# 3.4 Elements and Compounds Interesting facts about elements

```3.4 Elements and Compounds
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There are only about _________ elements which are known to occur naturally on Earth.
Seventy-five percent of the mass of the universe is estimated to be
_____________
The most abundant elements found in Earth’s crust are ______________,
_______________, _______________, and ________________
Most elements are _______________. These elements are usually
____________ (shiny) and can _______________________.
Periodic Table
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The modern periodic table is based on the work of ____________________
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Initially Mendeleev arranged the elements by increasing _______________. By
doing this he noticed a pattern and realized that elements with similar properties
occurred at regular intervals
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Mendeleev’s periodic table allowed him to
________________________________________________________________
___________________________________________________, and his
organizational structure is still in place today
Common Elements and their Symbols
Element symbols always _________________________________________________
Aluminum
Antimony
Argon
Barium
Beryllium
Bismuth
Boron
Bromine
Calcium
Carbon
Cesium
Chlorine
Chromium
Cobalt
Copper
Fluorine
Gold
Helium
Hydrogen
Iodine
Iron
Krypton
Lithium
Magnesium
Manganese
Mercury
Neon
Nickel
Nitrogen
Oxygen
Phosphorus
Platinum
Potassium
Silicon
Silver
Sodium
Sulfur
Tin
Tungsten
Uranium
Zinc
3.4 Elements and Compounds
Compounds and % by mass
Remember that in a compound, elements are _________________combined
The properties of the compound are usually very different from the individual
components: Example NaCl
Percent by mass =
Two substances are the same substance if the _______________________ is identical
for all corresponding elements in each substance.
Problems:
1. If hydrogen composes 11 % of a 100 gram sample of water, what is the mass of
the hydrogen in the sample?
2. If one gram of hydrogen reacts completely with 19.9 g of fluorine, what is the %
by mass of hydrogen in the compound that is formed?
3. If 3 % of a 100 g HCl solution is hydrogen, what is the mass of the chlorine in the
sample?
4. A 78.0 g sample of an unknown compound contains 12.4 g of hydrogen. What is
the percent by mass of hydrogen in the compound?
5. If 40% of a 300 g NaCl solution is sodium, what is the mass of the sodium
present in the sample?
6. Find the % by mass of the following elements in their compounds:
a. Hydrogen in H2O
b. Carbon in CO2
c. Carbon in CCl4
Answers: 11 g, 4.8 %, 97 g, 15.9 %, 120 g, a)11.1%, b) 27.3% c) 7.8%
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