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Elements, Atoms & Ions Chapter 4

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1 Elements, Atoms & Ions Chapter 4

2 Elements Over 112 known, of which 88 are found in nature
others are man-made Abundance is the percentage found in nature oxygen most abundant element (by mass) on earth and in the human body the abundance and form of an element varies in different parts of the environment 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 2

3 Table 4.1: Distribution (Mass Percent) of the 18 Most Abundant Elements in the Earth's Crust, Oceans, and Atmosphere

4 Table 4.2: Abundance of elements in the human body

5 Dalton’s Atomic Theory
Elements are composed of atoms tiny, hard, unbreakable, spheres All atoms of a given element are identical all carbon atoms have the same chemical and physical properties Atoms of a given element are different from those of any other element carbon atoms have different chemical and physical properties than sulfur atoms 3

6 Dalton’s Atomic Theory
Atoms of one element combine with atoms of other elements to form compounds. Law of Constant Composition all samples of a compound contain the same proportions (by mass) of the elements Chemical Formulas 4

7 Dalton’s Atomic Theory
Atoms are indivisible in a chemical process. all atoms present at beginning are present at the end atoms are not created or destroyed, just rearranged atoms of one element cannot change into atoms of another element cannot turn Lead into Gold by a chemical reaction 5

8 Formulas Describe Compounds
a compound is a distinct substance that is composed of atoms of two or more elements describe the compound by describing the number and type of each atom in the simplest unit of the compound molecules or ions 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 if there is only one atom, the 1 subscript is not written polyatomic groups are placed in parentheses if more than one 6

9 Figure 4.2: Dalton pictured compounds as collections of atmosphere NO, NO2, and N2O are represented

10 Are Atoms Really Unbreakable?
J.J. Thomson investigated a beam called a cathode ray he determined that the ray was made of tiny negatively charged particles we call electrons his measurements led him to conclude that these electrons were smaller than a hydrogen atom if electrons are smaller than atoms, they must be pieces of atoms if atoms have pieces, they must be breakable Thomson also found that atoms of different elements all produced these same electrons 7

11 The Electron Tiny, negatively charged particle
Very light compared to mass of atom 1/1836th the mass of a H atom Move very rapidly within the atom 8

12 Rutherford’s Results Over 98% of the  particles went straight through
About 2% of the  particles went through but were deflected by large angles About 0.01% of the  particles bounced off the gold foil 11

13 Figure 4.6: (a) The results that the metal foil experiment would have yielded if the plum pudding model had been correct; (b) Actual results

14 Rutherford’s Nuclear Model
The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume of the atom The nucleus is essentially the entire mass of the atom The nucleus is positively charged the amount of positive charge of the nucleus balances the negative charge of the electrons The electrons move around in the empty space of the atom surrounding the nucleus 12

15 Figure 4.9: A nuclear atom viewed in cross section

16 Structure of the Nucleus
The nucleus was found to be composed of two kinds of particles Some of these particles are called protons charge = +1 mass is about the same as a hydrogen atom Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons The other particle is called a neutron has no charge has a mass slightly more than a proton 13

17 The Modern Atom We know atoms are composed of three main pieces - protons, neutrons and electrons The nucleus contains protons and neutrons The nucleus is only about cm in diameter The electrons move outside the nucleus with an average distance of about 10-8 cm therefore the radius of the atom is about 105 times larger than the radius of the nucleus 14

18 Isotopes All atoms of an element have the same number of protons
The number of protons in an atom of a given element is the same as the atomic number found on the Periodic Table Atoms of an element with different numbers of neutrons are called isotopes All isotopes of an element are chemically identical undergo the exact same chemical reactions Isotopes of an element have different masses Isotopes are identified by their mass numbers mass number = protons + neutrons 15

19 Figure 4.10: Two isotopes of sodium

20 Elements Arranged in a pattern called the Periodic Table
Position on the table allows us to predict properties of the element Metals about 75% of all the elements lustrous, malleable, ductile, conduct heat and electricity Nonmetals dull, brittle, insulators Metalloids also know as semi-metals some properties of both metals & nonmetals 16

21 The Modern Periodic Table
Elements with similar chemical and physical properties are in the same column Columns are called Groups or Families Rows are called Periods Each period shows the pattern of properties repeated in the next period 17

22 Figure 4.11: The periodic table

23 The Modern Periodic Table
Main Group = Representative Elements “A” columns Transition Elements all metals Bottom rows = Inner Transition Elements = Rare Earth Elements metals really belong in Period 6 & 7 18

24 Important Groups Noble Metals Ag, Au, Pt
Group 8 = Noble Gases He, Ne, Ar, Kr, Xe, Rn all colorless gases at room temperature very non-reactive, practically inert found in nature as a collection of separate atoms uncombined with other atoms Noble Metals Ag, Au, Pt all solids at room temperature least reactive metals found in nature uncombined with other atoms 19

25 Important Groups - Halogens
Group 7A = Halogens very reactive nonmetals react with metals to form ionic compounds HX all acids Fluorine = F2 pale yellow gas Chlorine = Cl2 pale green gas Bromine = Br2 brown liquid that has lots of brown vapor over it Only other liquid element at room conditions is the metal Hg Iodine = I2 lustrous, purple solid 20

26 Allotropes Many solid nonmetallic elements can exist in different forms with different physical properties, these are called allotropes the different physical properties arise from the different arrangements of the atoms in the solid Allotropes of Carbon include diamond graphite buckminsterfullerene 21

27 Figure 4.18a: The three solid elemental (allotropes) forms of carbon

28 Figure 4.18b: The three solid elemental (allotropes) forms of carbon

29 Figure 4.18c: The three solid elemental (allotropes) forms of carbon

30 Electrical Nature of Matter
Most common pure substances are very poor conductors of electricity with the exception of metals and graphite Water is a very poor electrical conductor Some substances dissolve in water to form a solution that conducts well - these are called electrolytes When dissolved in water, electrolyte compounds break up into component ions ions are atoms or groups of atoms that have an electrical charge 22

31 Figure 4.20: (a) Pure water does not conduct a current; (b) Water containing a dissolved salt conducts electricity

32 Ions ions that have a positive charge are called cations
form when an atom loses electrons ions that have a negative charge are called anions form when an atom gains electrons ions with opposite charges attract therefore cations and anions attract each other moving ions conduct electricity compound must have no total charge, therefore we must balance the numbers of cations and anions in a compound to get 0 total charge 23

33 Figure 4.21a: The arrangement of sodium ions (Na+) and chloride ions (Cl-) in the ionic compound sodium chloride.

34 Figure 4.21b: Solid sodium chloride highly magnified.

35 Atomic Structures of Ions
Metals form cations For each positive charge the ion has 1 less electron than the neutral atom Na = 11 e-, Na+ = 10 e- Ca = 20 e-, Ca+2 = 18 e- Cations are named the same as the metal sodium Na  Na+ + 1e- sodium ion calcium Ca  Ca+2 + 2e- calcium ion The charge on a cation can be determined from the Group number on the Periodic Table for Groups IA, IIA, IIIA Group 1A  +1, Group 2A  +2, (Al, Ga, In)  +3 24

36 Atomic Structures of Ions
Nonmetals form anions For each negative charge the ion has 1 more electron than the neutral atom F = 9 e-, F- = 10 e- P = 15 e-, P3- = 18 e- Anions are named by changing the ending of the name to -ide fluorine F + 1e-  F- fluoride ion oxygen O + 2e-  O2- oxide ion The charge on an anion can be determined from the Group number on the Periodic Table Group 7A  -1, Group 6A  -2 25

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