Distinguishing Between Atoms

• Atomic Number: equals the number of protons in the nucleus of an atom of that element

• Atoms are electrically neutral, therefore the number of electrons must equal the number of protons

• How is the number of electrons for a neutral atom of a given element related to the atomic number of that element?– The atomic number is equal to the number of

electrons

Atoms of the First Ten Elements

Name Symbol Atomic Number

Composition of the Nucleus

Mass Number Number of electronsProtons

Neutrons

Hydrogen

H 1 1 0 1 1

Helium He 2 2 2 4 2

Lithium Li 3 3 4 7 3

Beryllium

Be 4 4 5 9 4

Boron B 5 5 6 11 5

Carbon C 6 6 6 12 6

Nitrogen N 7 7 7 14 7

Oxygen O 8 8 8 16 8

Fluorine F 9 9 10 19 9

Neon Ne 10 10 10 20 10

Element block

Review• The element Nitrogen

has an atomic number of what?

• How many protons and how many electrons are in a neutral nitrogen atom?

• 7

• 7, number of protons equals the number of electrons

How many protons and electrons are in each of the following atoms?

• Fluorine• 9,8• 9,9 • 8,9• 8,8• Aluminum• 12,13• 12,12• 13,12• 13,13

• Calcium• 19,19• 19,20• 20,20 • 20,19

Complete the Table

Element Atomic Number

Protons Electrons

K 19 19

5

16

23

Mass Number: the total number of protons and neutrons in an atom

• Mass # = protons + neutrons

• Example: Oxygen

• 16 = 8 + n

• n = 8

The following is shorthand for writing the # of protons and neutrons, you don’t need to write the # of electrons because they are the same as the # of protons

The large "X" represents where you will find the atom's elemental symbol.  The mass number, which is given the symbol "A", is located in the upper left-hand corner.  The atomic number (also called nuclear charge), which is given the symbol "Z", is found in the lower left-hand corner.

Number of neutrons  =  mass number  −  atomic number

• In the case of neon (Ne):

Number of neutrons  =  20- 10 = 10

• In the case of carbon (C):

Number of neutrons  =  12- 6 = 6

Number of neutrons  =  mass number  −  atomic number

More Examples

Elemental Symbol

# of Protons (equal to Z) 3 9

# of Neutrons(equal to A-Z) 4 10

# of electrons (equal to Z modified by

charge number)3 9

How many protons, electrons, and neutrons are in the following atoms?

Atomic Number

Mass Number

(a) Beryllium (Be)

4 9

(b) Neon (Ne) 10 20

(a) Sodium (Na) 11 23

How many neutrons are in each atom?

16 O

8

• 8• 16• 24

32 S

16

• 32• 48• 16

Express the composition of the following atom in shorthand form

Carbon-12

• 12 C 6

• 6 C 12

• 18 C 12

Fluorine-19

• 9 F 19

• 28 F 9

• 28 F 19

• 19 F 9

Isotopes: atoms that have the same number of protons but different numbers of neutrons

• Isotopes have different mass numbers because they have different numbers of neutrons

• They are chemically alike because they have identical numbers of protons & electrons, which are the subatomic particles responsible for chemical behavior.

Example: Hydrogen has three isotopes; each isotope has 1

proton in its nucleus.

Two isotopes of carbon are carbon 12 and carbon 13. Write the symbol for each isotope using superscripts and subscripts to represent mass #

and atomic number

Carbon-12 Carbon-13

___ ___

___C ___C

Determine the number of protons, neutrons, and electrons for each of the following

isotopes of Neon Neon-20

• 22• 10• 11

• Neon-21

• 21• 10• 11

Neon-22

• 22• 10• 12

Atomic MassProperties of Subatomic Particles

Particle Symbol Relative electrical charge

Relative mass(Mass of proton

=1)

Actual mass (g)

Electron e- 1- 1/1840 9.11x10-28

Proton P+ 1+ 1 1.67x10-24

Neutron n◦ 0 1 1.67x10-24

Actual mass of a proton or a neutron is very small (1.67 x10-24 g)Since about the 1920s, it has been possible to determine these masses (protons & neutrons) by using a mass spectrometer

• This is impractical to work with, so a reference isotope as a standard is used. The chosen isotope is C-12. It was assigned a mass of EXACTLY 12 atomic mass units or amu

• Carbon-12 has 6 protons and 6 neutrons, making up its mass

• Since the mass of an atom mainly depends on the number of protons and neutrons, shouldn’t the atomic mass of an element be a whole number? That’s usually not the case, Cl is 35.453 amu

• The reason for this involves the relative abundance of the naturally occurring isotopes of the element. Most occur as a mixture of two or more isotopes

• Each isotope has a fixed mass and a natural percent abundance

The atomic mass is a weighted average of all naturally occurring isotopes

• http://www.sisweb.com/referenc/source/exactmaa.htm

Example: Chlorine

35 Cl and 37 Cl 17 17

76% 24%

Three more 35 Cl for every one 37 Cl 17 17(Remember: the # of protons and neutrons = atomic mass)

% abundance:# of protons = 17 + 17+ 17+ 17 = 68# of neutrons = 18 +18 +18 +20 = 74 68 + 74 = 35.5 amu 4

Therefore, the Atomic Mass is a weighted average mass of the atoms in a naturally occurring sample of the element

ReviewWhich isotope of copper is more abundant:

copper – 63 or copper – 65? (the atomic mass of copper is 63.546 amu)

• Copper-63

• Boron has two isotopes: boron-10 and boron-11, which is more abundant

(atomic mass is 10.81 amu)?

• Boron -11

• Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element.

• 10X = 10.012 x 0.1991 = 1.993 amu

• 11X = 11.009 x 0.8009 = 8.817 amu

10.810 amu

• What is this element?

• Boron

Calculate the atomic mass of Bromine. The two isotopes of bromine have atomic masses and relative abundances of 78.92 amu (50.69%) and 80.92 amu (49.31%)

• 79.91 amu