Structure of the AtomStructure of the Atom

CHEMCHEM

Basic Parts of the AtomBasic Parts of the Atom

nucleusnucleus –the positively charged center of the –the positively charged center of the atomatom

protonsprotons – positive charged particle with a relative – positive charged particle with a relative mass of 1 mass of 1

neutronsneutrons – neutrally charged particle in the nucleus – neutrally charged particle in the nucleus with a relative mass of 1 with a relative mass of 1

electron cloudelectron cloud – negatively charged, mostly – negatively charged, mostly empty spaceempty space

electronselectrons – negatively charged particles with a – negatively charged particles with a relative mass of 1 / 1840relative mass of 1 / 1840

Atomic StructureAtomic Structure

Forces in AtomsForces in Atoms Electromagnetic Force:Electromagnetic Force:

Like charges Like charges repelrepel, opposite charges , opposite charges attractattract. . Strong ForceStrong Force::

Opposes the electromagnetic force between Opposes the electromagnetic force between protons in the nucleus. protons in the nucleus.

The The strongeststrongest of the four forces, but only acts over of the four forces, but only acts over short distances. short distances.

Weak ForceWeak Force:: The force responsible for radioactive decay. The force responsible for radioactive decay.

GravityGravity:: The force of attraction between all objects in the The force of attraction between all objects in the

universe. universe. The The weakestweakest of the four forces. of the four forces.

Atomic NumberAtomic Number

The The atomic numberatomic number is equal to the number is equal to the number of of protonsprotons in the nucleus of an atom in the nucleus of an atom

This number is expressed as a This number is expressed as a whole whole numbernumber on the periodic table. on the periodic table.

Since + and - charges are equal in atoms, Since + and - charges are equal in atoms, it is also the number of it is also the number of electronselectrons

Mass NumberMass Number

The The mass numbermass number is equal to the is equal to the sum of the sum of the protons and neutronsprotons and neutrons in the nucleus of an atom. in the nucleus of an atom.

This number is expressed as a This number is expressed as a

Number of neutrons in a atom = (mass number, Number of neutrons in a atom = (mass number, as a whole number) - (atomic number) as a whole number) - (atomic number)

How many neutrons are in an atom of zinc? . . . . How many neutrons are in an atom of zinc? . . . . 65 - 30 = 35 neutrons in Zn65 - 30 = 35 neutrons in Zn

Atomic Number and Mass NumberAtomic Number and Mass Number

IsotopesIsotopes

Atoms which have the same number of Atoms which have the same number of protons with different amounts of protons with different amounts of

neutronsneutrons..

Carbon-12 has 6 neutrons, while Carbon-Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. 14 has 8 neutrons.

The mass number for an element on the The mass number for an element on the periodic table represents the periodic table represents the averageaverage for for all the isotopes in a sample of the element. all the isotopes in a sample of the element.

Hydrogen has 3 common isotopes

Carbon also has 3 common isotopes nuclide name

1)6p, 6n mass # = 12 carbon - 122)6p, 7n mass # = 13 carbon - 133)6p, 8n mass # = 14 carbon - 14

1 2 3

IsotopesIsotopes

Atomic MassAtomic Mass

The average mass of the mixture of an elements The average mass of the mixture of an elements isotopesisotopes

Atomic mass is measured in Atomic Mass Units Atomic mass is measured in Atomic Mass Units where 1amu = (1/12) mass of carbon measured in where 1amu = (1/12) mass of carbon measured in grams. grams.

In order to find this average you need to know:In order to find this average you need to know:

1.1. number of stable isotopes of the atomnumber of stable isotopes of the atom

2.2. mass of isotopesmass of isotopes

3.3. percent abundance in nature of isotopespercent abundance in nature of isotopes

Atomic MassAtomic Mass

Example: A sample of cesium is 75%Example: A sample of cesium is 75%133133Cs, 20% Cs, 20% 132132Cs, and 5% Cs, and 5% 134134Cs. Cs.

What is its average atomic mass?What is its average atomic mass?

Answer: Answer: .75 x 133 = 99.75.75 x 133 = 99.75

.20 x 132 = 26.4 .20 x 132 = 26.4

.05 x 134 = 6.7.05 x 134 = 6.7

Total = 132.85 amuTotal = 132.85 amu