Chapter#1 Basic Concepts
Atom ایٹم
A 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 atoms, derived (اخذ کردہ) from the word “atomos” which means indivisible (ناقابل تقسیم). However (البتہ), the ideas of Greek philosophers were not based on experimental (تجرباتی) evidence (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) 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۔.
Modern research (جدید تحقیق) has clearly shown that an atom is further (مزید) composed of subatomic particles like electron الیکٹران, proton پروٹون, neutron نیوٹران, hyperon 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 the 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 (nm = 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 the wavelength of an electron is much shorter than that of visible light (نظر آنے والی روشنی).
Fig. (1.1) shows an electron microscopic photograph of a piece (ٹکڑا) of a graphite گریفائٹ which has been magnified (حیرت انگیز )about 15 millions times. The bright band (روشن بینڈ ) in the figure are layers (تہوں ) of carbon کاربن atoms. In the 20th century, X-ray work has shown that the diameter (قطر ) of atoms is 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 different kinds (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. Hemoglobin (ہیموگلوبن) is 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 hemoglobin 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.
Ions آئنز
Ions ( آئنز )are those species that carry either (یا تو) positive (مثبت)or negative (منفی) charge. Whenever (جب بھی) an atom of an
element loses (کھو دیتا ہے) one or more electrons, positive ions are formed (تشکیل دیا). A sufficient (کافی) amount of energy is to be provided (فراہم کی) to a neutral atom to ionize (آئنائز کرنا) it.
This A+ is called a cation (ایک مثبت چارج شدہ آئن، یعنی وہ جو الیکٹرولیسس میں کیتھوڈ کی طرف راغب ہو گا۔). A cation may carry (لے جا سکتا ہے) +1, +2, +3, etc.charge or charges. The number of charges (چارجز کی تعداد) present on an ion depends (انحصار کرتا ہے) upon the number of electrons (الیکٹران کی تعداد پر) lost by the atom. Anyhow (بہر حال), energy is always required to do so. Hence (اس لیے) the formation of the positive ions is an endothermic process (ایک اینڈوتھرمک عمل کوئی بھی عمل ہے جس میں سسٹم کے اینتھالپی ایچ میں اضافہ ہوتا ہے۔). The most common (نہایت عام) positive ions are formed by the metal atoms (دھاتی ایٹم) such as Na+
, K+, Ca2+, Mg2+ , Al3+, Fe3+, Sn4+, etc.
The chapter on chemical bonding (کیمیائی تعلقات) will enable us to understand the feasibilities of their formation (ان کی تشکیل کی امکانات). When a neutral atom picks up (اٹھاتا ہے) one or more electrons, a negative ion is produced, which is called an anion (ایک منفی چارج شدہ آئن، یعنی وہ جو الیکٹرولیسس میں انوڈ کی طرف راغب ہوگا۔).
Energy is usually released (توانائی عام طور پر خارج جاتی ہے۔) when an electron is added (جب ایک الیکٹران شامل کیا جاتا ہے۔) to the isolated neutral atom (الگ غیر جانبدار ایٹم, Therefore (اس لیے), the formation of an uninegative ion is an exothermic process (کیمیائی رد عمل سے خارج ہونے والی حرارت برقی مقناطیسی توانائی کی شکل اختیار کر لیتی ہے).The most common negative ions (سب سے زیادہ عام منفی آئن) are F, Br, S ,Cl , , −− − − etc.
The cations and anions possess altogether different properties (مجموعی طور پر مختلف خصوصیات) from their corresponding (متعلقہ) neutral atoms. There are many examples of negative ions which consist of a group of atoms (منفی آئنوں کی بہت سی مثالیں ہیں جو ایٹموں کے گروپ پر مشتمل ہیں۔) like OH-, CO3, SO4, PO4, MnO4, Cr2O7, etc. The positive ions having a group of atoms are less common e.g. NH4 +and some embrocations (گلے لگانا) in organic chemistry.
Molecular Ion مالیکیولر آئن
When an atom loses or gains an electron (جب ایک ایٹم الیکٹران کھو دیتا ہے یا حاصل کرتا ہے۔), it forms an ion. Similarly (اسی طرح), a molecule may also lose or gain an electron (مالیکیول الیکٹران کو کھو یا حاصل بھی کر سکتا ہے۔) to form a molecular ion, e.g., CH4+, CO+, N+ Cationic molecular ions are more abundant (وافر) than anionic ones. These ions can be generated (آئنوں کو پیدا کیا جا سکتا ہے) by passing high energy electron beam (ہائی انرجی الیکٹران بیم سے گزر کر) or αparticles or X-rays through a gas (ایک گیس کے ذریعے). The breakdown (ٹوٹنا) of molecular ions obtained (حاصل کیا) from the natural products (قدرتی مصنوعات) can give important information about their structure (ان کی ساخت کے بارے میں اہم معلومات دے سکتے ہیں۔).
RELATIVE ATOMIC MASS
Relative atomic mass (جوہری ماس) is the mass of an atom of an element as compared (موازنہ) to the mass of an atom of carbon taken as 12.
The unit used to express (اکائی اظہار کے لیے استعمال ہوتی ہے۔) the relative atomic mass is called atomic mass unit (amu) and it is 1/12th of the mass of one carbon atom, On the carbon -12 scale, the relative atomic mass of 12C is 12.0000 amu and the relative atomic mass of 1H is 1.008 amu. The masses of the atoms are extremely small (ایٹموں کا حجم بہت چھوٹا ہے۔). We don’t have any balance to weigh such an extremely small mass (ہمارے پاس اتنے چھوٹے بڑے وزن کے لیے کوئی توازن نہیں ہے۔), that is why we use the relative atomic mass unit scale (ایٹمی ماس یونٹ پیمانہ).