On this page you will find some key working definitions for this unit. Understanding and applying these concepts to solve problems, allow chemists to describe elements and compounds in a quantitative way.

Atomic Number (Z)

The atomic number of an element, represented by the letter "Z", refers to the number of protons all atoms of that element, have in their nucleus. The atomic number is the "fingerprint" of an element. That is, there are no two elements with the same atomic number. The number of protons in an atom is unique for a particular element. Therefore, a good way to identify them.

Example: The atomic number of carbon is 6 (₆C)

Mass Number (A)

The mass number of an atom, represented by the letter "A", tells the number of PROTONS + NEUTRONS in the nucleus of the atom.

If you subtract the atomic number (Z) from the mass number (A), you can obtain the number of neutrons in the nucleus of that atom .

Example: Carbon (Z=6; A=12) 12 - 6 = 6 neutrons .

Isotopes

Isotopes are atoms of the same element that have different mass. This difference in mass is due to the number of neutrons in the nucleus of an atom. The number of protons of all atoms of the same element, is always the same. But the number of neutrons may vary. Therefore their mass varies.

Example: Carbon-12 (6 neutrons); Carbon-13 (7 neutrons); Carbon-14 (8 neutrons)

Carbon has 3 isotopes (or atoms with 3 different masses)

Atomic Mass

Atomic mass, represents the average mass of the isotopes of an element. This mass is not only based on the arithmethic average, but also on the percentage abundance of the isotopes of an element.

Ex: ³⁵Cl % abundance = 75%

³⁷Cl % abundance = 25%

Atomic mass of Cl = 35.5

Relative Atomic Mass (RAM)

The relative atomic mass of an element, is how many times heavier, an atom of that element is compared to an atom of hydrogen. Hydrogen is the lightest element in the periodic table. It has a relative atomic mass of 1.

Example: Carbon (Z=12), has a RAM=12. One atom of carbon, is 12 times heavier than one hydrogen atom.

Formula Mass

In practice, this term means the same as the term relative molecular mass. The difference is that some substances do not exist as molecules. Therefore the need to differentiate them. For all practical purposes, the term is going to be used interchangeable in this course.

Percentage Composition

Compounds are made of two, or more elements. The proportion, by mass of the elements in a given compound is always the same. For example, all samples of water are 11% hydrogen, and 89% oxygen. The percentage composition of a compound is the percentage by mass of each of the elements in the compound.

Knowing the percentage composition of a compound is important in chemistry. It allows us to calculate how much of a compound must be decomposed to obtain a desired amount of a particular element

Empirical Formula

The word empirical means, determined by experimentation. The empirical formula of a compound tells us the elements present in that compound, and the simplest whole number ratio at which they combine.

The molecular formula is also called the true formula. It tells us the elements present in the compound, as well as the actual number of atoms of each element present in the compound.

Sometimes the empirical formula is the same as the molecular formula.

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