# Atoms and their structure

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Atoms and their structure

The Modern Model The modern atom is composed of two regions:
1. Electron cloud- Most of the volume of an atom and is the region where the electron can be found (extranuclear) 2. The nucleus – with protons and neutrons so that it has a Positive charge and almost all the mass

Major Subatomic Particles
Name Symbol Charge Relative Mass (amu) Actual Mass (g) Electron e- -1 1/1840 9.11x10-28 Proton p+ +1 1 1.67x10-24 Neutron no Atoms are measured in picometers, meters Hydrogen atom, 32 pm radius Nucleus is tiny compared to atom If the atom were a stadium, the nucleus would be a marble Radius of the nucleus is on the order of m Density within the atom is near 1014 g/cm3

Nuclear Symbols Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number E A Z elemental symbol mass number atomic number

Elemental Classification
Atomic Number (Z) = number of protons (p+) in the nucleus Determines the type of atom Li atoms always have 3 protons in the nucleus, Hg always 80 Mass Number (A) = number of protons + neutrons [Sum of p+ and nº] Electrons have a negligible contribution to overall mass In a neutral atom there is the same number of electrons (e-) and protons (atomic number)

W F Br 184 74 19 9 80 35 Find the number of protons number of neutrons
number of electrons atomic number mass number W 184 74 F 19 9 Br 80 35

If an element has 91 protons and 140 neutrons what is the:
If an element has an atomic number of 34 and a mass number of 78 what is the: number of protons in the atom? number of neutrons in the atom? number of electrons in the atom? complete symbol of the atom? If an element has 91 protons and 140 neutrons what is the: atomic number? mass number? number of electrons? complete symbol?

Isotopes Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers The atoms of the same element that differ in the number of neutrons are called isotopes of that element H 1 Protium 2 Deuterium 3 Tritium

Naming Isotopes When naming, write the mass number after the name of the element Examples: carbon- 12 carbon -14 uranium-235

Review: Write the nuclear symbols ( ) and name including mass for three isotopes of oxygen in which there are 8, 9, and 10 neutrons respectively.

Ions When atoms gain or lose electrons they form something called IONS. Energy is released when this happens. Ions are electrically charged atoms. (positive or negative)

Cations (+) CATIONS: are positively charged ions. Cations are formed when an atom loses electrons. Metals and Hydrogen form cations Ex: Na lose 1 electron  Na1+

Anions (-) ANIONS: are negatively charged ions. Anions are formed when atoms gain electrons. Most non-metals form anions. Ex: Cl gains one electron  Cl1-

Charges of Ions Group 1  loses 1 electron = +1
Group 2  loses 2 electrons = +2 Group 13  loses 3 electrons = +3 Group 15  gains 3 electrons = -3 Group 16  gains 2 electrons = -2 Group 17  gains 1 electron = -1 Group18  has a full outer shell = very stable 

Noble Gases Group 8A (He, Ne, Ar, etc.)
Have a full valence shell (complete octet) They neither want to gain or lose electrons Therefore, DO NOT form IONS ********Handout  chemical familes  discuss

(+) charge = subtract electrons from # protons
Only the number of electrons can change: (+) charge = subtract electrons from # protons (-) charge = add electrons from

How many protons and electrons do each of the following have?
Br-1 Ca+2 Se-2 e) Mg+2 f) Au3+ g) P-3 h) Mn+4

Average Atomic Mass Did you ever notice that the mass number on the periodic table is not a whole number? There are the decimal numbers on the periodic table. **This is because it is an average mass for all the isotopes of each atom

How heavy is an atom of oxygen?
There are different kinds of oxygen atoms (different isotopes) 16O, 17O, 18O We are more concerned with average atomic masses, rather than exact ones Based on abundance of each isotope found in nature We can’t use grams as the unit of measure because the numbers would be too small Instead we use Atomic Mass Units (amu) Standard amu is 1/12 the mass of a carbon-12 atom Each isotope has its own atomic mass

Calculating Averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = x 50 g + 1 x 60 g = 260 g Average mass = (4 x 50 g + 1 x 60 g)/5 = (260 g)/5 = (4 x 50 g)/5 + (1 x 60 g)/5 = 260 g/5

Calculating Averages Average mass = 0.8 x 50 + 0.2 x 60
80% of the rocks were 50 grams 20% of the rocks were 60 grams Average = (% as decimal) x (mass1) (% as decimal) x (mass2) (% as decimal) x (mass3) + …

Calculating Averages Calculate the av atomic mass of copper if copper has two isotopes 69.1% has a mass of amu The rest (30.9%) has a mass of amu Magnesium has three isotopes 78.99% magnesium 24 with a mass of amu 10.00% magnesium 25 with a mass of amu The rest magnesium 26 with a mass of amu What is the atomic mass of magnesium?

Average Atomic Masses If not told otherwise, the mass of the isotope is the mass number in amu The average atomic masses are not whole numbers because they are an average mass value Remember, the atomic masses the decimal numbers on the periodic table