# Counting Atoms - PC\|MAC

Counting Atoms Chapter 3 Section 3 Atomic Number Atomic number: the number of _______ of each atom Represented by ____ The atomic number ____________

the element. Isotopes Isotopes: atoms of the ______element that have ____________masses Isotopes have the same number of ______ and ____________, but a different number of ____________. Most elements naturally have

__________ isotopes Mass Number Mass number: the total number of _____ and ________that make up the _______ of an isotope Mass number = ____ + _____ Isotopes are usually identified by specifying their ________number.

Designating Isotopes There are two methods to specify isotopes Method 1 is called ___________: write the name of the element, then a hyphen, then the mass number Method 2 is called the ____________: in front of the symbol of the element, _________the mass number and _______ the atomic number

Hydrogen Hydrogen is special in that it also has specific names for each isotope Hydrogen with only 1 p+ and no n0 is called ________, hydrogen-___, Hydrogen with 1 p+ and 1n0 is called __________, hydrogen-___, Hydrogen with 1 p+ and 2 n0 is called __________, hydrogen-___,

Practice: name the isotope both ways helium with 1 neutron __________, carbon with 7 neutrons __________,

oxygen with 8 neutrons uranium with 142 neutrons __________, ___________, More Practice How many protons, electrons, and

neutrons are there in an atom of chlorine-37? _______________________________ How many protons, electrons, and neutrons make up an atom of bromine80 _______________________________ Relative Atomic Masses

If we express the mass of atom in _____, it would be extremely ________. __________mass is used instead. ___________is the standard atom and has been assigned a mass value of exactly 12 ______________ (______) More about Relative Atomic Mass

All other atomic masses are determined by comparing it with the mass of the carbon-12 atom See Table 4 in this section for some typical atomic masses The masses could also be expressed by adding up the relative masses of the e-s, p+s, and n0s Average Atomic

Mass ______atomic mass: the __________of the atomic masses of the naturally occurring __________of an element. The average atomic mass depends on both the ________and the ______________of each of the elements isotopes. Calculating Average

Atomic Mass Copper consists of 2 naturally occurring isotopes A sample of copper contains 69.15% of copper-63 (62.929601 amu) and 30.85% of copper-64 (64.927794 amu) Express the percentages as _______values. 52.3% is expressed as ________

Continued Solve like this: _________________________________ _________________________________ ________________________________ Can round value to 2 decimal places The Mole The SI unit for the _______of a

substance ________: the amount of a substance that contains as many particles as there are atoms in exactly _____g of _________. It is a ___________unit like dozen, score, gross, etc. Avogadros Number _____________________: the

number of particles in exactly one mole of a pure substance NA = _________________particles Conversions with Avogadros Number Since _______________is the number of particles in a mole, that can be used as a _________________factor.

Work practice problems and see the book for sample problems. Basic Mole Problems How many atoms are found in an iron sample that is 3.5 moles? How many moles are in a sample of lead that contains 4.56 x 1025 atoms?

Molar Mass Molar mass: the mass of one _______of a pure substance Usually written in units of _________. The molar mass of an element is numerically equal to the _____________of the element in atomic mass units (found on the periodic table).

More about Molar Mass Molar mass is usually rounded to two decimal places. What is the molar mass of carbon? What is the molar mass of chlorine? Gram/Mole

Conversions The molar mass can be used as a ________________factor. Work practice problems Next Slide See the book for sample problems Simple Practice

What is the mass in grams of 3.50 mol of copper? A chemist produced 11.9 g of Al. How many mol of Al were produced?

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