Periodic Trends Trends in Atomic Size Atomic radius
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Periodic Trends
Trends in Atomic Size • Atomic radius is one half the distance between the nuclei of two atoms of the same element when the atoms are joined. • Two atoms of the same element that are attached to each other or two or more nonmetals that are bonded are called molecules. • In general, atomic size increases from top to bottom within a group and decreases from left to right across a period.
Who Has a Larger Radius? • Sodium or Potassium? – Potassium • Sodium or Aluminum? – Sodium • Carbon or Neon? – Carbon • Rank the following in order of increasing atomic radius: Boron, Nitrogen, Fluorine – Fluorine, Nitrogen, Boron • Rank the following in order of increasing atomic radius: Sr, Ba, Ca – Ca, Sr, Ba
Ions • An Ion is an atom or group of atoms with a positive or negative charge. • Positive and negative ions form when electrons are transferred between atoms. Metallic elements tend to lose electrons. If they lose electrons, what type of charge would this atom carry? – Positive. If sodium loses an electron, a written representation would be Na+ Nonmetallic elements tend to gain electrons. What type of charge must they carry? – Negative. Oxygen tends to gain two electrons and would be written as O 2 - • An ion with a positive charge is called a cation. • An ion with a negative charge is called an anion.
Trends in Ionic Size • Cations are always smaller than the atoms from which they form. • Anions are always larger than the atoms from which they form Why do you think this is? – When an atom loses an electron to become a cation, the remaining electrons are pulled in closer to the nucleus by the protons within. As the number of electrons increases, the attraction to the nucleus decreases.
Ionization Energy • The energy required to remove an electron from an atom. • First ionization energy, the energy required to remove one (the first) electron, tends to decrease down a group and increase across a period. How does the trend in ionization energy compare the trend of atomic radius? – They are opposite
Try these out! • Predict which element in each of the following pairs has the larger first ionization energy. (a) Na or Mg (b) Al or Si (c) F or Cl a. Mg b. Si c. F
Trends in Electronegativity • The ability of an atom of an element to attract electrons when the atom is in a given compound. • In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right.
Try These! • Which of the following has a higher electronegativity value: Lithium or Potassium? - Lithium Boron or oxygen? - Oxygen Bromine and Gallium? - Bromine
6. 3 Section Assessment pg. 182 Questions 21, 22, 24, 25 Study for chapter 3 quiz