Isotopes & Atomic Mass

The Proton

Protons are a positively charged particle that is located in the nucleus of an atom. The number of protons is very important in the atom as it gives the atom its identity. An atom with only 1 proton will be identified as a hydrogen atom; an atom with 6 protons will be identified as a carbon atom and so on. The number of protons in an atom determines its elemental identity. NO TWO ELEMENTS HAVE THE SAME NUMBER OF PROTONS IN THEIR ATOMS.

The number of protons in an atom is called the ATOMIC NUMBER of the element. Each element therefore has a unique atomic number.

The Neutron

The neutron is an uncharged particle that is also found in the nucleus of atoms. The number of neutrons is variable, meaning that an atom of a particular element can have more or less neutrons than another atom of that same element. When the number of neutrons is different for the atoms of an element, those atoms are called ISOTOPES of the element.

Shown below are the three isotopes of hydrogen (H). Notice that in each atom the number of protons is the same (1), but that the number of neutrons is different. Hydrogen is somewhat unique in that each of its isotopes has been given special names. This is not true for the isotopes of other elements.

Isotopes are important because we can often use them in spectroscopy. Deuterium can be used to "label" compounds that have hydrogen atoms so that the spectroscope can see their location better. Because of the additional mass of the extra neutron in the nucleus, this makes the atom easier to "see".

The number of neutrons plus the number of protons is called the ATOMIC MASS NUMBER.

Since the number of protons for a specific element is always the same and given by the atomic number, calculating the number of neutrons in an atom is easy; just subtract the number of protons from the atomic mass number. Conversely, if you are given the number of protons and neutrons in an atom, you can easily determine the identity of the element from the periodic table using the number of protons (atomic number) and adding the two amounts together will tell you the atomic mass number.

The Electron

Some might say that the electron is the most important part of the atom. The electron is the smallest of the atomic particles and carries a negative charge. While the mass of the electron is so negligible it is not included in the atomic mass number, it is very important in determining the charge of the atom and how reactive the atom will be with the world around it.

In an atom where the number of electrons is exactly the same as the number of protons, the atom is said to be neutral. But if the number of electrons is greater or less than the number of protons, then you have what is called an ion.

If the number of electrons is greater than the number of protons, the atom is an ANION.

If the number of electrons is less than the number of protons, the atom is a CATION.

Electrons are found in clouds that surround the nucleus of an atom. Those clouds are located specific distances from the nucleus. Because electrons move so quickly, it is impossible to know the exact location of the electron around the nucleus but scientists have determined specific areas where electrons are more likely to be found which we call SHELLS. The overall shape of the shells depends on the number of electrons in the atom. The higher the atomic number, the more shells and electrons an atom will have. The overall shell shape becomes more complex as the number of electrons increases.

Electrons and Bonding

Electrons are the glue that hold molecules together. Compounds either share electrons (covalent bonds) or are held together by differences in charge (ionic bonds). In either case an electron is the reason for the bond.

Covalent Bonds:

Ionic Bonds:

Elements

In each of the examples above we have used symbols like Cl and Li to represent the elements being used. If we look at the following reaction:

C(s) + O₂(g) → CO₂(g)

In the language of atomic theory we would say that one atom of carbon joined with two atoms of oxygen to form one molecule of carbon dioxide. Pretty cumbersome in words but much easier to both say and read using symbols:

1 C atom reacts with 2 O atoms to give one molecule of CO₂.

Or, better yet, C(s) + O₂(g) → CO₂(g)

The use of these symbols, which we call chemical symbols makes statements regarding chemical changes much easier. Each element has its own unique symbol. We can't think about chemistry or work most chemical problems without knowing the symbols.

Symbols

We can represent an element or the atom of an element by its symbol.

• C = carbon
• O = oxygen
• B = boron
• Pb = lead

We also use these symbols to show how atoms (in molecules) change partners in a chemical reaction.

• CO + PbO → CO₂ + Pb
• NH₃ + HCl → NH₄Cl

How do the elements themselves exist?

Monatomic elements (single elements)

He, Ne, Ar, Kr, Xe, Rn
(called Noble Gases, they used to be called the inert gases)

Diatomic elements (Two of the same element bonded together)

H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂
(Oxygen also exists as O₃ = ozone. This is called an allotropic form; many elements exist in several allotropic forms.)

Other forms of elements (Polyatomic Elements = multiple atoms of the same element bonded together)

P₄, S₈, C₆₀, etc
Diamond, graphite, and buckminsterfullerene ("buckyball," C₆₀) are all forms of the same element, carbon.

We always write reactions involving elements using their actual formulas.

You should become familiar with the first few rows of the periodic table as those are the elements most commonly used in reactions. Just like any other language, you must know the alphabet (elements) and words (compounds) to be able to speak it fluidly.