Proton, Neutron, Electron Counting

• Protons (p+ ) are positively charged and located in the nucleus

• The number of protons in each atom can be found on the periodic table. This number is known as the atomic number.

Proton, Neutron, Electron Counting

• Electrons (e-), as you know, are negatively charged and located outside the nucleus in the electron cloud.

• In neutral atoms (no charge) the number of protons should equal the number of electrons.

• EXAMPLE: Carbon has a atomic number of 6. Therefore it has 6 protons. Since it is a neutral atom it also has 6 electrons.

Proton, Neutron, Electron Counting

• Neutrons (N0 ) do not have a charge.

• The number of neutrons is found by subtracting atomic number from the mass number.

• Example: In carbon-12 there are 6 protons, 6 electrons, and 6 neutrons (MASS # - ATOMIC #)

Proton, Neutron, Electron Counting

• The number of neutrons in an atom can change. This will not change the type of atom it is.

• These are called isotopes:

Example: carbon-12: 6p, 6n, 6e; carbon -13: 6p, 7n, 6e

Carbon-12 and Carbon-13 are isotopes of carbon.

Relative Abundance and Mass Number

• Since isotopes exist, we must account for them when we calculate the mass number for an atom.

• This is done by a weighted average. Basically, the more of a certain isotope there is for an atom will have more of an effect on the average atomic mass.

(THINK ABOUT HOW YOUR GRADE IS CALCULATED HERE BIGGER ASSIGNMENTS “COUNT” FOR MORE)

EXAMPLE PROBLEMS ON CHALKBOARD

Relative Abundance and Mass Number

avg. atomic mass = (% in decimal form of isotope1 ) X (mass of isotope1 )

+ (% in decimal form of isotope2 ) X (mass of isotope2 )

EXAMPLE: Carbon has two isotopes. Carbon-12 has a mass of 12.000 amu and is 98.89% abundant. Carbon-13 has a mass of 13.001 amu and is 1.11% abundant. What is the average atomic mass for carbon?