Atomic Structure and Dalton's Atomic Theory

Unit 2: Atoms, Ions and Ionic Compounds Atoms Nuclear Symbols Ions Names and Formulas of Ionic Compounds Atomic Models

Atoms • Atom:the smallest representative particle of an element • Democritus (460-370 BC) first suggested the concept of atoms • Rejected by Aristotle and the Greek thinkers in favor of fire, earth, air, water & heavenly ether as the basic components of matter • Dalton’s Atomic Theory (1803 – 1807) led to the re-emergence of the idea that atoms are the building blocks of all matter.

Dalton’s Atomic Theory • Dalton:Each element is composed of extremely small, indivisible, indestructible particles called atoms. • We now know that atoms are not indivisible. Each atom contains subatomic particles including: • Protons • Neutrons • Electrons

Dalton’s Atomic Theory • Dalton: All atoms of a given element are identical to each other in mass and other properties but different from the atoms of all other elements. • Dalton wasn’t quite correct. Some atoms of the same element have different masses. They are, however, chemically identical to each other.

Dalton’s Atomic Theory • Dalton: Atoms of one element cannot be changed into atoms of another element. • Dalton was correct when talking about ordinary chemical reactions. • Nuclear reactions, however, can convert one element into another.

Dalton’s Atomic Theory • Dalton: Compounds are formed when atoms of more than one element combine. • A given compound contains atoms combined in definite whole number ratios. • Fe2O3(rust) • H2O (water) • H2O2 (hydrogen peroxide) • These ratios are the same for any sample of a particular pure substance.

Dalton’s Atomic Theory • Dalton’s Atomic Theory explained several laws that were already known: • Law of Conservation of Mass (Matter) • Matter cannot be created or destroyed. • The total mass of substances present before and after a chemical reaction is constant. 5.0 g of reactants  5.0 g of products

Dalton’s Atomic Theory • Dalton’s Atomic Theory explained several laws that were already known (cont.): • Law of Constant Composition • A given compound always contains the same relative numbers and kinds of atoms. Ethyl alcohol C2H6O Methane always contains 25% H and 75% C

Dalton’s Atomic Theory • Dalton’s atomic theory was based on evidence gained in the macroscopic world. • Today, scientists can actually see individual atoms using special techniques. • See figure 2.2 of your text for a scanning tunneling micrograph of gallium arsenide • http://www.almaden.ibm.com/vis/stm/gallery.html

Modern View of Atomic Structure • Atoms are tiny spherical particles with a: • nucleus: • The small positively charged center of an atom • contains most of the mass of the atom • contains two important subatomic particles: protons and neutrons • diffuse, negatively charged outer layer that contains a third type of subatomic particle, the electron

Modern View of Atomic Structure • Most of the volume of an atom is empty space. Diffuse layer containing electrons in constant motion

Modern View of Atomic Structure • Important subatomic particles • Protons • positively charged subatomic particles found in the nucleus • Neutrons • neutral (uncharged) subatomic particles found in the nucleus • Electrons • very small, negatively charged subatomic particles found in a diffuse layer surrounding the nucleus

Modern View of Atomic Structure Relative Particle Symbol Charge Mass Proton p +1 1.0073 amu Neutron n 0 1.0087 amu Electron e- -1 5.468 x 10-4 amu amu (atomic mass unit): • a unit used to express very small masses • 1.66054 x 10-24 g

Modern View of Atomic Structure • If all atoms are composed of protons, neutrons, and electrons, then why is an atom of gold different from an atom of carbon? • The atoms of each element have a unique number of protons in the nucleus. • Atomic number: the number of protons in the nucleus • Carbon: 6 p (atomic # = 6) • Gold: 79 p (atomic # = 79)

Modern View of Atomic Structure • Atoms are electrically neutral (i.e. the total charge on an atom is zero). • Each atom contains the same number of protons and electrons. • Helium: 2 p, 2 e- • Magnesium: 12 p, 12 e- • Iron: 26 p, 26 e- • The atomic number tells you both the number of protons and electrons in an atom.

Modern View of Atomic Structure • The number of neutrons found in atoms of the same element can vary. • Hydrogen atoms: • Always 1 p and 1 e- • May have 0, 1, or 2 n • Atoms that have the same number of protons but different numbers of neutrons are referred to as isotopes of an element.

Modern View of Atomic Structure • Isotopes: • Atoms with the same number of protons but different numbers of electrons • Atoms with the same atomic number but different mass numbers • Mass number = # protons + # neutrons • a calculated value • A hydrogen atom with 2 neutrons: • Mass number = 1 p + 2 n = 3

Modern View of Atomic Structure • Atoms are small pieces of matter and therefore have mass. • Atomic weight (atomic mass) • The average atomic mass of an element expressed in amu • Calculated using the weighted average of each of the naturally occurring isotopes of an element

Atomic number Atomic weight (atomic mass) 5 10.81 B Elementsymbol electronegativity en = 2.04 boron Element name Modern View of Atomic Structure • Finding information on the periodic table: • So where do you find mass number???

Nuclear Symbols • Information about a particular isotope of an element is often depicted using its nuclear symbol: • For example, the nuclear symbol for a hydrogen atom with 2 neutrons would be: • Since atoms are electrically neutral, the charge is zero and is not shown.

Nuclear Symbols Example: Use the following nuclear symbol to determine the information listed below: Mass number = Atomic number = # protons = # neutrons = # electrons = Charge =

Nuclear Symbols Example: An atom has 21 neutrons and 19 protons. Identify the missing information. Mass number = Atomic number = # protons = # neutrons = # electrons = Charge = Nuclear symbol:

Ions • An atom can gain or lose electrons to become a charged particle called an ion. • A positively or negatively charged chemical particle that is formed when an atom (or group of atoms) gains or loses one or more electrons • Cation:a positively charged ion • Formed when an atom loses one or more electrons • Anion:a negatively charged ion • Formed when an atom gains one or more electrons

3p 4n 3p 4n e- e- e- e- e- Ions Formation of a cation + e- Lithium atom 3 p, 4 n, 3 e- Lithium ion (cation) 3 p, 4 n, 2 e- Net charge = 0 Net charge = +1

8p 8n 8e- 8p 8n + 2e- 10e- Ions Formation of an anion Oxygen atom 8p, 8 n, 8e- Oxide ion (anion) 8p, 8n, 10e- Net charge = -2 Net charge = 0

137 56 2+ Ba Nuclear Symbols of Ions • Since protons  electrons for an ion, the charge of the ion must be shown in the nuclear symbol. • Charge = # p - # e- • Show the magnitude of the charge first and then show the sign (+ or -)

137 56 2+ Ba Nuclear Symbols of Ions Example: Use the following nuclear symbol to determine the missing information. Mass number = Atomic number = # protons = # neutrons = # electrons = Charge =

Nuclear Symbols of Ions Example: Write the nuclear symbol. 1) 53 p, 74 n, 54 e- • 23 e-, 30 n, charge = +3 • 10 e-, 10 n, charge = -1

Atomic Structure and Dalton's Atomic Theory