2.. FUNDAMENTALS OF CHEMISTRY.pdf

Name: _______________________________________ Date: ____________________ Section: _____________ Roll No.: _________________ La Martiniere Girls’ College...

Name: _______________________________________ Date: ____________________ Section: _____________ Roll No.: _________________ La Martiniere Girls’ College Class VII Chemistry Chapter 2 FUNDAMENTALS OF CHEMISTRY THE ATOM An atom is the smallest unit of a substance that retains the properties of that substance. Neils Bohr visualized the ‘modern concept’ of an atom concluding that: An atom consists of a central nucleus containing protons (positively charged particles) and neutrons (neutral particles with no charge). Electrons revolve around the nucleus in different shells or orbits which are imaginary paths surrounding the nucleus. Atoms are made up of three types of particles electrons , protons and neutrons. These particles have different properties. Electrons are tiny, very light particles that have a negative electrical charge (-). Protons are much larger and heavier than electrons and have the opposite charge. They have a positive charge (+). The number of protons in an atom is equal to the number of electrons in it. p+ = e- Neutrons are large and heavy like protons; however, neutrons have no electrical charge. Normally, atoms are electrically neutral because the number of electrons is equal to the number of protons (total negative charge = total positive charge). Electrons orbit around the nucleus. Particles Symbol Charge Location Protons p+ Positive charge Found in the nucleus Neutrons n0 Neutral (no) charge Found in the nucleus Electrons e- Negative charge Found orbiting the nucleus LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 1 of 7 ATOMIC NUMBER Atomic number (Z) is the number of protons present in the nucleus of an atom. ATOMIC NUMBER = Number of protons = Number of electrons Hence, Z= p+ = e- MASS NUMBER Mass number or atomic mass (A) is the total number of protons and neutrons present in the nucleus of an atom. MASS NUMBER = Number of protons + Number of neutrons Hence, A = p++ n0 ELECTRONIC CONFIGURATION Electrons revolve around the nucleus in imaginary paths called orbits or shells. The different shells are K shell (first shell), L shell (second shell), M……... For the first eighteen elements, there are some easy rules. The K-shell only holds two electrons. The L-shell only holds eight electrons. The M-shell only holds eight electrons (for the first eighteen elements). The distribution of electrons in various shells of an atom is known as electronic configuration. Example: Sodium atom p+ = 11, n0 = 12, e- = 11 Electronic configuration = 2(K), 8(L), 1(M) LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 2 of 7  Atom Symbol Atomic Number K Shell L Shell M Shell Hydrogen H 1 1 Helium He 2 2 Lithium Li 3 2 1 Beryllium Be 4 2 2 Boron B 5 2 3 Carbon C 6 2 4 Nitrogen N 7 2 5 Oxygen O 8 2 6 Fluorine F 9 2 7 Neon Ne 10 2 8 Sodium Na 11 2 8 1 Magnesium Mg 12 2 8 2 Aluminium Al 13 2 8 3 Silicon Si 14 2 8 4 Phosphorus P 15 2 8 5 Sulphur S 16 2 8 6 Chlorine Cl 17 2 8 7 Argon Ar 18 2 8 8 CHEMICAL SYMBOL An atom of any element is denoted by a symbol. A symbol is the short form or abbreviated name of the element. A symbol is generally represented by the first capital letter or alphabet of the atom of that element. In cases, where the name of more than one element begins with the same alphabet, the first two alphabets are chosen. Some of the abbreviations for the names of elements make sense--H for Hydrogen, C for Carbon, He for Helium. But some of them seem completely random. Some elements have been given symbols Element Symbol Latin name based on their names in Latin, which was sodium Na natrium once commonly used by scientists as an international language. For example, here potassium K kalium are some familiar elements with their Latin iron Fe ferrum names: copper Cu cuprum silver Ag argentum tin Sn stannum gold Au aurum mercury Hg hydrargyrum lead Pb plumbum LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 3 of 7 RADICALS A radical is a group of atoms of elements that behave like a single unit and show valency. Example: Positive radical- Ammonium (NH41+); Negative radical-Hydroxide (OH1-) VALENCY Valency is the numberof electrons an atom can donate, accept or share so as to achieve stable electronic configuration of the nearest noble gas. The valency of an element is always a whole number. The valency of all metals (and hydrogen) is considered positive. Example: Na+ The valency of all non- metals is considered negative. Example: Cl- Elements having valency of one are said to be monovalent. Example: Na+ Elements having valency of two are said to be divalent. Example: Zn2+ Elements having valency of three are said to be trivalent. Example: Al3+ Certain elements exhibit more than one valency or variable valency. Example: Cu1+, Cu2+ In most chemical reactions, when the atoms combine, they lose their neutral nature. This happens because the atoms lose or gain electrons in order to become stable. In some cases, electrons are shared. In an atom, the positively charged protons are equal in number to the negatively charged electrons. If an atom loses one electron it means that: The atom now has one proton more than the number of electrons, or, the atom now has one positive charge more than the number of negative charges. Therefore, the atom now carries an overall positive charge. The ‘charged atom’ is called an ion. (cation) If an atom gains one electron it means that: The atom now has one electron more than the number of protons, or, the atom now has one negative charge more than the number of positive charges. Therefore, the atom now carries an overall negative charge. The ‘charged atom’ is called an ion. (anion) An ion is any atom or a group of atoms carrying a positive or negative charge due to loss or gain of electrons. Example: Sodium ion (Na+) and Chloride ion (Cl-) The positively charged ion is called cation. (Example: Na+) The negatively charged ion is called anion. (Example: Cl-) LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 4 of 7  VALENCY CHART VALENCY POSITIVE VALENCIES NEGATIVE VALENCIES (Includes all metals, hydrogen and (Includes all non-metals and non- ammonium radical) metallic radicals) ELEMENT SYMBOL ION ELEMENT/ SYMBOL ION RADICAL VALENCY Potassium K K1+ Chlorine Cl Cl1-(chloride) 1 Sodium Na Na1+ Bromine Br Br1-(bromide) Hydrogen H H1+ Iodine I I1-(iodide) Ammonium NH4 NH41+ Nitrate NO3 NO31- Silver Ag Ag1+ Hydroxide OH OH1- Bicarbonate HCO3 HCO31- Bisulphite HSO3 HSO31- Bisulphate HSO4 HSO41- VALENCY Calcium Ca Ca2+ Oxygen O O2-(oxide) 2 Magnesium Mg Mg2+ Sulphur S S2-(sulphide) Zinc Zn Zn2+ Sulphite SO3 SO32- Sulphate SO4 SO42- Carbonate CO3 CO32- VALENCY Aluminium Al Al3+ Phosphate PO4 PO43- 3 Nitrogen N N3-(nitride) ELEMENTS EXHIBITING VARIABLE VALENCY ELEMENT SYMBOL NAME ION NAME ION Copper Cu Cuprous Copper[I] Cu1+ Cupric Copper[II] Cu2+ Mercury Hg Mercurous Mercury[I] Hg1+ Mercuric Mercury[II] Hg2+ Iron Fe Ferrous Iron[II] Fe2+ Ferric Iron[III] Fe3+ Tin Sn Stannous Tin[II] Sn2+ Stannic Tin[IV] Sn4+ Lead Pb Plumbous Lead[II] Pb2+ Plumbic Lead[IV] Pb4+ LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 5 of 7 CHEMICAL FORMULA A formula is the representation of the chemical name of a substance by means of symbols. It denotes the number of atoms of each element present in a compound. HOW IS A CHEMICAL FORMULA WRITTEN? For writing a chemical formula The symbol of the elements should be known. The valency of the elements should be known. ❖ The positive and negative ions are written side by side (positive ion is written first). Their charge or valency is written at the top (superscript), on the right-hand side. ❖ The valencies are then interchanged and written at the bottom (subscript), on the right-hand side without the signs. ❖ The final formula is written as. If the valency is one (1), then it is not written in the final formula. If the valencies are the same for both the ions, they cancel each other and only the symbols are written. Writing the formula of Magnesium chloride: Mg2+ Cl1- Mg1 Cl2 MgCl2 LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 6 of 7  EXERCISES 1) The number of protons is 4. Find the a. Number of electrons b. Atomic number 2) The number of electrons is 8. Find the a. Number of protons b. Atomic number 3) The number of protons is 6 and the number of neutrons is 6. Find the a. Number of electrons b. Atomic number c. Mass number 4) The number of electrons is 7 and the number of neutrons is 7. Find the a. Number of protons b. Atomic number c. Mass number 5) Draw the structure of an atom with atomic number 6 and number of neutrons 6. Write its electronic configuration. 6) Draw the structure of an atom with atomic number 12 and number of neutrons 13. Write its electronic configuration. 7) Study the given figure and answer the following questions with reference to the diagrams (i) and (ii): i) ii) a) What is the number of electrons and protons in the given atom? b) Calculate the atomic number c) Calculate the mass number. d) What will this atom do to become stable? e) What charge will it acquire? f) What is its valency? 8) State the symbol and valency of the following: a) Potassium b) Zinc c) Chlorine d) Sulphite e) Bromine 9) Write the formula of the following: a) Sodium chloride b) Calcium sulphate c) Aluminium oxide d) Plumbous sulphate e) Cupric hydroxide LMGC/CL 7/CHEMISTRY/Ch.No.2 Page 7 of 7