2.1.4State the relative masses and relative charges of protons, neutrons and electrons. Calculate the number of protons, neutrons and electrons in atoms from the identity, mass number, atomic number and/or charge.

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2.1.5 State the position of protons, neutrons and electrons. Draw and interpret planetary models for elements up to Z=18.


The incredible green lights in this cold northern sky consist of charged particles known as ions (see image below).


Their swirling pattern is caused by the pull of Earth’s magnetic north pole. Called the northern lights, this phenomenon of nature shows that ions respond to a magnetic field.Do you know what ions are? Read on to find out.



Atoms Are Neutral

The northern lights aren’t caused byatoms, because atoms are not charged particles. A neutral atom always has the same number of electronsasprotons. Electrons have a charge of -1 and protons have a charge of +1. Therefore, the charges of an atom’s electrons and protons “cancel out.” This explains why atoms are neutral in charge.

Atoms to Ions

So, what would happen to an atom’s charge if it were to gain extraelectrons?If an atom were to gain extra electrons, it would have more electrons thanprotons. This would give it a negative charge, so it would no longer be neutral.Atoms can also lose electrons resulting in it taking on a charge. In either case, they become anion. Ions are atoms that have a positive or negative charge because they have unequal numbers ofprotonsand electrons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions(see the image below).


For example, consider an atom of fluorine.A fluorine atom has nine protons and nine electrons, so it is neutral (see top diagram in the image below). There is one spot open for an electron to be added to the outermost energy level (maximum capicity is 8 electrons). If an electron is added to the atom, it becomes a fluoride ion with a charge of -1 (see bottom diagram in the image below), and stable like that of neon (see middle diagram in the image below).

Note: Ion charge is written number then charge; however, if 1 is the number it is often omited. For example, the flouride ion can be written F1- or F-.


Like fluoride, other negative ions usually have names ending in–ide. Positive ions, on the other hand, are just given the element name followed by the wordion. For example, when a sodium atom loses an electron, it becomes a positive sodium ion. The charge of an ion is indicated by a plus (+) or minus sign (-), which is written to the right of and just above the ion’s chemical symbol. For example, the fluoride ion is represented by the symbol F-, and the sodium ion is represented by the symbol Na+. If the charge is greater than one, a number is used to indicate the magnitude of the charge. For example, iron (Fe) may lose two electrons to form an ion with a charge of plus two. This ion would be represented by the symbol Fe2+. This and some other common ions are listed with their symbols in the tablebelow.

Some Common IonsCationsAnions
Name of IonChemical SymbolName of IonChemical Symbol
Calcium ionCa2+ChlorideCl-
Hydrogen ionH+FluorideF-
Iron(II) ionFe2+BromideBr-
Iron(III) ionFe3+OxideO2-

How Ions Form

The process in which an atom becomes an ion is called ionization. It may occur when atoms are exposed to high levels of radiation. The radiation may give their outer electrons enough energy to escape from the attraction of the positive nucleus. However, most ions form when atoms transfer electrons to or from other atoms or molecules. For example, sodium atoms may transfer electrons to chlorine atoms. This forms positive sodium ions (Na+) and negative chloride ions (Cl-).

You can see an animation of this process at the URL below:http://www.youtube.com/watch?v=xTx_DWboEVs

Why do you think atoms lose electrons to, or gain electrons from, other atoms?Atoms form ions by losing or gaining electrons because it makes them more stable and this state takes less energy to maintain. The most stable state for an atom is to have its outer mostenergy levelfilled with the maximum possible number of electrons. As in example above with the nonmetal fluorine, a more stable state can be achieved by gaining one electron and filling up the outer energy level.In the case of metalssuch as lithium, with just one electron in the outermost energy level, a more stable state can be achieved by losing that one outer electron.

Some Common Ions

All the elements in column 1 of the periodic table (group 1A) (shown in the figure below) have have one singleelectron in the outermost energy level. For that reason, all members of the 1A family will tend to lose only one electron when ionized to become stable. The entire family forms +1 ions: Li1+, Na1+, K1+, Rb1+, Cs1+, Fr1+.

Note: Although hydrogen (H) is in this same column, it is does not always behave like the other elements in the column. Yes, there are times when hydrogen acts like the other group one elements and forms +1 ions, but most of the time it shares electrons with other elements.


The elements in family 2 (group 2A) (shown in the figure below) all have two electrons in the outermost energy level. This entire family will form +2 ions: Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+. All members of family 2 (2A) form ions with +2 charge.


Column 13 (group 3A) members (shown in the figure below) have three electrons in the outermost energy level. When these atoms form ions, they will almost always form +3 ions: Al3+, Ga3+, In3+, Tl3+.

Note: Boron is omitted from this list. Boron generally doesn’t lose all of of its valence electrons during chemical reactions, generally boron shares electrons.


For Column 14 (group 4A), the larger atoms in the family (germanium, tin, and lead) often form ions with charges of +4. However,tin and lead also have the ability to also form +2 ions. You will learn later that some atoms have the ability to form ions of different charges, and the reasons for this will be examined later. The other atoms in the group generally share electrons with other elements, and therefore, do not generally ionize.

Like column 14 (group 4A), the elements of column 15 (group 5A) are also divided into two groups. The elements in this group have five outermost energy level electrons. During ionization, three electrons are added to the smaller elements to form -3 ions: N3-, P3-, As3-.The other elements in the group generally share electrons with other elements, and therefore, do not generally ionize.

The elements in column 16 (6A) (shown in figure below) have 6 outer electrons. These atoms generally attract two more electrons to fill their outermost energy level. They form -2 ions: O2-, S2-, Se2-, Te2-.

Column 17 (group 7A) are all nonmetals with seven valence electrons. When these atoms form ions, they form -1 ions: F1-, Cl1-, Br1-, I1-.

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Column 18 (group 8A) is made up of the noble gases, which have no tendency to either gain or lose electrons. With 8 valence electrons these elements are stable.