Atoms, Molecules and Ions Chapter 2

The Modern Atomic Theory Modern Atomic theory has four assumptions: 1. Atoms make up all matter. 2. The atoms of one element are different from the atoms of another element. 3. Atoms combine in definite ratios to make compounds. 4 Combinations of atoms in compounds can change 4. only when a chemical reaction happens. This means reactions alter atom combinations, but the identity of the atoms themselves remain the same. 2

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Structure of Atoms • Atoms are made up of three main particles, neutron, electron, and proton. Particle

Symbol

Charge

Mass

electron

e-

-1

0.0005486 amu

proton

p+

+1

1.007276 amu

neutron

no

0

1.008665 amu

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Atomic number, Z: The identity of an element is controlled by the number of protons in the nucleus. In the neutral atom: number of protons inside the nucleus is the same number of electrons around the nucleus. Atomic number = # of Protons = # of Electrons Every element has its own unique atomic number. Example What is the atomic number for nitrogen, N? Nitrogen is in the seventh position in the periodic table. This means nitrogen atoms have 7 protons in the nucleus, 7 electrons around the nucleus, and they have an atomic number of 7. 4

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• Mass number, A: It is equal to the sum of neutrons and protons inside the nucleus, because the "massive" particles in the atom are protons and neutrons. M Mass number b = # off Protons P t + # off neutrons t # of neutrons = Mass number - # of Protons # of neutrons = A – Z 23 How many neutrons, electrons and protons are in an atom of Na? Sodium, Na, has atomic number 11. # of Protons =11 # of Electrons = 11 N b off neutrons Number t =A-Z Number of neutrons = 23 - 11 = 12

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An atom with a mass number of 39 contains 20 neutrons. What is the atomic number and identity of the element? The atomic number is Z = 39- 20 =19. The identity is potassium because K is element 19

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Isotopes All the atoms of the same element have the same atomic number, but they can have different: numbers of neutrons and mass numbers

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Atomic Mass The atomic mass of an element is the mass average of the atomic masses of the different isotopes of an element. For example, naturally occurring carbon, for example, is a mixture of two isotopes, 12C (98.89%) and 13C (1.11 %). Individual carbon atoms therefore have a mass of either 12.000 or 13.03354 amu. But the average mass of the different isotopes of carbon is 12.011 amu.

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Isotope abundances The isotopes of an element do not occur with equal frequency. The relative abundance depends on the relative stability of the isotope. The isotopes contribute to the average atomic mass based on their abundance. The atomic weights in the periodic table are weighted averages. This means the tabulated value doesn't doesn t match any actual atom atom, but is closer to the most common isotope.

Average weight = % First isotope abundance x its mass + % Second isotope abundance x its mass What is the average atomic mass for thallium, Tl, if there are two isotopes with the following masses and abundances? (Tl-203 (203Tl) has a mass of 203.059 amu with ith an abundance b d off 29.52 29 52 %, % Tl Tl-205 205 (205Tl) h has a mass off 205.059 205 059 amu with ith an abundance of 70.48 %) Step 1: Convert percents to decimals 29.52 % to 0.2952 and 70.48 % to 0.7048 Step 2: Average weight = 0.2952 x ( 203.059 amu) + 0.7048 x ( 205.059 amu) 204.466 amu rounded off to 204.5 amu with 4 significant. 8

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The Periodic Table • The modern periodic table (PT) represents the arrangements of elements according to the building up (Aufbau principle), where each element has one electron more than the previous element. element • The periodic table can be classified in many different ways:  Metallic character: metals, nonmetals, and metalloids.  Position in the periodic table: horizontal rows (Periods) vertical columns (Groups) 9

The periodic table contains the following important groups:

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A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces

H2

H2O

NH3

CH4

A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, HCl CO

A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 11

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive charge If a neutral atom loses one or more electrons i b it becomes a cation. i Na

11 protons 11 electrons

Na+

11 protons 10 electrons

anion – ion with a negative g charge g If a neutral atom gains one or more electrons it becomes an anion. Cl

17 protons 17 electrons

Cl-

17 protons 18 electrons

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A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3-

A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3-

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Do You Understand Ions?

H How many protons t and d electrons l t are iin

27 3+ 13 Al ?

13 protons, 10 (13 – 3) electrons How many protons and electrons are in

78 234 Se ?

34 protons, 36 (34 + 2) electrons

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A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance An empirical formula shows the simplest whole-number ratio of the atoms in a substance molecular

empirical

H2O

H2O

C6H12O6

CH2O

O3

O

N2H4

NH2 16

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ionic compounds consist of a combination of cations and an anions • the formula is always the same as the empirical formula • the sum of the charges g on the cation(s) ( ) and anion(s) ( ) in each formula unit must equal zero

The ionic compound NaCl

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Formula of Ionic Compounds 2 x +3 = +6

3 x -2 = -6

Al2O3

Al3+ 1 x +2 = +2

Ca2+ 2 x +1 = +2

Na+

O22 x -1 = -2

CaBr2

Br1 x -2 = -2

Na2CO3

CO3218

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Chemical Nomenclature • Ionic Compounds – often a metal + nonmetal – anion (nonmetal), add “ide” to element name

BaCl2

barium chloride

K2O

potassium oxide

Mg(OH)2

magnesium hydroxide

KNO3

potassium nitrate 19

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