Counting atoms. atomic number - # of protons in atom of an element identifies element tells also #...

ChemistryChapter 3

Counting atoms

atomic number- # of protons in atom of an elementidentifies

element

tells also # of e-

Au, K, C, V

remember atoms of the same element ALWAYS HAVE the same number of protons, but may have different number of neutrons

if atom is neutral, then it has same # of p & e-

Isotopes

protium deuterium

tritium

Isotopes of Hydrogen

Mass Number-the total number of protons and neutrons in nucleus

always whole number

H isotopes have special names; all other elements are indicated by their mass number

hyphen notation- name written followed by mass #chlorine-37 hydrogen-3

nuclear symbol- name of element written w/ mass number as superscript & atomic number as subscript

Designating Isotopes

What’s the difference?

average atomic mass- the weighted average of the atomic masses of the naturally occurring isotopes of an element

usually reported on p. tableround to nearest whole number to become mass number

mass #- atomic # = # neutrons

Li p ___e-___n ___

find for Ag, F, H

Find p, e-, & n

atomic masses are VERY SMALL O atoms have a mass of 2.657 x 10-

23

scientists use relative atomic masses

one atom was arbitrarily chosen to be the STANDARD by which all other atoms are measured

carbon-12 atomit’s been assigned a mass of 12

amu, atomic mass units

atomic mass unit- is exactly 1/12 the mass of a carbon-12 atom

most elements occur as a mixture of isotopes

some isotopes are more prevalent than others

average atomic mass – the weighted average of the atomic masses of the naturally occurring isotopes of an element

at mass = (mass of isotope x abundance) + (any others)

total abundance

ex: Copper-63 makes up 69.15% and copper-65 has an abundance of 30.85%. What is its avg at mass?

ALL ATOMIC MASSES ARE GOING TO BE ROUNDED TO TWO DECIMAL PLACES BEFORE USED IN CALCULATIONS!

for ex: N is 14.01Ar= 39.95

IMPORTANT MESSAGE FROM OUR SPONSORS!

atoms too small to weigh individually

will use a unit to describe a particular # of objects

SI unit chosen is mole, molit is the amount of sub that contains as many elementary particles as there are atoms in 12 g of C-12

the mass of 1 mol of atoms of a pure element in grams is numerically equal to the atomic weight of that element in amu

for ex: 47.88g of Ti= 1 mol Ti atoms

1 mol = 6.022 x 1023 particles (atoms, molecules,

ions)aka Avogadro’s number

named in honor of Italian chemist Lorenzo Romano Amedeo Carlo Avogadro de Quaregna e di Cerreto

the mass of 1 mol of molecules, atoms, ions, or formula units

used to RELATE MASS IN Grams TO # ATOMS, MOLECULES, ETC

set it up as a conversion factor in calculations

molar mass = # grams of substance1 mol of sub

Molar mass

How many g of oxygen contains Avogadro’s # of O atoms?

Au?

K?

Cu

How many grams in 2.25 mol of Fe?

Use Avogadro’s # to show # of atoms in a given amount of a substance:

Avogadro’s constant = 1 mol of any substance

6.022 x 1023 particles

# grams of substance1 mol of sub

and

1 mol of any substance6.022 x 1023 particles

We’ve learned about 2 conversion factors:

How many mol of lead are equal to 1.57 x 1042 atoms of Pb?

How many atoms of Na are in 3.80 mol of Na?

What is the mass in grams of 5.0 x 109 atoms of Ne?

1. What is the mass in grams of 17.8 mol of Li?

2. How many mol of Ag are found in 351g?

3. If you have 25.3 mol of Ga, how many atoms are there?

Molar mass, Avogadro’s constant Quiz

calculate mol/mass/#molecules of compounds

H2O