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Understanding Atomic Number and Mass Number in Chemistry

This overview explains the concepts of atomic number (Z) and mass number (A) in atomic structure. The atomic number represents the total number of protons in an atom's nucleus, directly correlating to the number of electrons in neutral atoms. Using practical examples, we clarify how to determine the number of neutrons by applying the formula A = Z + N. Learn how to visualize and draw atoms according to the Bohr-Rutherford model while exploring chemical families and their characteristics. Engaging activities are included to reinforce these concepts.

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Understanding Atomic Number and Mass Number in Chemistry

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  1. Atomic Notation http://www.chemicool.com/

  2. Atomic Number Z • The atomic number Z of an element is the number of protons found in the nucleus of the atom of this element. • Example : • The atomic number of lithium is Z = 3. It means that the nucleus of lithium atom contains 3 protons.

  3. Atomic Number Z • According to Bohr-Rutherford model of atomic structure, the atom is neutral. Then, if an atom contains Z protons, it should contain Z electrons. • Example : • The atomic number of chlorine is Z = 17. Chlorine atom contains 17 protons and 17 electrons. • Rule : for a neutral atom, Z = number of protons = number of electrons.

  4. Mass Number A • Lets designate by N the numbre of neutrons in the nucleus of an atom. • And by A the total number of protons and neutrons in the nucleus. • This number A is called Mass Number. (See page 247). • By definition: A = Z + N

  5. Examples • The atomic number of sodium is Z = 11. Its mass number A = 23.Determine the number of protons, electrons and neutrons of this atom. Draw this atom. Solution The atomic number Z = 11, then this atom contains 11 protons. The atom is neutral, then the number of electrons = the number of protons = Z = 11. The mass number A = Z + N, then the number of neutrons N = A – Z = 23 -11 =12 neutrons.

  6. Cont’d • Sodium atom contains 11 p+, 11 e- and 12 no. 2 e- on the first shell (K) 8 e- on the second shell (L) 1 e- on the third shell (M)

  7. Notation of the structure of an atome • The proper notation of the structure of an atom uses the chemical symbol X with both the mass number A (upper left) and the atomic number Z (lower left). X A Z

  8. calculate the number of protons, electrons and neutrons of each of the following atoms:

  9. Families of elements • In chemistry, the term chemical family is used to describe a group of related elements, elements can have common characteristics or proporties. • To understand how to classify elements in a family, you work individually the ScienceInquiry Activity, page 216.

  10. Your task • Page 216, Sciences Inquiry Activity : «Patterns of Proporties in a Chemical Family» • Time : 10 to 15 minutes • Answer by writting the questions. • Note : A coinage metal is a metal used to make coins.

  11. Hoomework • Determine the number of protons, electrons and neutrons of each atom. Draw each atom according to Bohr-Rutherford model. What one common charactristic of these atoms do you notice.

  12. Réponses • Hydrogen atom • 1 proton, 1 electron, 0 neutron • Lithium atom • 3 protons, 3 electrons, 4 neutrons • Sodium atom • 11 protons, 11 electrons, 12 neutrons • Potassium atom • 19 protons, 19 electrons, 20 neutrons • Common characteristic : one electron on the outer shell.

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