Name: Date: Blk: Dalton Thomson Rutherford Bohr THOMSON

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1 Name: Date: Blk: NOTES: ATOMIC STRUCTURE I. History of the Atom Dalton Thomson Rutherford Bohr DALTON - everything is made of atoms - different elements combine to form compounds in simple whole ratios - each element has its own unique type of atom with a characteristic weight - small indivisible solid particle model 2. THOMSON - discovery of subatomic particle (electron) Cathode ray experiment - rays are particles, not waves - new particles are negative, lighter than smallest atom (hydrogen) - plum pudding model 3. RUTHERFORD Thin gold foil experiment - alpha particles mostly pass through foil - but sometimes deflected back when they hit something massive - thus, atom made mostly of empty space - core of positively charged material, where most of the mass of the atom is (nucleus) - why don t negative electrons give in to the pull of the positive nucleus and collapse inwards?

2 4. BOHR - negatively charged electrons found in concentric circular orbits around the positive charged nucleus - electrons found at fixed energy levels orbiting at fixed distances from the nucleus - path closest to nucleus = lowest energy level - energy higher the farther the orbits are from the nucleus - the farther the electron is from the nucleus, the less attraction it feels - electrons can jump from one energy level to another, but are not found between levels - they lose or gain a discrete package of energy (quantum of energy) every time it jumps levels à quantum leap - explains why each element has a certain number of electrons available for reactions - the electrons found on the outer most orbit à valence electrons - planetary model II. Atomic Number and Atomic Mass (review!) Particles which make up the atom are called subatomic particles. 1. Protons: positively charged particles in the nucleus 2. Electrons: small negative particles outside the nucleus 3. Neutrons: neutral particles in the nucleus The Properties of the Atomic Particles PARTICLE SYMBOL CHARGE MOLAR MASS LOCATION electron e g (=0g) outside nucleus proton p g (=1g) inside nucleus neutron n g (=1g) inside nucleus The chemical elements are different from one another by the number of protons in the nucleus.

3 ATOMIC NUMBER = the # of protons in the nucleus - found on top of element symbol A NEUTRAL atom has NO OVERALL CHARGE: NUMBER OF ELECTRONS = NUMBER OF PROTONS EXAMPLE: 1. Any atom containing 3 protons must be? Lithium 2. A neutral sodium atom has 11 electrons If electrons are added to or subtracted from a neutral atom, the resulting particle is called: ION à # of protons # of electrons Electrons have a NEGATIVE charge so: - SUBTRACTING a NEGATIVE charge produces a POSITIVE ion - Cation = # of e - < # of p s - ADDING a NEGATIVE charge produces a NEGATIVE ion - Anion = # of e - > # of p s Example: 1. If an electron is added to a neutral F atom, then the ion is written as: 2. If two electrons are removed from a neutral Ba atom, then the ion is written as:

4 Since both neutrons and protons have a molar mass of approximately 1 g, then: - Total mass of an atom = total number of protons and neutrons *the electrons are too light to make an appreciable contribution to the mass of an atom ATOMIC MASS = the total # of protons and neutrons, thus: NUMBER OF NEUTRONS = ATOMIC MASS ATOMIC NUMBER Example: Find the number of protons, neutron and electrons in the following atoms: a) Al b) C But, then why is it that the atomic mass of some elements is not a whole number? *See Titanium above III. Natural Mixtures: Isotopes and Mass Number Example: ISOTOPES OF HYDROGEN What s the difference between Hydrogen-1, Hydrogen-2 and Hydrogen-3? The number of neutrons ISOTOPES: different forms of the same element, with the SAME # of protons but with DIFFERENT # s of neutrons, or - atomic species having the same atomic number, but different atomic masses

5 MASS NUMBER = the total # of protons and neutrons IN AN ISOTOPE OF AN ELEMENT Hydrogen-3 To find the p s and n s from nuclear notation: Calcium-41 Find the # of protons and # of neutrons in each of the following isotopes:

6 Now try the other way! Now try it with ions! 1) 56Fe 3+ 2) 76As 3-3) 201Au + 4) 82Br -

7 EXTRA PRACTICE: Write the nuclear notation: 1) An isotope has 46 protons, 58 neutrons and 42 electrons. 2) An isotope has 52 protons, 79 neutrons and 54 electrons. NATURAL MIXTURES - The molar mass of chlorine is 35.5 g. Since there can t be 0.5 of a proton or neutron, then this atomic mass MUST represent an AVERAGE value of a MIXTURE of isotopes. Example: Find the average atomic mass of: 1) Cl, given that it is composed of 75.77% Cl- 35 and 24.23% Cl ) B, given that it is composed of 18.8% B- 10 and 81.2% B- 11. BONUS: 3) Naturally occurring silicon consists of 92.23% Si- 28 (mass = g), 4.76% Si- 29 (mass = g) and 3.10% Si- 30 (mass = g). What is the expected average molar mass (atomic mass) of a sample of natural silicon, expressed to 4 decimal places? (Ans: g)