Grade 8 Chemistry - Structure of the Atom PDF

Garden City School Grade: 8 Name: --------------------------------- Scholastic Year: 2024 - 2025 Subject: Chemistry Instructor: Suhaib Audi Chapter...

Garden City School Grade: 8 Name: --------------------------------- Scholastic Year: 2024 - 2025 Subject: Chemistry Instructor: Suhaib Audi Chapter 2: Structure of the Atom 1- Definition of an atom: Matter is made up of tiny particles called atoms. An atom is the smallest particle (tiny particle) that cannot be subdivided. It cannot be seen by the naked eye. An atom consists mainly of two parts, a nucleus and energy levels separated by an empty space. 2- Composition of atom: (+ ve) (neutral) (- ve) 3- Fundamental particles of an atom:( composition of the atom) The atom consists of 2 parts: The nucleus is the central part and the most passive part that contains:  Positively charged particles called protons (𝑷+ ).  Non-charged particles called neutrons (𝒏𝟎 ). Proton and neutron particles are called nucleons.  The energy levels (electron clouds) are external part that contains:  Negatively charged particles; the electrons (𝒆− ). Composition of atom: protons Composition of nucleus Neutrons Electrons 4- The characteristics of the fundamental particles of an atom Particle Symbol Relative Mass Location charge Proton 𝑃+ 1+ 1U Inside the nucleus Neutron 𝑛0 0 1U Inside the nucleus Electron 𝑒− 1− 1/1836 U In the electron cloud 1u = atomic mass unit= mass of proton or neutron = 1.67 × 10− 27 𝐾𝑔. 5- The atom is neutral Since the number of protons is equal to the number of electrons, atoms are electrically neutral, and the total electric charge of an atom is Zero. (q atom=0). 6- Atomic Symbol or atomic representation of an atom (also called nuclide): The atom of an element is represented by the atomic symbol: Where:  X: is the symbol of the element. (ex: Mg, Cl, O…).  Z: is the atomic number = number of protons found in the nucleus. It identifies each element.  A: is the mass number: number of nucleons = number of protons + number of neutrons. A=Z+N Z=A-N N=A-Z  Example: Cl has 17 protons and 18 neutrons. (atomic number Z = number of protons = 17) (A= Z+N = 17 + 18 = 35) Atomic representation of Cl: 𝟑𝟓 𝟏𝟕𝑪𝒍 Short-hand notation: X-A X: symbol of the element. A: the mass number. ex: Cl-35 7- Atomic mass Mass of the atom = (Mass of nucleus) + (Mass of electron cloud) Mass of the atom = (Mass of protons + Mass of neutrons) + (Mass of electrons) Because the mass of electrons is negligible as compared to the mass of protons and neutrons (nucleons), then the mass of electrons is considered as equal to zero, therefore the mass of an atom is concentrated inside its nucleus. Atomic Mass = A × (1u) = A u The atomic mass is equal to the mass number ‘’A’’. The unit of atomic mass is ‘’a.m.u’’ or ‘’u’’. 8- Charge of the atom: 8.1. Charge of nucleus (relative nuclear charge) This is the charge of protons since that of neutrons is zero. Relative Charge of Nucleus = number of protons × relative charge of 1 proton = Z × (𝟏+ ) = Z+ Where the relative charge of 1 proton = 1+. 8.2. Relative Charge of Electron Cloud Since the atom is electrically neutral, then the number of protons = number of electrons= atomic number Z. Relative Charge of Cloud =number of electrons × relative charge of 1 electron = Z × (1− ) = z- Where the relative charge of 1 electron= 1−. 8.3. Charge of the atom Charge of the atom = Charge of the nucleus + Charge of the electron cloud = (z+) + (z-) = 0 The atom is electrically neutral; the total electric charge of an atom is zero. Conclusion: Charge of atom= charge of nucleus + charge of electron cloud = (number of protons × relative charge of 1 proton) + (number of electrons × relative charge of 1 electron) = [ Z × (𝟏+ ) ] + [ Z × (𝟏− ) ] = (𝒁+ ) + (𝒁− ) = 0 The charge of an atom is expressed in coulomb “C”. 8- Isotopes: They are atoms of the same element having same number of protons but different number of neutrons. Or they are atoms of the same element with the same atomic number (Z), but different mass number (A).  Isotopes of the same element have the same chemical properties because they have the same atomic number Z. o Isotopes of the same element exist in nature with constant % called % relative abundances. Example: The table below groups chlorine isotopes with their % abundance: Short notation Abundance % Atomic number Cl-35 75% 17 Cl-37 25% 17 - The most abundant isotope is the one with the highest relative % abundance (Cl-35). - The least abundant isotope is the one with the smallest relative % abundance (Cl-37). - The densest isotope (heaviest) is the one with the highest atomic mass (Cl-37). - The least dense isotope (lightest) is the one with the smallest atomic mass (Cl-35). Exercise 1: Choose the true statements and correct the false ones 1. The center of the atom is called the electron cloud. 2. The region that surrounds the nucleus is the electron cloud. 3. “A” represents the atomic number of an atom. 4. Atom is the smallest unit of matter. Exercise 2: Write the symbolic representation of each of the following atoms. 1. An oxygen atom that has an atomic number equals to 8 and a mass number equals to 16. 2. A carbon atom that contains 12 nucleons and 6 protons. Exercise 3: Given a sodium atom whose nucleus contains 12 neutrons, and has 11 electrons in its electron cloud. 1. Find the number of protons of sodium atom, then deduce its atomic number. 2. Determine its mass number. 3. Represent this atom symbolically. Exercise 4 A chlorine atom has an atomic number Z= 17 and contains 18 neutrons in its nucleus. 1. Specify the number of protons. 2. Calculate the mass number (A). Exercise 5 A calcium atom has 20 positively charged particles in its nucleus, and its mass number (A) is 40. 1. Specify the atomic number (Z). 2. Deduce the number of electrons. 3. Calculate the number of neutrons. 4. Write the atomic symbol for calcium. Exercise 6 A sodium atom (Na) has an atomic number (Z) = 11 and a mass number (A) = 23. 1. Determine the composition of the sodium atom. 2. Write the atomic symbol for sodium. Exercise 7 An atom (X) contains 7 electrons outside its nucleus. Determine its atomic number (Z). Exercise 8 A helium atom (He) with a mass number of 4 contains 2 neutrons in its nucleus. Determine the number of protons. Exercise 9: A calcium atom 𝟒𝟎 𝟐𝟎𝑪𝒂 has 20 electrons outside its nucleus. Determine the composition of Ca atom. Exercise 10: Exercise 11:  Exercise 12: Exercise 13: Part A Part B Exercise 14: Consider the following atom: 𝟏𝟔𝟖𝑶. 1- Determine the composition of this atom. 2- Determine its nuclear charge. 3- Calculate the charge of its cloud. 4- Deduce that the atom is neutral. 5- Calculate the mass of the atom in amu. Given: charge of a proton = 1+ charge of an electron =1- Exercise 15: Consider the following atom: 𝟐𝟕 𝟏𝟑𝑨𝒍. 1- Determine the composition of this atom 2- Determine its nuclear charge. 3- Calculate the charge of its cloud. 4- Deduce the charge of Al atom. 5- Calculate the mass of the atom in amu. Given: charge of a proton = 1+ charge of an electron =1- Exercise 16: Consider the sodium atom: 𝟐𝟑 𝟏𝟏𝑵𝒂. Given: charge of a proton = 1+ charge of an electron =1- 1. Determine the composition of Na atom. 2. Calculate the nuclear charge of Na atom. 3. Calculate the relative charge of the electron cloud of Na atom. 4. Deduce that charge of Na atom. 5. Calculate the atomic mass of Na atom. Exercise 17: In nature, magnesium exists as the form of three isotopes. The table below shows the relative abundances in percentages of magnesium: Relative abundance Isotopes of Magnesium Number of protons Number of electrons Number of neutrons 80 % 𝟐𝟒 𝟏𝟐𝑴𝒈 10% 𝟐𝟓 𝟏𝟐𝑴𝒈 X% 𝟐𝟔 𝟏𝟐𝑴𝒈 1- show that these 3 atoms of magnesium are isotopes. 2- Calculate the relative abundance of the isotope Mg-26. 3- Complete the above table. 4- Identify the heaviest isotope of magnesium. 5- Calculate the nuclear charge of the isotope Mg-25. 6- calculate the relative charge of the electron cloud of Mg-26 7- calculate the atomic mass of heaviest isotope.