11

Atomic Structure & Compounds

(loosely based on Chapter 3 Sec 1 thru 4of Jespersen 6th ed)

Dr. C. YauFall 2013

22

Dalton's Atomic Theory (p.10)1.Matter consists of tiny particles called atoms.

2. In any sample of a pure element, all the atoms are identical in mass and other properties.

3.The atoms of different elements differ in mass and other properties.

4.When atoms of different elements combine they form compounds with the elements in a fixed ratio by mass. Reactions are merely rearrangement of atoms to form different compounds.

5.Atoms are indestructible . In chemical reactions, the atoms rearrange but they do not themselves break apart.

(?)

(?)

Chemical

333

The 3 major subatomic particlesKnow this well !

Name &Symbol

Mass Charge Location

proton (p) ~ 1 u +1 inside nucleus

neutron (n) ~ 1 u 0 inside nucleus

electron (e-) ~ 0 u -1 outside nucleus

u (amu) = atomic mass unit (a very small unit of mass)1 gram = 602,000,000,000,000,000,000, 000 u

4

• Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weight measured in u.

• Atomic masses are based on a standard mass, that of an atom of C.

• 1 atom of carbon-12 weighs 12 u exactly.• Thus 1 u = 1/12 the mass of a carbon-12

atom. This is the definition of u.

Atomic Mass

5

• Most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses.

• For example: There are 3 isotopes of hydrogen, 4 isotopes of iron and 10 isotopes of tin.

• Isotopesare atoms of the same element with different masses.

• Isotopesare atoms with the same #p but with different #n.

• Chemically, isotopes have virtually identical properties. (There are slight differences that we will note be concerned with at this level of chemistry.)

Isotopes

KNOW WELL

6

Atomic Notation• An element is a substance whose atoms all

contain the identical number of protons, called the atomic number (Z)

• Isotopes are distinguished by the mass number (A):

– Atomic number, Z = number of protons– Mass number, A = (# of protons) + (# of neutrons)– Note that for atoms, A is greater than Z: the symbol

is "top-heavy."• For neutral atoms, the number of electrons and

protons must be equal.

XAZ

7

From the nuclear symbol we can determine the # protons, electrons and neutrons: # protons = 92# electrons = 92 (since no charge is shown)#neutrons = 235 – 92 = 143

Mass number, (protons + neutrons) Chemical Symbol Atomic number, (number of protons)

235 U 92

Example: uranium-235

This indicates the mass number.

8

Fill in the blanks:symbol neutrons protons electrons60Co 81Br

36 29 29

33 27 2746 35 35

Cu6529

9

How many neutrons arethere in Fe?Trick question! You can’t tell!How many neutrons are therein 53Fe?A) 26B) 55C) 27D) none of theseAns. 53 – 26 = 27mass number - # protons = # neutrons

This is how Fe appears in the periodic table.

26 Fe 55.85

This is NOT the atomic mass number.

101010

Hydrogen has 3 isotopes:

H11 H2

1 H31

protium deuterium tritium99.985% abundance

0.015 % abundance

0.000 % abundnace

1 proton

no neutron

1 proton

1 neutron

1 proton

2 neutrons

approx. mass of each atom = 1 u 2 u 3 u

111111

What we call "heavy water" is D2O.

(Regular water being H2O, with practically all the hydrogen atoms being protium.)

One molecule of H2O weighs 18 u.

One molecule of D2O weighs 20 u.

Hence D2O is known as “heavy water.”Tritium is the only radioactive isotope of hydrogen. Such isotopes are referred to as a radioisotope.

NOTE: The term "isotope" does NOT mean that it is radioactive.

12

Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine.

35.45 u

0.7577 x 34.9689 u = 26.495 u (4 sig. fig.)0.2423 x 36.9659 u = 8.9568 u (4 sig. fig.) atomic mass = 35.4518 u (2 decimal places)

17 Cl 35.45 This is the atomic mass

or atomic weight.

13

There are 2 isotopes of element Z. The first is 56.5% in abundance and has a mass of 152.3 u. If the atomic mass is 155.5 u, what is the mass of the other isotope?

Hint: Let x be the mass of the other isotope.What % in abundance is this other isotope?

159.6 u = 160. u (3 sig.fig.)

14

•A groups = representative elements or main group elements

I A = alkali metals II A = alkaline earth metalsVII A = halogens VIII = noble (inert) gases

•B groups = transition elements• Inner transition elements = elements 58 – 71 and 90 – 103

58 – 71 = lanthanide elements90 – 103 = actinide elements

Some Important Classifications:

15

Metals, Nonmetals, And Metalloids

16

Metals...•reflect light (have metallic luster)•can be hammered or rolled into thin sheets (are malleable) and can be drawn into a wire (are ductile)•are solids at room temperature(except Hg) •conduct electricity and heat

Properties Of Metals

17

Nonmetals And MetalloidsNonmetals...

– lack the properties of metals– tend to pulverize when struck with a hammer– Are non-conductors of electricity and heat– Many are gases, a few solids,

and one liquid (Br2)– react with metals to form (ionic) compounds

•Metalloids– have properties between metals and nonmetals

18

EXTREMELY IMPORTANT!

nonmetal + nonmetal

nonmetal + metal

metal + metal

covalent molecules

ionic compounds

alloys

Note: Alloys are not compounds. They are homogeneous mixtures of metals, sometimes with a small amount of nonmetals, such as C in steel.

19

Learning Check: Count The Atoms In A Chemical Formula

• Na2CO3

• (NH4)2SO4

• Mg3(PO4)2

• CuSO4•5H2O

• ___Na, ___ C, ___ O• ___N, ___H, ___S, ____O• ___Mg, ___P, ____O• ___Cu, ___S, ___O, ___H

32 12 8 1 43 2 81 1 9 10

What exactly does this dot tell you?

Covalent MoleculesThe subscripts in the formula of covalent

molecules tell you exactly how many atoms of each element is present.

They do not merely state a ratio.

e.g. CO2 tells us the molecule contains one C atom and two O atoms.

21

Ionic Compounds• Positively charged ions are called cations • Negatively charged ions are called anions • Subscripts in the formula always specify the

smallest whole-number ratio of the ions needed to make a neutral combination (formula unit, or f.u.)

It does not mean 2 Fe ions are bonded to 3 O ions.Subscripts tell us the ions are in a ratio of 2Fe to 3 O.

Fe O O2-Fe3+3232 +

The subscripts in the formula of• covalent molecules tell you the exact

number of the atoms of each element is present.

• ionic compounds tell you the ratio of the atoms of each element that is present.

23

Overview: Molecules vs. Formula Units

• Electrically neutral, discrete particles called molecules. (H2O)

• Neutral groups of charged particles called formula units. (NaCl)

24

Why are ions charged?• # p+ = # e- if neutral• # p+ < # e- if negative• # p+ > # e- if positive• The number of p+ never changes when ions form.• Ions are formed when the atom gains or loses

electrons. Protons are not affected.

How does Ca form Ca2+? Ca lost 2 electrons

How is N3- formed? N gained 3 electrons

25

Fill in the blanks:Symbol neutrons protons electrons60Co3+

81Br-

36 29 27

33 27 24

46 35 36

26529Cu

26

Hardness and brittleness–Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other.–Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions

Summary of Properties

27

• To melt the a solid, there must be sufficient kinetic energy to overcome the attractions between particles– Molecular compounds have weak attractions

between particles and so tend to have low melting points.

– Many molecular compounds are gases at room temperature.

– Ionic compounds tend to have strong attractions so they have high melting points.

– Nearly all ionic compounds are solids at room temperature.

Melting Points

28

• Requires the movement of electrical charge• Ionic compounds:

– Do not conduct electricity in the solid state– Do conduct electricity in the liquid and aqueous

states-The ions are free to move.

• Molecular compounds:– Do not conduct electricity in any state.– Molecules are comprised of uncharged particles.– Exception: Strong acids are molecular

substances that become ionic when dissolved in water.

Electrical Conductivity

29

Which of the following is likely true of NO2?

a) It conducts electricity well.b) It has a low melting point.c) It is likely a solid in its pure form.d) None of these

The formula tells you it’s a molecular compound, so it is not expected to conduct electricity, should have a relatively low mp, and therefore not likely to be a solid in its pure form.

You should know how the formula tells you that it’s a molecular formula. See Slide 18!