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Atomic Structure & Compounds (loosely based on Chapter 3 Sec 1 thru 4 of Jespersen 6 th ed). Dr. C. Yau Fall 2013. 1. Dalton's Atomic Theory (p.10). Matter consists of tiny particles called atoms. In any sample of a pure element, all the atoms are identical in mass and other properties.
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Dr. C. Yau
Know this well !
u (amu) = atomic mass unit
(a very small unit of mass)
1 gram = 602,000,000,000,000,000,000, 000 u
are atoms of the same element with different masses.
are atoms with the same #p but with different #n.
This indicates the mass number.
From the nuclear symbol we can determine the # protons, electrons and neutrons:
# protons = 92
# electrons = 92 (since no charge is shown)
#neutrons = 235 – 92 = 143
Mass number, (protons + neutrons)
Atomic number, (number of protons)
symbol neutrons protons electrons
36 29 29
How many neutrons are
there in Fe?
Trick question! You can’t tell!
How many neutrons are there
D) none of these
Ans. 53 – 26 = 27
mass number - # protons = # neutrons
This is NOT the atomic mass number.
0.015 % abundance
0.000 % abundnace
approx. mass of each atom = 1 u
(Regular water being H2O, with practically all the hydrogen atoms being protium.)
One molecule of H2O weighs 18 u.
One molecule of D2O weighs 20 u.
Hence D2O is known as “heavy water.”
Tritium is the only radioactive isotope of hydrogen. Such isotopes are referred to as a radioisotope.
NOTE: The term "isotope" does NOT mean that it is radioactive.
0.7577 x 34.9689 u = 26.495 u (4 sig. fig.)
0.2423 x 36.9659 u = 8.9568 u (4 sig. fig.)
atomic mass = 35.4518 u (2 decimal places)
This is the atomic mass
or atomic weight.
Hint: Let x be the mass of the other isotope.
What % in abundance is this other isotope?
159.6u = 160. u (3 sig.fig.)
or main group elements
I A = alkali metals II A = alkaline earth metals
VII A = halogens VIII = noble (inert) gases
58 – 71 = lanthanide elements
90 – 103 = actinide elements
nonmetal + nonmetal
nonmetal + metal
metal + metal
Note: Alloys are not compounds. They are homogeneous mixtures of metals, sometimes with a small amount of nonmetals, such as C in steel.
What exactly does this dot tell you?Learning Check: Count The Atoms In A Chemical Formula
The subscripts in the formula of covalent molecules tell you exactly how many atoms of each element is present.
They do not merely state a ratio.
e.g. CO2 tells us the molecule contains one C atom and two O atoms.
Negatively charged ions are called anions
Subscripts in the formula always specify the smallest whole-number ratio of the ions needed to make a neutral combination (formula unit, or f.u.)
It does not mean 2 Fe ions are bonded to 3 O ions.
Subscripts tell us the ions are in a ratio of 2Fe to 3 O.
How does Ca form Ca2+?
Ca lost 2 electrons
How is N3- formed?
N gained 3 electrons
Symbol neutrons protons electrons
36 29 27
Hardness and brittleness
The formula tells you it’s a molecular compound, so it is not expected to conduct electricity, should have a relatively low mp, and therefore not likely to be a solid in its pure form.
You should know how the formula tells you that it’s a molecular formula. See Slide 18!