Lecture 17 Atomic Number and Isotopes Ozgur Unal NIS – CHEMISTRY Lecture 17 Atomic Number and Isotopes Ozgur Unal

Atomic Number There are more than 110 elements in the Periodic Table. What makes these elements different? The number of protons in the nucleus of an atom identifies the atom as a particular element. The number of protons in an atom is called the atomic number. atomic number = number of protons = number of electrons

How many protons and electrons are in each atom? A) radon B) magnesium Atomic Number How many protons and electrons are in each atom? A) radon B) magnesium Radon has 86 protons and 86 electrons Magnesium has 12 protons and 12 neutrons

Isotopes Dalton was incorrect about atoms being indivisible and stating that all atoms of an element are identical. For example, there are 3 types of potassium atoms that occue naturally. Al these 3 types has 19 protons and 19 electrons. However, one type of potassium atom contains 20 neturons, another 21 neutrons and still another 22 neutrons. K-39 K-40 K-41

Isotopes and Mass Number Isotopes containing more neutrons are heavier, but they have the same chemical properties with the other atoms of the same type. Chemical behavior is determined by the number of electrons an atom has. Each isotope of an element is identified with a number called mass number. The mass number is the sum of the atomic number and neutrons in the nucleus. mass number = atomic number + number of neutrons

Isotopes and Mass Number Example: Hydrogen and its isotopes

Natural Abundance of Isotopes In nature, most elements are found as mixtures of isotopes. The relative abundance of each isotope is constant. For example, in banana 93.26% of potassium atoms have 20 neutrons, 6.73%have 22 neutrons and 0.01% have 21 neutrons. In another banana, these percentages will still be the same.

Lecture 18 Mass of Atoms Ozgur Unal NIS – CHEMISTRY Lecture 18 Mass of Atoms Ozgur Unal

Mass of Atoms What do you think the number below an atom’s symbol is in the periodic table? Why is this number not an integer? These numbers show the atomic mass of that element. Since we have different isotopes that belong to each element, how can we calculate the mass of one atom of an element? We should take the weighted average mass of the isotopes of that element, as well see later.

Mass of Atoms Remember the masses of protons and neutrons. mp ≈ mn ≈ 1.67 x 10-27 kg These numbers are so small To help us in our calculations, we define a new unit for mass, called atomic mass unit. Atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom. 1 amu is close to the mass of a proton or a neutron, but not exactly the same.

Atomic Mass An atom’s mass depends on the number of protons and neutrons it has. The masses of a proton and a neutron is very close to 1 amu. However, the mass of an atom is not an ineteger number. This is because elements have isotopes with different abundances. In order to find the mass of an element, we calculate the abundance and the mas of each isotope of that element. The atomic mass of an element is the weighted average mass of the isotopes of that element.

Atomic Mass Example: Chlorine has two istotopes: Cl-35 and Cl-37 Cl-35 has an abundance of 75.78 %, and Cl-37 has 22.24 %. If the masses of one Cl-35 atom and one Cl-37 atom 34.969 amu and 36.966 amu, respectively, what is the atomic mass of Chlorine mCl-35 = 34.969 amu Percent abundance: 75.78 % mCl-37 = 36.966 amu Percent abundance: 22.24 % Mass contribution of Cl-35: 34.969 * 75.78 % = 26.496 amu Mass contribution of Cl-37: 36.966 * 22.24 % = 8.957 amu Atomic mass of Chlorine = (26.496 + 8.957) amu = 35.453 amu

Atomic Mass - Examples Nitrogen has two naturally occuring isotopes: N-14 and N-15 The atomic mass of N if 14.007 amu. Which isotope is more abundant? N-14 is more abundant because the atomic mass is closer to 14 than 15.

Atomic Mass - Examples Atomic mass of F (Fluorine) is extremely close to 19 amu. What can you say about isotope abundance of F? Fluorine has only one form: F-19. Fluorine has no isotopes. Bromine (Br) has an atomic mass of 79.904 amu. What can you say about the abundance of Br isotopes? Bromine has two istotopes: Br-79 and Br-81. There is no Br-80