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Ch2−p1 Chapter 2 - Atoms & Elements (Chapter 2 in 1st Edition) Consider pouring some sugar in water. How would you describe this action? A Chemist says: Sugar is being dissolved in water One chemical mixing with another chemical One compound getting near to another compound Chemists write in symbolic language: Chemists envision: Molecules bump into each other Atoms in the molecules vibrating Chemists draw water as: or -letter abbreviations for Chemical symbols are the names of the elements. Water is made up of two elements: hydrogen (H) and oxygen (O). Chemists need to know: How many elements are there? What are they? How do they behave? Are they similar or different? Ch2−p2 Periodic Table Dmitri Mendeleev (late 1800's) outlined first modern Periodic Table. 88 elements occur in nature 115 elements are currently known Gases (11) Helium, Neon, Argon, Krypton, Xenon, Radon He Ne Ar Kr Xe Rn (noble gases) , and Oxygen, , are the main Nitrogen, components of air(78% and 21% respectively). The other 1% is from other gases. A formula is the symbol for a stable compound. Fluorine (symbol Chlorine (symbol Hydrogen (symbol the presence of O2) Liquids (2) Bromine (symbol Mercury (symbol Remainder are Solids , formula , formula , formula , highly reactive) , reactive) , explosive in , formula , formula , reactive) , poisonous) Ch2−p3 Organization: Representative and Transition elements Groups and Periods Group number and period numbers Metals versus Non-metals versus Metalloids Human body composition (by mass): Oxygen ( , 65%) , 18.3%) Carbon ( , 10%) Hydrogen ( Nitrogen ( , 3.3%) , 1.7%) Calcium ( , 1%) Phosphorous ( many elements are essential in trace quantities for maintaining proper health e.g. iodine ( , need ca. 50 mg/year) , , need ca. 1 g/year) copper & fluorine ( , need ca. 5 g/year) iron & zinc ( Ch2−p4 Atoms What do we know about the composition of elements? Greeks Philosophers believed that matter must contain small particles called atomos. John Dalton developed the first atomic theory in 1808. Postulates of Atomic Theory: 1. All matter is made up of tiny particles called atoms. 2. All atoms of a given element are similar to one another but different from atoms of other elements. 3. Atoms of two or more elements combine to form compounds. The composition of a particular compound always has same kinds of atoms in the same ratios. 4. A chemical reaction involves rearrangement, separation or combination of atoms. Atoms are never created or destroyed during a chemical reaction. eg. ___H2 + O2 → ___H2O What’s wrong with the above equation? Ch2−p5 Equations must always be What is inside an atom? . subatomic particles: protons (p or p+, +) electrons (e-, -) neutrons (n or n0, 0) Where are the particles inside the atom? From the famous 1911 gold foil experiment, Ernest Rutherford proposed that the mass of the atom was made up of a small, dense region at the centre of the atom. This region is called the nucleus. The rest of the atom is essentially empty space which is occupied by the electrons. IMPORTANT: The # protons defines the element. If the # protons changes, then it is not the same element. eg. The carbon atom has 6 protons in the nucleus. If you remove 1 proton from the carbon nucleus, you change the nature of the element. C - p → B if you add 1 proton to the carbon nucleus you get nitrogen. C + p → N These are nuclear reactions. It takes a nuclear reaction to change an element. Ch2−p6 Atomic Number and Mass Number mass number element atomic number 12 6 C Atomic number = # protons Mass number = # protons + # neutrons If an atom is neutrally charged, then # electrons = # protons NOTE: different periodic tables use different formats. Mass number is not typically shown. Questions: Give the number of protons, neutrons and electrons for each neutral atom? 1 1 16 8 H O 4 2 56 26 He Fe 24 12 Mg 197 79 Au Ch2−p7 Atoms of an element can be different if the number of neutrons they have differ. This will result in different mass numbers. eg. 1 1 12 6 H C 2 1 13 6 H C 3 1 14 6 H C These are called isotopes of the element. All isotopes coexist in the stable form of the element. Under normal conditions, it is OK to treat the element as an “averaged atom” with an “averaged mass”. Atomic Mass (as shown on the periodic table) = Weighted average of all naturally occurring isotopes Example: chlorine has 2 isotopes Ch2−p8 35 17 Cl 75.8 % 34.97 amu The average mass of Cl is: 37 17 Cl 24.2 % 36.97 amu Ch2−p9 The Electrons “electrons = chemistry” Electrons are the particles that interact with each other to forms bonds. When the interaction is broken, so is the bond between atoms. Electrons are constantly moving through the space surrounding the nucleus and therefore must have Do all electrons have the same energy? . What are the consequences of electrons having different energies? Electron Energy Levels Electrons are arranged in specific energy levels called shells. The energy levels are numbered by “n”, n = 1, 2, 3, ...7. As the energy levels increase so does the energy of the electrons. . Ch2−p10 Within each shell, electrons are grouped into subshells. The different subshells are labelled s, p, d, and f. The number of subshells is determined by the shell number. Thus: Shell Number n=4 n=3 n= 2 n=1 Number of subshells Types of subshells Within each subshell are orbitals. Orbitals are three dimensional regions of space around the nucleus where the electrons are most likely to be found. 2 IMPORTANT POINTS REGARDING ORBITALS: 1. The shapes of the orbitals represent electron density and NOT the pathway the electron follows. 2. Orbitals may only hold up to two electrons. The s subshell contains 1 s-orbital. It is spherical in shape. Ch2−p11 The p subshell contains 3 p-orbitals. They are dumbbell shaped. z z z y y y x x x The d subshell contains 5 d-orbitals and the f subshell contains 7 f-orbitals. You don’t need to know their shapes right now. Within a subshell, the order of increasing energy is: 5s 5p 5d 5f 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s 5g Ch2−p12 Electron Configuration The electron configuration of an atom describes the organization of its electrons. Electrons are always arranged starting with the lowest energy subshell, followed by the next highest subshell and so on. In the Electron Configuration notation, the number of electrons in a subshell is written as a superscript. e.g. hydrogen atom ( Number of e = )? , E. C. = carbon atom ( Number of e = )? , E. C. = chlorine atom ( Number of e = )? , E. C. = Remember; always start with the lowest energy subshell and only a maximum of two electrons in each orbital. The subshells of lower numbers are the inner shells and the subshells with the highest number represent the outermost shell. Ch2−p13 Shorthand Notation. What is the E.C. for Ar? What is the E.C. for Br? Therefore, we can write the Noble Gas notation for Br as: Periodic Law Elements in the vertical columns or groups of the periodic table exhibit similar physical and chemical properties. As we go from one group to the next there is also a regular change in physical and chemical properties. This pattern of change is known as the periodic law. There is also a similarity in the electron arrangements for elements within the same group. What is the E.C. for: H Li Na K Ch2−p14 Therefore, among elements within a group, their similar chemical and physical properties can be attributed to them having the same number of electrons in the outermost shell. Ch2−p15 Important Concepts from Chapter 2 • Element name and symbols • Periodic Table Organization (groups and periods) • Atomic Number and Mass Number • Calculating Average Atomic Mass • Isotopes • Electron Organization – Shells, Subshells, and Orbitals (shape and energy) • Electron Configuration