Chapter 2.docx

Structure of Atoms  An atom is made up of a dense center called the nucleus, and a cloud of electrons surrounding it.  The nucleus has two types of particles called protons and neutrons.  The protons are positively charged and the neutrons have no charge (neutral).  The electrons are negatively charged.  The number of protons is always equal to the number of electrons, so an atom is electrically neutral.  The relative mass and charge of each particle is as shown in the table below.

Proton number and Nucleon number.  The proton number of an atom is equal to the number of protons in the atom.  The proton number is also called the atomic number.  The nucleon number of an atom is equal to the total number of protons and neutrons in the atom.  The nucleon number is also called the mass number since the protons and neutrons give the atom its mass.

The proton number of the atom shown above is 3. The nucleon number for the atom shown is 7.

Atomic symbol The structure of an atom can be written in symbol form as:

Mass Number (Nucleon number)

Atomic Number (Proton Number)

A B

X

 E.g. What are the atomic particles found in an atom of fluorine, 199 F?

Electron arrangement  The electrons surround the nucleus like a cloud and move around it in fixed orbits or shells.  Each shell is numbered 1, 2, 3, 4 and so on, going outwards from the nucleus. Each shell can hold a certain maximum number of electrons.  The maximum number of electrons each shell can hold is given by: 1st shell = 2 2nd shell = 8 3rd shell = 8 (for the first 20 elements in the Periodic Table)  The electron arrangement of an atom is also called its electronic structure  The electronic structure can also be stated as a series of numbers called the electron configuration.

 The electronic configuration of the sodium atom shown is: 2.8.1. The electrons in the outermost shell are called the outer electrons or valence electrons.

Isotopes  Isotopes are atoms of the same element which have the same number of protons but different numbers of neutrons.  Isotopes are chemically alike in all aspects, except for their masses. However, some isotopes are radioactive.  Hydrogen has three isotopes 11H, 21H and 31H.

Isotopes of hydrogen  Chlorine exists as two main isotopes: 3517Cl (75%) and 3717Cl (25%).  The average atomic mass of chlorine is given by the weighted mean of the two isotopes: 0.75 x 35 + 0.25 x 37 = 35.5  Hence, in the Periodic Table the atomic mass of chlorine is given as 35.517Cl.

Ions

 In an atom, the number of electrons is equal to the number of protons, so an atom is electrically neutral.  However, atoms can lose or gain electrons to form charged particles called ions.

 Atoms form ions so that they can have the stable electronic structure of a noble gas such as helium, neon or argon.  If an atom gives away its outer electrons, then it has more protons than electrons and becomes a positively charged ion.  If an atom takes in electrons, then it has more electrons than protons, and it becomes a negatively charged ion.  Examples: 

Mg  Mg2+ + 2e(Mg loses two electrons to form a positive ion, Mg2+)



Cl + e-  Cl(Cl gains one electron to form a negative ion, Cl-)

Ions of metals and non-metals.

 Metals e.g. sodium, calcium and aluminum which have less than 4 electrons in their outermost shells, always give away electrons to form positive ions.  Non-metals like chlorine, oxygen and nitrogen which have more than 4 electrons in their outer shell, take in electrons to form negative ions.

Molecules of elements and compounds.  A molecule is made up of two or more atoms chemically joined together.  Molecules of elements are made up of the same kind of atoms.

Molecules of elements  Molecules of compounds are made up of two or more different kinds of atoms.

Molecules of compounds.