101A short lecture on

Isotopes by Mr. C

Isotopes and the Periodic Table

• An element is identified by the number of its protons

• An atom’s mass is determined by the number of protons + neutrons

• Therefore, two atoms of the same element can have different masses (ISOTOPES)

HOW?

Isotopes:Once again:

• An element is identified by the number of its protons

• And atom’s mass is determined by the number of protons + neutrons

• Therefore, two atoms of the same element can have different masses (ISOTOPES) if they have the same number of but different numbers of

PROTONSNEUTRONS

Isotopes:• So: 2 atoms of the same element can have

different masses (ISOTOPES) if they have the same number of protons but different numbers of neutrons.

• Example - HYDROGEN • Hydrogen-1 and Hydrogen-3 p+ p+ n n• Hydrogen-1 is the most common hydrogen isotope;

Hydrogen-3 (also known as tritium) is part of “heavy water” waste from nuclear power plants

• Example - CARBON

C-12 occurs most often in nature, • C-13 occurs in about 5 % of all carbon, • C-14 is used in carbon dating.

Carbon-12 (6 p+ 6 n)

Carbon-13 (6 p+ 7n)

Carbon-14 (6 p+ 8n)

p+p+p+p+p+p+ p+p+p+p+p+p+ p+p+p+p+p+p+

n n n n n n n n n n n n n n n n n n n n n

ISOTOPES – so what?• Because of the existence of 3

isotopes of hydrogen its atomic mass is 1.0079 amu and not exactly 1 amu.

• This is why in the periodic table most elements’ atomic masses are not whole numbers (they have 2 or more isotopes).