atom has 6 electrons

• A carbon atom also contains neutrons, but unlike electrons and protons the number of neutrons can change from carbon atom to carbon atom

• Some carbon atoms have 6 neutrons, others have 7 neutrons and still others have 8 neutrons

ISOTOPES• Dalton’s atomic theory states that all

atoms of a given element are identical. This is mostly true

• Atoms of the same element can differ in the number of neutrons

• most elements have two or more isotopes

• Isotopes are atoms of the same element with different numbers of neutrons (and therefore different masses)– atoms have the same atomic number but

different mass numbers

IDENTIFYING ISOTOPES• To distinguish one isotope from another an

atom’s mass number is added after the element’s name:– carbon-12; carbon-14; uranium-235

• Remember the mass number of an isotope represents the sum of the neutrons and protons in the nucleusPotassium-

39Potassium-

40Potassium

-41

protons 19 19 19

neutrons

20 21 22

electrons

19 19 19

isotopeproton

selectrons neutrons

Hydrogen–1 (protium)

1 1 0

Hydrogen-2(deuterium)

1 1 1

Hydrogen-3(tritium)

1 1 2

WRITING SYMBOLS FOR ATOMS

A

XZ

MASS NUMBER = # PROTONS + # NEUTRONS

SYMBOL

ATOMIC NUMBER = # PROTONS

PRACTICE

56

Fe26

26 protons

26 electrons

30 neutrons

PRACTICE: FILL IN THE BLANKS

symbol

atomic number

mass numbe

r

number of

protons

number of

electrons

number of

neutrons

Fe 56

60 144

102 45 45

59 31

Al 27

U235

92 U238

92

uranium-235 uranium-238

used for nuclear reactors and atomic

bombs

TURN TO THE PERSON NEXT TO YOU:

• You have an atom each of two isotopes of carbon:– What is the same about the two

atoms?– What is different about the two

atoms? – How can we tell the two atoms apart?

IMPORTANT TO NOTE:

• Isotopes with more neutrons have greater mass, but are chemically the same (under normal conditions)

• The number of protons determines the identity of the element and electrons are responsible for an element’s chemical properties

THE MODERN PERIODIC TABLE

*Elements are arranged according to atomic number

HHydrogen

1

1.008

Atomic number (Z): number of protons

Element name

Symbol: one or two letters

Average atomic mass: weighted average of element’s isotopes masses

ATOMIC MASS

• The mass of an atom depends on the number of electrons, protons and neutrons it contains

• because atoms are extremely small particles (even the smallest speck of dust can contain 10,000,000,000,000,000 atoms) it’s impractical to measure the mass of atoms in grams

Particle Actual Mass (g)

Electron 9.11 x 10-28

Proton 1.673 x 10-24

Neutron 1.675 x 10-24

MEASURING ATOMIC MASS• Because the actual masses of protons and neutrons

are very small chemists have developed a way to measure atoms based on the Carbon-12 atom as the standard

• Instead of grams, the unit we use is the Atomic Mass Unit (amu)– Carbon-12 is exactly 12 amu– 1 amu is exactly 1/12 of the mass of a carbon-12

atom• Protons and neutrons have a mass slightly greater

than 1 amu

Particle Mass (amu)

Electron 0.000549

Proton 1.007276

Neutron 1.008665

AVERAGE ATOMIC MASS• When one consults the periodic

table, the atomic mass of carbon is not 12.00 amu but 12.011 amu

• This is because most elements have more than one naturally occurring isotope and the atomic mass on the periodic table is the weighted average of the mass of each of the element’s isotopes

AVERAGE ATOMIC MASS

• In nature elements exist as a mixture of their isotopes, for example chlorine:

• 75% chlorine-35• 25% chlorine-37• Atomic mass of chlorine is 35.453 amu

• So the average atomic mass is calculated by taking the weighted average of the isotopes’ masses– This is why an element’s atomic mass

is not a whole number

AVERAGE ATOMIC MASS OF CARBON

IsotopeMass of isotope

Abundance of isotope

(%)

carbon-12

12.00000 amu

98.90%

carbon-13

13.00335 amu

1.10%mass x abundance

carbon-12: 12.000 x 0.9890 = 11.868 amu

carbon-13: 13.00335 x 0.0110 = + 0.143 amu

12.011 amu

PRACTICE: CALCULATE ATOMIC MASS FOR ELEMENT X. IDENTIFY

THE ELEMENT

Isotope Mass (amu)%

abundance

X-6 6.015 7.5%

X-7 7.016 92.5%

Isotope Mass (amu)%

abundance

6X 6.015 7.5%

7X 7.016 92.5%

1. Mass contribution = (mass)(% abundance)

6X Mass contribution = (6.015)(.075) = 0.451 amu

7X Mass contribution = (7.016)(.925) = 6.490 amu

2. Sum of the mass contributions:

0.451 amu + 6.490 amu = 6.941 amu

Which element is this?