Summary of the Atom• atoms are the smallest particles that can be

uniquely associated with an element• each element has unique atoms • atoms are composed of e-, p and n• atoms are electrically neutral (# of e- = # of p)• for a single element, isotopes differ only in

number of n (neutrons)• atoms have characteristic masses (atomic

weights)• atoms combine with one another in definite,

whole number proportions to make compounds

~ 10-10 m

nucleusMass 9 x 10Mass 9 x 10-31-31 kg kg Mass > 10Mass > 10-26-26 kg kg

electron

~ 1 – 7 x 10-15 m (1 – 7 fermi)

Microcosms of our solar system, atoms are dominantly empty space:

If an oxygen atom had a total radius of 100 km, the nucleus

would be a ~1 m diameter sphere in

the middle.

electron orbits

The Spacious Atom

In a simplistic model, electrons float around the nucleus in energy levels called shells.

As the number of

electrons increases,

they start to fill shells

farther out from the

nucleus.

In most cases,

electrons are lost or

gained only from the

outermost shell.

electron orbits

Electrons in Orbit

AtomAtom

NucleusNucleus

The Nuclear Modelof the atom

Subatomic Particles

Particle Symbol Charge RelativeMass

Electron e- 1- 0

Proton p+ 1+ 1

Neutron n 0 1

Atomic Number

11

Na

Atomic Number

Symbol

All atoms of an element have the same number of protons

11

Na

11 protons

Sodium

Number of Electrons

An atom is neutral The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons in a

neutral atom

Mass Number

Counts the number

of

protons and neutrons

in an atom

Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

Notation for Atoms

12C13C C

only one isotope of carbon

all isotopes of carbon

only one isotope of carbon

Basic Definitions• “atomic number” = number of protons in the

nucleus; • “atomic weight” = average mass of an atom

calculated from the masses and natural abundances of all isotopes

(use atomic weights to calculate the molecular weights of compounds from their constituent elements!)

• “mass number” = sum of protons + neutrons in the nucleus

• “isotopic mass” = mass of a single isotope

More Atomic Symbols

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-

Isotopes

Atoms with the same number of protons,

but different numbers of neutrons.

Atoms of the same element (same atomic

number) with different mass numbers

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37

Mass spectrometry

Atomic weight measurements

How was the atomic weight measured?

• By mass spectrometry– This also measures

% natural abundance

for a given isotope

Atomic weight calculation There are three naturally occuring isotopes

of neon (Ne):

20Ne isotopic mass = 19.99244018 amu21Ne isotopic mass = 20.9938467 amu22Ne isotopic mass = 21.9913855 amu

the atomic weight is reported in text as:20.1797 amu

Learning Check 1

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

12C 13C 14C 6 6 6

#P _______ _______ _______

#N _______ _______ _______

#E _______ _______ _______

Solution

12C 13C 14C 6 6 6

#P __6___ _ 6___ ___6___

#N __6___ _ _7___ ___8___

#E __6___ _ 6___ ___6___

Learning Check 2

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom

1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons?

1) 37 2) 65 3) 67

Solution

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30

B. Number of neutrons in the zinc atom

2) 35

C. What is the mass number of a zinc isotope

with 37 neutrons?

3) 67

Learning Check 3

Write the atomic symbols for atoms with the following:

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

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

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

Solution

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

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

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

Learning Check 4

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

1) 14 2) 16 3) 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 is

1) 34X 2) 34X 3) 36X 16 14 14

Solution

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

1) 14

B. It has a mass number of3) 34

C. The element is1) Si

D. Another isotope of this element would be

3) 36X 14

Masses of Atoms

A scale designed for atoms gives their small

atomic masses in atomic mass units (amu)

An atom of 12C was assigned an exact mass of

12.00 amu

Relative masses of all other atoms was

determined by comparing each to the mass of 12C

An atom twice as heavy has a mass of 24.00

amu. An atom half as heavy is 6.00 amu.

Atomic Mass

Gives the mass of “average” atom of each

element compared to 12C

Average atom based on all the isotopes and their

abundance %

Atomic mass is not a whole number

Na22.99

Calculating Atomic Weight or Mass

Percent(%) abundance of isotopes

Mass of each isotope of that element

Weighted average =

mass isotope1(%) + mass isotope2(%) + …

100 100

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%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

Atomic mass calculation

How was the atomic mass calculated?

• multiply each isotopic mass by the reported natural abundance for the isotope, then:

• add these individual contributions for each isotope to get the average atomic mass for the element

Atomic mass calculation

There are three naturally occuring isotopes of neon (Ne):20Ne mass # = 19.99244018 amu (90.51%)21Ne mass # = 20.9938467 amu (0.27%)22Ne mass # = 21.9913855 amu (9.22%)

the atomic mass is reported in text as:

20.1797 amu

18.10 + 0.057 + 2.03 = 20.19 amu

Learning Check 5

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?

Solution

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga

100

Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for 71Ga

100

Atomic mass Ga = 69.7 amu

Finding An Isotopic Mass

A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?

Assign X and Y values:X = % 10B Y = % 11B

Determine Y in terms of XX + Y = 100Y = 100 - X

Solve for X:X (10.0) + (100 - X )(11.0) = 10.8

100 100

Multiply through by 10010.0 X + 1100 - 11.0X = 1080

Collect X terms

10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B

- 1.0

Y = 100 - X

% 11B = 100 - 20% = 80% 11B

Learning Check 6

Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check Zumdahl or any other chemistry text for atomic mass)

1) 30% 2) 70% 3) 100%

Solution

2) 70%

Solution

62.9X + 6490 = 64.9X = 6350

-2.0 X = -140

X = 70%

Atomic Masses

13C12C

13.00335 amu (1.11%)

12.0000 amu (98.89%)

atomic weight of C = 12.01115 amu WHY?

Calculating masses of atoms relative to 12C

(mass of 12C atom) * 1.58320 = mass of F atom

= 18.99840

reported atomic weight of F = 18.9984

Charged Atoms: Ions

Left to their own devices, atoms are electrically neutral.

That means that they have an equal number of

protons and electrons.

During the course of most natural events,

protons are not gained or lost, but electrons may be.

Atoms with more or fewer electrons than protons are

electrically charged. They are called ions:

an atom that loses electrons takes on a positive charge

(cation);

an atom that gains electrons takes on a negative charge

(anion).

Complex cations and anions can also occur: (NH4)+1, (SO4)-2

An ISOTOPE is one of a set of nuclides with the same Z and consequently different A. (ie isotopes are the same chemical element but different masses). e.g.

  C126 C136 C146

An ISOBAR is one of a set of nuclides with the same A but different N and Z.

e.g C146 O148N147

An ISOTONE is one of a set of nuclides with the same N and consequently different A. e.g.

21412021

401921

3918 A,K,A

• More on atomic notation, which is based on the nuclear structure:– Isotope: same Z, different A and N– Isobar: same A, different Z and N– Isotone: same N, different Z and A

Example: From the following list of atoms, which are isotopes, isobars, and isotones?

Component

Atom

A Z N

Xe

I

Cs

I

Xe13154 I130

53 Cs13255 I131

53