Ch 19 Properties of Atoms and the Periodic Table Section 2 Masses of Atoms

The Essential Question – What does the periodic table tell us about the structures of different atoms?

Ponder the following A Bohr model of a helium atom and a beryllium atom are given below. Helium, HeBerylium, Be (cont.)

– List three similarities and three differences. – How do you think a gold atom is different from a copper atom? (cont.)

You will be able to: – Use the periodic table to identify the properties of an elements atom.

– Atomic Mass Unit is one-twelfth the mass of a carbon atom containing six neutrons and six protons – Atomic number is the number of protons in the nucleus of an atom. – Mass number is the mass of an individual atom. The sum of the neutrons and protons in the nucleus. Notes

Unit 1 Investigation III Activity Purpose: This lesson will formally introduce you to atomic structure. Beryllium AtomFluorine AtomCarbon Atom (cont.)

elementchemical symbol atomic number # of protons # of electrons # of neutrons mass number atomic weight beryllium5 fluorine chlorine lead126 potassium1939 tin70 tungsten gold118 (cont.)

Making Sense – If you know the atomic number of an element, what other information can you figure out about the atoms of that element? – If you know the atomic number of an element, can you figure out how many neutrons an atom of that element has? Can you come up with a close guess? Explain.

– Mass number is the number of protons plus the number of neutrons. – Atomic mass is the “weight” or mass of a single atom. – Atomic weight is the decimal number on the periodic table. Notes

Check-In Use your periodic table to identify the following elements: a) Atomic number 18 b) Has three electrons c) Atomic mass of 16.0

Wrap-Up – Each successive element has one more proton than the element preceding it. – The atomic number is equal to the number of protons. (cont.)

– The number of electrons is equal to the number of protons (as long as the atom is neutral). – The mass number is equal to the number of protons plus the number of neutrons (most of the mass is found in the nucleus). (cont.)

Isotopes

The Essential Question – How do isotopes of an atom account for the atomic weight of an element?

Ponder this A chemist investigating a sample of lithium found that some atoms have a lower mass than other atoms. The chemist determined that the structures of the two types of atoms would be similar to the two drawings below. (cont.)

– What is different about the two atoms? – What is the atomic number of each atom? – What is the mass number of each atom? – Do you think they are both lithium atoms? Why or why not?

– Atoms of the same element that have different numbers of neutrons are called isotopes. – Atomic mass units (amu) are “invented” measurement units of the atomic mass. Notes

Unit 1 Investigation III

Activity Purpose: In this lesson you will investigate isotopes and how they affect atomic weight. (cont.)

Unit 1 Investigation III boron atom # protons # neutrons # electrons (cont.)

Unit 1 Investigation III ElementChemical Symbol Atomic Number Atomic Weight # of protons # of electrons # of neutrons BoronB5 or 6 Chlorine17 Lithium6.94 VanadiumV23 Nitrogen7 Magnesium ArgonAr or 22 (cont.)

Unit 1 Investigation III Making Sense – Explain why the atomic weights listed in the periodic table are not usually whole numbers.

Unit 1 Investigation III – While the element iron is defined as being made up of neutral atoms with 26 protons and 26 electrons, not every iron atom has the same number of neutrons. – Atoms that have the same number of protons but different numbers of neutrons are called isotopes. Notes (cont.)

Unit 1 Investigation III – What we call the atomic weight on the periodic table is actually the average atomic mass of that element’s naturally occurring isotopes. – Isotopes have similar chemical properties in that they combine with other elements to form similar compounds. Notes (cont.) (cont.)

Unit 1 Investigation III – Atomic Weight is the weighted average of the atomic masses of different isotopes taking into account their abundance. (cont.)

Unit 1 Investigation III (cont.)

Unit 1 Investigation III Check-In – Predict the isotopes of carbon, C. Which isotope is more abundant? How do you know?

Unit 1 Investigation III Wrap-Up – Elements may have anywhere from 2 to 10 naturally occurring isotopes. – The atomic weight of an element listed on the periodic table is actually the average mass of the naturally occurring isotopes of that element. – Isotopes have the same number of protons and electrons, but different numbers of neutrons. (cont.)

Unit 1 Investigation III – Isotopes of a single element exhibit similar properties in that they form similar compounds. – Isotopes are referred to by their mass numbers. (cont.)