نوفمبر 23rd, 2013

 

The Periodic Table

Elements can be grouped into classes according to the chemical compounds they form. If the elements are written in a table in order of increasing atomic weights, they fall naturally into a series of columns, with members of a given chemical group in the same column. This is called the periodic table of the elements.The term periodic means recurring at regular intervals. The original periodic tables were…

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Mole and chemical formulas

Because the individual atoms in a compound are combined in integer ratios, and each element has a specific atomic mass, it is possible to use mole calculations to determine chemical formulas. The two types of chemical formulas are molecular formulas and the empirical formulas.

– Often, the empirical formula is the same as the molecular formula.

The molecular formula shows the actual number of atoms in a substance….

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Percentage Composition of Compounds

The percentage composition of a compound is the percent by mass of each element in the compound.

How do we calculate the percent composition? Here are the steps:

Calculate the molar mass of the compound.

      2.Divide the molar mass of each element by the molar mass of the compound and multiply by 100%.

For example, here is how you calculate the percent by mass of…

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Molecular Mass of the Elements and compounds

Molar mass  means the mass of one mole. The molar mass concept can be applied to elements and to compounds. Molar mass is expressed in grams. For example, one mole of carbon has amass of 12 grams. Therefore, carbon’s molar mass is 12 grams. This is equivalent to the statement that 6. 02 x 1023 atoms of carbon have a mass of 12 grams. Consider this example: chemists know that…

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The Mole Concept

The concept of mole is extremely useful in chemistry. The mole is a term like dozen. Dozen means 12. The mole means 6.02 x 1023. It is convenient for chemists to use a concept like mole which represents such a large number since they deal in very small and very numerous quantities. The mole is also called Avogadro’s number.

Avogadro’s number: It is the number of molecules present…

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Chemical Formulas

There are two types of compounds: molecular and ionic compounds. For molecular compounds, a molecular formula represents the number and kinds of atoms in the molecule. The molecular formula also tells us the lowest whole number ratio between the atoms in the compound. For example, water is represented by the formula H2

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compounds

Compounds

Compounds are pure substances that differ from elements because they contain more than one kind of atom. The Law of Definite Composition (or the Law of Definite Proportions) applies to compounds.

For example, sodium chloride contains one atom of sodium for each atom of chlorine. Water contains two atoms of hydrogen for each atom of oxygen.

It is known that two elements can combine with one…

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isotopes and isobars

Isotopes: Isotopes are atoms of the same element having the same atomic number but different atomic masses. Note that the difference in the masses of isotopes is due to the different number of neutrons in their nuclei. Isotopes have the same chemical properties . because they have the same number of electrons. Isotopes have different physical properties because they have different number of neutrons.

Isobars: Nuclei of atoms of…

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Atomic Number and Mass Number

Atomic Number: Atomic number of an element is equal to the number of protons present in the nucleus of an element.

Atomic number = number of protons in one atom of element.

Since the atom as a whole is neutral therefore atomic number is the same as the number of electrons around the nucleus of an atom. Atomic number of an element does not change during…

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Structure of the Atom

Each atom is made up of an inner core called the nucleus.

 – Most of the atom’s mass is located in the relatively small nucleus.

 – The rest of the atom is comprised of a relatively large region outside the nucleus where the electrons are found.

– The atom is made of 3 subatomic particles:

Positively charged protons Neutral particles called neutrons Negatively charged electrons

– Protons and neutrons…

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