Chapter#1 Basic Concepts

Atom ایٹم

Long time ago, it was thought that matter (مادہ) is made up of simple, indivisible (ناقابل تقسیم) particles (ذرات). Greek philosophers thought that, matter could be divided into smaller and smaller particles (ذرات) to reach a basic unit, which could not be further (مزید) sub-divided. Democritus ڈیموکریٹس (460-370 B.C.) called these particles atomos, derived (اخذ کردہ) from the word “atomos” means indivisible (ناقابل تقسیم). However (البتہ), the ideas of Greek philosophers were not based on experimental (تجرباتی) evidences (شواہد).

In the late 17th century, the quantitative (مقداری) study of the composition (مرکب) of pure substances (مادہ) disclosed (انکشاف) that a few elements (عناصر) were the components (اجزاء) of many different substances. It was also investigated (چھان بین) that how, elements combined to form compounds (مرکبات) and how compounds could be broken down (ٹوٹ) into their constituent (اجزاء) elements.

In 1808, an English school teacher, John Dalton جان ڈالٹن, recognized (تسلیم کیا) that the law of conservation (تحفظ قانون) of matter and the law of definite proportions (یقینی تناسب کا قانون) could be explained by the existence of atoms. He developed an atomic theory (جوہری نظریہ); the main postulate of which is that all matter is composed (مشتمل) of atoms of different elements, which differ in their properties (خصوصیات).

Atom is the smallest particle of an element ایٹم کسی عنصر کا سب سے چھوٹا ذرہ ہوتا ہے, which can take part in a chemical reaction کیمیائی رد عمل. For example, He and Ne, etc. have atoms, which have independent (آزاد) existence (وجود) while atoms of hydrogen ہائیڈروجن, nitrogen نائٹروجن and oxygen آکسیجن do not exist independently آزادانہ طور پر موجود نہیں ہے۔.

The modern researches (جدید تحقیق) have clearly shown that an atom is further (مزید) composed of subatomic particles like electron الیکٹران, proton پروٹون, neutron نیوٹران, hypron ہائپرون, neutrino نیوٹرنو, anti-neutrino اینٹی نیوٹرنو, etc. More than 100 such particles are thought to exist (موجود ہے) in an atom. However, electron, proton and neutron are regarded (سمجھا جاتا ہے) as the fundamental (بنیادی) particles of atoms.

A Swedish chemist J. Berzelius (سویڈش کیمسٹ جے برزیلئس) – (1779-1848) determined ((طے کیاthe atomic masses of elements. A number of his values are close to the modern (جدید) values of atomic masses. Berzelius also developed (بنایا) the system of giving element a symbol(علامت).

Evidence of Atoms (ایٹوں کا ثبوت)

It is not possible actually (اصل میں ) to see the atoms but the nearest possibility (امکان) to its direct evidence (ثبوت ) is by using an electron microscope. A clear and accurate (درست ) image of an object (چیز)  that is smaller than the wavelength of visible light (نظر آنے والی روشنی), cannot be obtained (حاصل). Thus an ordinary (عام) optical microscope can measure the size of an object up to or above 500 nm (lnm = 10-9m). However (البتہ), objects of the size of an atom can be observed (مشاہدہ کیا) in an electron microscope. It uses beams of electrons instead (اس کے بجائے )of visible light, because wavelength of electron is much shorter than that of visible light (نظر آنے والی روشنی).

Fig. (1.1) shows electron microscopic photograph of a piece (ٹکڑا) of a graphite  گریفائٹ which has been magniied (حیرت انگیز )about 15 millions times. The bright band (روشن بینڈ ) in the igure are layers (تہوں ) of carbon کاربن  atoms. In the 20th century, X-ray work has shown that the diameter (قطر ) of atoms are of the order 2x 10-10 m which is 0.2 nm. Masses of atoms range رینج from 10-27 to 10-25 kg. They are often expressed (اظہار) in atomic mass units یونٹس (amu) when 1 amu is = 1.661 x 10-27 kg. The students can have an idea (خیال ) about the amazingly (حیرت سے ) small size of an atom from the fact (حقیقت) that a full stop may have two million atoms present(موجودہ) in it.

Molecule

A molecule is the smallest (سب سے چھوٹا ) particle (ذرہ) of a pure substance (مادہ) which can exist independently آزادانہ طور پر موجود ہے. It may contain (پر مشتمل ہے) one or more(one or more) atoms. The number of atoms present in a molecule determines (کا تعین) its atomicity جوہری. Thus molecules can be monoatomic (واحد ایٹم), diatomic (دو ایٹم) and triatomic (تین ایٹم), etc., if they contain (پر مشتمل ہے )one, two and three atoms respectively (بالترتیب). Molecules of elements (elements) may contain one, two or more same type (ایک ہی قسم ) of atoms. For example (مثال کے طور پر), He, Cl2 , O3 , P4 , S8 . On the other hand (دوسری طرف), molecules of compounds (مرکبات) consist (مشتمل) of diferent kind (مختلف قسم ) of atoms. For example (مثال کے طور پر), HCl, NH3 , H2 SO4 ,C6H12O6 . The sizes of molecules are definitely (ضرور ) bigger (بڑا ) than (مقابلے) atoms. They depend (انحصار) upon the number of atoms present (موجود)in them and their shapes (شکلیں). Some molecules are so big (اتنا بڑا ) that they are called (کہا جاتا ہے) macro-molecules. Haemoglobin (ہیموگلوبن) is such a macro-molecule found in blood (خون). It helps (مدد کرتا ہے) to carry oxygen (لے جانا آکسیجن) from our lungs (پھیپھڑوں ) to all parts of our body (ہمارے جسم کے تمام حصے). Each (ہر ایک) molecule of haemoglobin  ہیموگلوبن is made up (بنا ہوا ) of nearly (قریب ترین )10,000 atoms and it is 68,000 times heavier (بھاری) than a hydrogen atom.

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