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As all substances must be electrically neutral, the total number of positive charges on the cations of an ionic compound must equal the total number of negative charges on its anions. The formula of an ionic compound represents the simplest ratio of the numbers of ions necessary to give identical numbers of positive and negative charges. For example, the formula for aluminum oxide, Al 2 O 3 , indicates that this ionic compound contains two aluminum cations, Al 3+ , for every three oxide anions, O 2− [thus, (2 × +3) + (3 × –2) = 0].

It is important to note, however, that the formula for an ionic compound does not represent the physical arrangement of its ions. It is incorrect to refer to a sodium chloride (NaCl) “molecule” because there is not a single ionic bond, per se, between any specific pair of sodium and chloride ions. The attractive forces between ions are isotropic—the same in all directions—meaning that any particular ion is equally attracted to all of the nearby ions of opposite charge. This results in the ions arranging themselves into a tightly bound, three-dimensional lattice structure. Sodium chloride, for example, consists of a regular arrangement of equal numbers of Na + cations and Cl anions ( [link] ).

Two diagrams are shown and labeled “a” and “b.” Diagram a shows a cube made up of twenty-seven alternating purple and green spheres. The purple spheres are smaller than the green spheres. Diagram b shows the same spheres, but this time, they are spread out and connected in three dimensions by white rods. The purple spheres are labeled “N superscript postive sign” while the green are labeled “C l superscript negative sign.”
The atoms in sodium chloride (common table salt) are arranged to (a) maximize opposite charges interacting. The smaller spheres represent sodium ions, the larger ones represent chloride ions. In the expanded view (b), the geometry can be seen more clearly. Note that each ion is “bonded” to all of the surrounding ions—six in this case.

The strong electrostatic attraction between Na + and Cl ions holds them tightly together in solid NaCl. It requires 769 kJ of energy to dissociate one mole of solid NaCl into separate gaseous Na + and Cl ions:

NaCl ( s ) Na + ( g ) + Cl ( g ) Δ H = 769 kJ

Electronic structures of cations

When forming a cation, an atom of a main group element tends to lose all of its valence electrons, thus assuming the electronic structure of the noble gas that precedes it in the periodic table. For groups 1 (the alkali metals) and 2 (the alkaline earth metals), the group numbers are equal to the numbers of valence shell electrons and, consequently, to the charges of the cations formed from atoms of these elements when all valence shell electrons are removed. For example, calcium is a group 2 element whose neutral atoms have 20 electrons and a ground state electron configuration of 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 4 s 2 . When a Ca atom loses both of its valence electrons, the result is a cation with 18 electrons, a 2+ charge, and an electron configuration of 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 . The Ca 2+ ion is therefore isoelectronic with the noble gas Ar.

For groups 12–17, the group numbers exceed the number of valence electrons by 10 (accounting for the possibility of full d subshells in atoms of elements in the fourth and greater periods). Thus, the charge of a cation formed by the loss of all valence electrons is equal to the group number minus 10. For example, aluminum (in group 13) forms 3+ ions (Al 3+ ).

Questions & Answers

A given amount of gas occupies 10.0dm5 at 4atm and 273°C. The number of moles of the gas present is? [ Molar volume of gas at s.t.p= 22.4dm3]
Michael Reply
how do u derive this fundamental constants
Bertram Reply
am not sure how
do you have an idea
that's why am asking
could you elaborate a little more on your question?
or reword it perhaps. I think I understand what you're trying to ask but the wording of the question makes it confusing
what fundamental constants?
you see Bertran. if you can do a little more explaining on what you are trying to have answered there wouldn't be so much confusion and you'd most likely get your answer your searching for
this value would be a little hard to understand so I thought you should know how it is derived
I need a deeper explanation to this value
you're still being very ambiguous. please tell me what fundamental constant you are referring to.
am asking this question under fundamental physical constant
ok how do u derive the value of Avogadro's number without cramming
fundamental physical constants are dimensionless and cannot be derived and have to be measured.
dimensionless quantities are obtained as ratios of quantities that are not dimensionless
if you want to understand better then I suggest you read up on "dimensional analysis", "dimensionless quantity", and "dimensionless physical constant"
what is an atom
Idah Reply
An atom is the smallest indivisible particle of an element that is capable of independent existence
That's conventional thinking since the Greeks and our current teaching, but it's been discovered that atoms are made up of even smaller subatomic particles called "quarks". Until fully understood let's stick with the current knowledge that an atom is the smallest unit of mass being indivisible
definition of ions
an ion is an atom or molecule that has a net electrical charge. the charge of the electron atom as negative and is equal and opposite to that of the proton that item which is positive. the net charge of an ion is non-zero.
this is due to the total number of electrons being on equal to its total number of protons. a cation is a positively charged ion with fewer electrons than protons while an anion is negatively charged with more electrons and protons.
because of their opposite electric charges cations and anions attract each other and readily form ionic compounds.
so please who can throw more light on acids and base for me
an acid is a molecule or ion capable of donating a proton (hydrogen ion H+) it is a substance that increases the concentration of hydronium ions (H3O+) when added to water or decreases the hydroxide concentration water.
how separating acid to gas
a base is a proton acceptor and it's a substance that dissociates in water to form hydroxide ions (OH-). thus the base decreases the aqueous hydronium concentration in water and increases aqueous hydroxide concentration in water.
also a reaction between an acid with a base is called a neutralization reaction and the products of the reaction are salt and water and not an acid or base
is the smallest particle of elements
What is rightful definition of element
angela Reply
an element is the group of (vertical columns) of the periodic table exhibit similar chemical behaviour.
any other information
a chemical element is a species of atom having the same number of protons and it's atomic nuclei
for example let's use oxygen the atomic number of oxygen is 8 so the element oxygen describes all atoms which have 8 protons
I hope I was helpful an answering your question
is alkanes a saturated hydrocarbon?
Faith Reply
it's saturated cos it has single bonds
yh....because they don't undergo additional reactions which hydrogen and other atoms can add across the carbon-carbon or triple bond
and me...I'm I wrong?
how does metal looses electron
Sammy Reply
By oxidation and reduction
by oxidation loss
An acid is a proton donor.
Eric Reply
what is an acid
Amara Reply
an acid is a substance when dissolved in water produces hydrogen ion or hydroxonium ion
is a substance which dissolves in water to produce hydrogen ions as the only positively charged ions
what is ionic bonding
Kylian Reply
It involves the transferring of electron from a metal to a non mental
that's right
bonding between a metal and a non metal
calculate the hydrogen ion concentration of the solution when pH=5
Adamu Reply
answer pls
What is thermodynamics
prince Reply
what is the meaning this word twentieth
Is the branch of physics that deal with heat and temperature and their relation to work, energy and properties of matter
There are no topics on hydrocarbons
Zina Reply
they are there please check under organic chemistry in the contents.
its not making sense to me I still don't understand
megan Reply
How and why
we need diagram for easy going and understand
How can we easily differentiate between the 5 gas laws
Favour Reply
first amd foremist me i only know 3 gas laws, so please list them here
the gas laws i know include pressure law boyles law charles law i differentiate these with this formular big take classy pork pigs. viral i read thus as 1 big classy pigs take pork viral big.....take means in boyles law,temp is constant clasy.....prk mns in chrls lw, press cons
classy.....pork means in charles law pressure is constant pigs.....viral means, in pressure law volume is kept constsng ,the rest is the same boyles states that vol of a gas is inversely prop to volume keeping temp constant charles law, state vol of a gas is directly prop to temp keepn pressure cns
how many carbon is present in alkene
Alase Reply
it's the carbon to carbon being double bonded to each other that makes it an alkene, not the amount of carbon itself. ex: C=C, C=C=C. both are alkenes.
I need more light on alkene
an alkene is a hydrocarbon, you can find this under the sub topic of organic chemistry in this textbook, check it out please,but before you know about alkenes you should first know about alkanes, alkanes are saturated and dnt easily react while alkenes have double bonds and can react .
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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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