Atomic

Structure

Modern Atomic Theory

• Atom – smallest particle of an element that retains the properties of the element

• Subatomic Particles– Protons– Neutrons– Electrons

Protons

• Positive Charge (+1)

• Located in the nucleus

• Mass = 1 amu (atomic mass unit)– 1 amu = 1.66 x 10-24 grams

• Symbol: p+ or H+

Neutrons

• Neutral Charge

• Located inside the nucleus

• Mass = approximately 1 amu

• Symbol: n

Electrons

• Negative Charge (-1)

• Located outside the nucleus

• Mass = 1/1836 of a proton (basically mass less)

• Symbol: e- or e-1

Nucleons

• Particles located inside the nucleus– Protons and Neutrons

• All mass is located in the center of the atom (inside the nucleus), nucleus is very dense

Atomic Number

• The number of protons in an atom

• Bold number on the periodic table

• For a neutral atom: Number of protons = number of electrons

Why???

Atomic Mass/Mass Number

• The mass of an atom (expressed in amu)

• Equal to the number of protons + the number of neutrons in an atom

Why aren’t electrons included in the mass?

Nuclear Symbol

• The nuclear symbol consists of three parts

– the symbol of the element– the atomic number of the element– the mass number of the specific isotope

Examples

Indicate how many protons, neutrons and electrons are in each of the following atoms.

1. Aluminum-27

2. Xe-131

3. 56Fe

Gram Atomic Mass

• The mass of 1 mole of the element– 1 mole = 6.02 x 1023 particles

• Expressed in grams instead of amu

Mass Examples

1. 1 atom of K =

2. 1 mole of K =

3. 1 CO2 molecule =

4. 1 mole of CO2 =

Isotopes

• Same element, same atomic number, number of protons are the same

• Different masses, because the number of neutrons varies

Example: indicate the number of protons, neutrons, and electrons in carbon-12 and carbon-14

Weighted Average Atomic Mass

• Mass found on the periodic table is the weighted average of the naturally occurring isotopes

• Accounts for the percent abundance and mass of each of the elements isotopes

How to Calculate

• Each exact atomic mass is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures.

Example: Nitrogen

Mass Number

Exact Mass (amu)

Percent Abundance

14 14.003074 99.63

15 15.000108 0.37

Examples

• Calculate the weighted average atomic mass for Magnesium.

Mass Number

Exact Mass (amu)

Percent Abundance

24 23.985043 78.99

25 24.985837 10.00

26 25.982593 11.01

Ions

• Occur when a neutral atom gains or loses a electrons

• Caused by the transfer of electrons• Results in a charged atom• Protons are never lost or gained

Cation

• Positive Ion• Results from the loss of an electron

• Elements on the left side of the table (metals) tend to lose electrons to form cations

Anion

•Negative ion• Results from the gain of an electron

• Elements on the right side of the table (nonmetals) tend to gain electrons to form anions

Ion Examples

Indicate the number of protons, neutrons and electrons in each:

1. 35Cl atom:

2. 35Cl- ion:

3. 39K atom:

4. 39K+ ion:

5. 40Ca atom: 6. 40Ca2+ ion: