1 Elements, Atoms & Ions Chapter 4

2 4.1Elements Over 112 known: 88 are found in nature, rest are man-made Abundance is the percentage found in nature  oxygen is most abundant element (by mass) on earth and in the human body 4.2Symbols for Elements Each element has a unique symbol The symbol of an element may be one letter or two  if two letters, the second is lower case

3 4.3Skip

4 4.4Formulas Describe Compounds a compound is a distinct substance that is composed of atoms of two or more elements A formula describes a compound by describing the number and type of each atom in the simplest unit of the compound Rules for writing formulas  each element represented by its letter symbol  the number of atoms of each element is written to the right of the element as a subscript  polyatomic groups are placed in parentheses (if more than one)

5 4.5(skip) 4.6The Modern Concept of Atomic Structure Atoms are composed of three main parts: protons, neutrons and electrons. The nucleus contains protons (+) and neutrons (0). The electrons (-) are arranged in energy levels (orbitals) outside the nucleus. In a neutral atom, # protons = # electrons

6 The number of protons in the nucleus is the atomic number The number of protons plus neutrons in the nucleus is the mass number.

7 4.7Isotopes All atoms of an element have the same number of protons Atoms of an element with different numbers of neutrons are called isotopes All isotopes of an element are chemically identical and behave the same in chemical reactions Isotopes of an element have different masses Isotopes are identified by their mass numbers mass number = protons + neutrons See examples on page 96

8 4.8 Introduction to the Periodic Table See handout for this section

9 4.9 Natural States of the Elements Elements are usually not found in pure form in nature (on earth). Matter around us consists mostly of compounds and mixtures. Most elements are reactive and form compounds with other elements.  Ex: Na + Cl  NaCl Exceptions: the noble gases (Group 8A) and the noble metals (gold, silver, platinum).  Noble gases exist as single atoms.

10 Some gases exist as diatomic molecules at 25 o C: O 2, N 2, H 2 O 2 and N 2 are components of air The Group 7A halogens are also diatomic molecules at 25 o C: F 2, Cl 2, Br 2, I 2  However, hydrogen and the halogens are rarely present on earth in elemental form – always form compounds with other elements. Ex: Na + Cl  NaCl Mg + 2Br  MgBr 2 2H 2 + O 2  2H 2 O

11 Only two elements are liquids at 25 o C: bromine (Br 2 ) and mercury (Hg) metal. Cesium and Gallium melt at ~30 o C. All other elements are solids at 25 o C.

12 Allotropes  Forms of a solid nonmetallic element with different physical properties.  The different physical properties arise from the different arrangements of the atoms in the solid.  Allotropes of carbon include: diamond (hard) graphite (slippery) buckminsterfullerene –large, soccer-ball-shaped molecules See fig 4.18

Ions A neutral atom has zero net charge because the number of its protons (+) equals the number of its electrons (-). Adding or removing electrons from an atom creates an ion – an atom with a net positive or negative charge. When one or more electrons are lost by a neutral atom, an ion with a positive charge is formed, called an cation.  Examples:

14  A cation is named using the parent name of the atom: Na + is the sodium ion (or sodium cation) Mg 2+ is the magnesium ion (or magnesium cation) When one or more electrons are gained by a neutral atom, an ion with a negative charge is formed, called an anion. Examples:

15  An anion is named by taking the root name of the atom and adding the suffix –ide. Cl - is the chloride ion F - is the fluoride ion O 2- is the oxide ion Ions are never formed by adding or removing protons to a nucleus. Isolated atoms do not form ions on their own.

16 Ions form when metallic elements react with nonmetallic elements. The metal atoms lose one or more electrons, which are in turn gained by the atoms of the nonmetal.

17 Ion Charges and the Periodic Table See fig 4.19 Group 1A metals all form 1+ ions. Group 2A metals all form 2+ ions. Group 3A metals form 3+ ions. Transition metals form cations with various charges. Examples: Fe 2+ and Fe 3+ Cu + and Cu 2+ Pb 2+ and Pb 4 + Group 7A nonmetals all form 1- ions. Group 8A nonmetals all form 2- ions.