Chapter Two Atoms, molecules and Ions

Chapter Two / Atoms, molecules and Ions Dalton’s atomic theory has four assumptions: 1.Atoms are the building blocks of elements. 2.For the same element all atoms are identical. The atoms for one element are different from the atoms of all other elements. 3.Atoms combine in definite ratio to makes compound. 4.A chemical reaction involves only the separation, combination and rearrangement of atoms. It dos not result in their creation or destruction. H 2 O  2H + O What is atom? Atom is the basic unit of an element that can enter into chemical combination. The Atomic Theory

Chapter Two / Atoms, molecules and Ions Structure of atom protonNeutronElectron

Chapter Two / Atoms, molecules and Ions Structure of atom Nucleus protonNeutronElectron

Chapter Two / Atoms, molecules and Ions Structure of atom Protons and neutrons are together in the nucleus of an atom, whereas electrons are in motion in orbits around the central nucleus. Protons carry a positive electrical charge, electrons carry a negative charge, and neutrons carry no charge. Most atoms are electrically neutral, meaning that they have an equal number of protons and electrons MassChargeSymbolParticle amue-e- Electron amu+1p+p+ Proton amu0n0n0 Neutron

Chapter Two / Atoms, molecules and Ions Structure of atom J.J. Thomson is the scientist who determine the ratio between electronic charge to the mass of an electron using a cathode ray tube experiment

Chapter Two / Atoms, molecules and Ions Structure of atom What is amu? amu is atomic mass unit 1 amu = × kilograms

Chapter Two / Atoms, molecules and Ions Atomic Number and Mass Number All atoms can be identified by the number of proton and neutrons they contain. Atomic number is the number of proton in the nucleus. In neutral atom the number of proton is equal to the number of electron thus the atomic number also indicate to the number of electron. The chemical identity of an atom can be determined solely by the atomic number. Example: the atomic number of Fluorine is 9 this means that each fluorine atom have 9 protons and 9 electrons. Any atom have 9 proton is Fluorine. Atomic Mass is the total number of neutrons and protons present in the nucleus of an atom of an element.

Chapter Two / Atoms, molecules and Ions Structure of atom Thus, number of neutrons = mass (weight) number - atomic number Isotopes are atoms that have the same atomic number (proton number) but different mass numbers. This mean the number of neutrons is the only difference between isotopes. For example there are 3 isotopes for hydrogen 1 H 2 H 3 H HydrogenDeuteriumTritium

Chapter Two / Atoms, molecules and Ions Structure of atom Example: Carbon (atomic number 6) has three natural isotopes with atomic weights of 12, 13 and 14. With exception of hydrogen, which has different names for each of its Isotopes, isotopes of element are identified by their mass number. For example carbon isotopes are called : carbon-12, carbon 13 and carbon 14 X A Z Mass Number Atomic Number Element Symbol isotope #p #n ====== == == C-1266 C-1367 C-1468

Chapter Two / Atoms, molecules and Ions Structure of atom Important notice : as chemical properties of element determine by the number of proton and electron. Thus isotopes of the same element have similar chemistries, forming the same type of compound and displaying similar reactivity. Do you understand Isotopes? How many protons, neutrons, and electrons are in ? 6 protons, 8 (14 - 6) neutrons, 6 electrons C 14 6

Chapter Two / Atoms, molecules and Ions Periodic Table Between 1800 and 1900 more than half the elements were discovered. Scientist tried to organize all the elements based on similarities which led to the porn of the periodic table. The periodic table is a chart in which elements having similar chemical and physical properties are grouped together.

Chapter Two / Atoms, molecules and Ions Periodic Table Period Group Alkali Metal Alkali Earth Metal Halogen Noble Gas

Chapter Two / Atoms, molecules and Ions Periodic Table Elements in the periodic table are divided into three categories: Metal: (in green colour, Most elements) is a good conductor of heat and electricity Nonmetal: (in blue colour, 17 elements) is a poor conductor of heat and electricity Metalloid: (in brown colour, 8 elements) has properties that are intermediate between those of metals and nonmetals

Chapter Two / Atoms, molecules and Ions Molecules Molecules : is an aggregate of at least two atoms or more in a definite arrangement held together by chemical forces. What is the difference between molecule and compound? Molecules may contain two atoms of the same element or atoms of different elements. Compound only contain two or more elements Thus all compounds are molecules but not all molecules are compounds. Molecules or compounds ? H 2 O, I 2, NaCl, H 2, O 2, NaOH compound H 2 O, NaCl, NaOH All molecules

Chapter Two / Atoms, molecules and Ions Molecules Diatomic MoleculesPolyatomic molecules Molecules contain only two atoms. Example : H 2, O 2 N 2, HCl, CO Molecules contain more than two atoms. Example : H 2 O, NH 3, O 3

Chapter Two / Atoms, molecules and Ions Ions Ion is an atom or a group of atoms that has a net of positive or negative charge. CationAnion An ion with positive charge An ion with negative charge If a neutral atom loses one or more electrons it becomes a cation. If a neutral atom gains one or more electrons it becomes an anion. Na + 11 protons 10 electrons Cl 17 protons 17 electrons Cl - 17 protons 18 electrons Na 11 protons 11 electrons

Chapter Two / Atoms, molecules and Ions SALT is an ionic compound because it formed from cations and anions - a Cation and an Anion held together by an ionic bond.

Chapter Two / Atoms, molecules and Ions Ions Monatomic ionsPolyatomic ions Ions contain only one atom. Example : Na +, Mg +2, S -2, Ions contain more than one atom. Example : NH 4 +, CN -, OH -,

Chapter Two / Atoms, molecules and Ions Ions Do You Understand Ions? How many protons and electrons are in Proton = 13, electron = 13-3 = 10 Neutron = 27 – 13 = 14 Al ? 3+3+ How many protons and electrons are in Proton = 34, electron = = 36 Neutron = 78 – 34 = 44 Se ?