Section 4-3

Section 4.3 How Atoms Differ

• Explain the role of atomic number in determining the identity of an atom.

• Define an isotope.

• Explain why atomic masses are not whole numbers.

• Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

Section 4-3

Section 4.3 How Atoms Differ (cont.)

atomic number

isotopes

mass number

The number of protons and the mass number define the type of atom.

periodic table: a chart that organizes all known elements into a grid of horizontal rows (periods) and vertical columns (groups or families) arranged by increasing atomic number

atomic mass unit (amu)

atomic mass

Section 4.3 Subatomic Particles

POSIT IVECHARG E

PROT ONS

NEUT RALCHARG E

NEUT RONS

NUCLEUS

NEG AT IVE CHARG E

ELECT RONS

AT OM

Most of the atom’s mass.

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

QUARKSAtomic Numberequals the # of...

equal in a neutral atom

Section 4-3

Atomic Number

• Each element contains a unique positive charge in their nucleus.

• The number of protons in the nucleus of an atom identifies the element and is known as the element’s atomic number.

Section 4-3

Isotopes and Mass Number

• All atoms of a particular element have the same number of protons and electrons but the number of neutrons in the nucleus can differ.

• Atoms with the same number of protons but different numbers of neutrons are called isotopes.

Section 4-3

Isotopes and Mass Number (cont.)

• The relative abundance of each isotope is usually constant.

• Isotopes containing more neutrons have a greater mass.

• Isotopes have the same chemical behavior.

• The mass number is the sum of the protons and neutrons in the nucleus.

Atomic & Mass Numbers

• Mass Number = protons + neutrons

• Atomic Number = protons only

• # of Neutrons = mass # - atomic #• Mass Number

– always a whole number

– NOT on the Periodic Table!

© Addison-Wesley Publishing Company, Inc.

Section 4.3 Isotopes

C126Mass #

Atomic #

• Nuclear symbol:

• Hyphen notation: carbon-12

4.3 Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 8

- 33 42

- 31 15

8 8 1616

Arsenic 75 33 75

Phosphorus 15 3116

Atomic #

Section 4.3 Isotopes

• Chlorine-37

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20

Cl3717

Section 4-3

Mass of Atoms

• One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.

• One amu is nearly, but not exactly, equal to one proton and one neutron.

• 12C atom = 1.992 × 10-23 g

Section 4-3

Mass of Atoms (cont.)

• The atomic mass of an element is the weighted average mass of the isotopes of that element.

Avg.AtomicMass

• EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Avg.AtomicMass

• EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.40 amu

A. A

B. B

C. C

D. D

Section 4-3

A B C D

0% 0%0%0%

Section 4.3 Assessment

An unknown element has 19 protons, 19 electrons, and 3 isotopes with 20, 21 and 22 neutrons. What is the element’s atomic number?

A. 38

B. 40

C. 19

D. unable to determine

A. A

B. B

C. C

D. D

Section 4-3

Section 4.3 Assessment

A B C D

0% 0%0%0%

Elements with the same number of protons and differing numbers of neutrons are known as what?

A. isotopes

B. radioactive

C. abundant

D. ions