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Matter: Elements and Compounds
Matter
is defined as anything that takes up space and has mass. Matter
exists in many diverse forms, each with its own characteristics. Rock, metals,and glass are just few examples of what seems an endless assortment of mater. Element
is a substance that cannot be broken down by other substances by
chemical reactions. Today chemists recognize 92 elements occurring in naturee.g gold, copper, carbon, and oxygen. Two or more elements may be combined ina fixed ratio to produce a compound. Table salt, for example, is actually sodiumchloride (NaCl), a compound that is composed of elements sodium (Na) and chlorine(Cl).
Elements Essential to Life
About 25 of the 92 natural elements are known to be essential to life,but four of these-
carbon
(C),
oxygen
(O),
hydrogen
(H), and
nitrogen
(N)-make up
90% of living matter. Phosphorus (P), sulfur (S), calcium (Ca), potassium (K) anda few other elements account for most of the remaining 4% of an organism’s weight.^ Trace elements
are those required by organisms in minute quantities. However,
because they are mandatory for good health, trace elements are not nutrient ofmarginal importance to the organisms.
The Structure and Behavior of Atoms ^ The units of matter are called atoms. ^ An atom is the smallest possible amount of element that retains
that element’s properties. ^ Each atom has at its center a dense
positively charged nucleus,
which is surrounded at some distance by a cloud ofnegatively charged electrons. Electrons held in orbit by electrostatic attraction to the nucleus.
The neutron and proton are almost identical in mass, each about 1.7x
-24^ grams
and other conventional units are not very useful for describing the massof the objects. Thus, for atoms and subatomic particle, scientists use a unitof measurement called dalton.
Particle
Weight (Daltons)
Charge
Neutron
Proton
Electron
The atomic weight or mass number is the sum of protons and neutrons located in thenucleus of the atom
Atoms that have unfilled electron shell are less stable and have tendency tointeract with other atoms to complete their unfilled electrons.This electron exchangecan be achieved either a by transferring electrons from one atom to another or bysharing electrons between two atoms.These two strategies generate the two typesof chemical bonds:
Ionic bond and covalent bond.
Ionic Bonds Form by the Gain and Loss of Electrons
Ionic bonds are most likely to be formed by atoms that have just one or two electronsin their unfilled outer shell or just one or two electrons short of acquiring a filled outershell. For example, Na with atomic number 11 can donate one electron from its 3th shellto chlorine with atomic number17. This event leads formation of compound called NaClor table salt. When electrons jump from Na to Cl, both atoms become electricallycharged
Na lost electron and has one less electron than it has proton in its nucleus
Na
Cl gain electron and has one more electron than it has protons
Cl
There Are Different types of covalent Bonds
With six electrons in its second electron shell, oxygen needs two more electrons tocomplete its outermost shell.Two oxygen atoms form a molecule by sharing two pairsof outermost electrons.The atoms are joined by what is called a double covalent bond.Nitrogen has five electrons in outermost shell, three less than it needs forcomplete outermost shell. Two nitrogen atoms will join together by a
triple covalent
bond
and share pairs of outermost electrons. The molecules we have looked at so far are H
, O 2
, and N 2
Water is a compound whose molecular formula is H
O; it takes 2
two atoms of hydrogen to satisfy outermost electron shell of one oxygen. Anothermolecule that is also compound is methane, a component of natural gas with amolecular formula CH
. Carbon has four unshared electrons, so its bonding 4
capacity is 4.
