Protons, Neutrons, Electrons/Isotopes/Average

Atomic Mass

If an atom was the size of a football stadium …

• the nucleus (protons + neutrons) would be a marble on the 50 yard line

• the electrons would be smaller than gnats out in the stands

An Atom is Like…Gnat-sized electron

Marble -sized nucleus

Important things to remember about atoms!

Most of an atom is empty space

Electrons are so small, we can pretend that their mass is zero

The mass of an atom is from its nucleus (protons & neutrons)

Atom Facts!

Tells you the identity of the element

(Each element has a different atomic #)

Atomic Number

Atomic Mass

Given the following atomic numbers, determine the identity of the elements:

Atomic # Element

16

9

53

Practice!

Sulfur

Fluorine

Iodine

Also tells you the number of protons

(Each element has a different # of protons)

Protons

Atomic Mass

Determine the number of protons for each element:

Element # Protons

chlorine

arsenic

oxygen

Practice!

17

33

8

Isotopes = Different versions of the same element

Isotopes of Carbon

Isotopes

Important things to remember about the isotopes of an element!

Same Element

Same # Protons

Different # Neutrons

Different Masses

Isotope Facts!

Relative Percent Abundance = The overall percent of each isotope of an element

Isotopes of Carbon

% Abundance

1

1H or Hydrogen-1 1 proton 0 neutrons

Mass number

Hydrogen-3OR

21H or Hydrogen-2

1 proton 1 neutron

31H or Hydrogen-3

1 proton 2 neutrons

Isotope Symbols

The mass number = # protons + # neutrons (So, if you know the # protons, you can figure out the #

neutrons, and vice versa!)

Example: 31H or Hydrogen-3

# Protons = 1

Mass Number = 3

Mass number = # protons + # neutrons

3 = 1 + x

x = # Neutrons = 2

Mass Number

Determine the # of protons and neutrons for each of the following isotopes:

168O

2110Ne

Flourine-19

Boron-11

Practice!

/8 protons, 8 neutrons

10 protons, 11 neutrons

9 protons, 10 neutrons

5 protons, 6 neutrons

The Atomic Mass is the average

weighted mass of the isotopes of an

element

Atomic Mass

Atomic Mass

To calculate the average weighted mass (atomic mass of an element:

Step 1Multiply each isotope’s mass x its %

abundance. (Make sure the % abundance is in decimal form!)

Step 2Add all of the answers from step 1 above.

Ave. Weighted Mass

Calculate the average weighted mass (the atomic mass) of nitrogen.

Isotope Mass Relative % Abund.

Nitrogen-14 14.003 amu 99.63

Nitrogen-15 15.000 amu 0.37

Step 1

(14.003)(0.9963) = 13.951 amu

(15.000)(0.0037) = 0.055 amu

Step 2

13.951 + 0.055 = 14.006 amu

Mass % Abund. (as a decimal)

Ave. Weighted Mass

Calculate the average weighted mass (the atomic mass) of Silicon.

Isotope Mass Relative % Abund.

Silicon-28 27.977 amu 92.23

Silicon-29 28.976 amu 4.67

Silicon-30 29.974 amu 3.10

Step 1:

(27.977)(0.9223) = 25.803 amu

(28.976)(0.0467) = 1.353 amu

(29.974)(0.0310) = 0.929 amu

Practice!

Step 2:

25.803+1.353+0.929 =

28.085 amu

Tells you the number of electrons too!

Remember! The atomic # also tells you the number of protons

(so…the # protons = # electrons in an atom)

Electrons

When an atom loses or gains one or more electrons, it becomes an ion.

There are two kinds of ions:• Cations: Ions formed when atoms lose

one or more electrons – have a positive charge

Ex. Ca2+, Na1+, Al3+

• Anions: Ions formed when atoms gain one or more electrons – have a negative charge

Ex. F1-, P3-, O2-

Ions!

Determine the number of electrons for each element:

Element # Electrons

S

S2-

Al

Al3+

Practice!

16

18

13

10

Finished!