# Atomic Structure Protons, Neutrons, and Electrons Section 4.3.

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Atomic Structure Protons, Neutrons, and Electrons Section 4.3

Distinguishing Among Atoms Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes. 4.3

Protons Protons- determine the identity of an atom –Elements are different because they each have different #s of protons The atomic number of an element is the number of protons in the nucleus of an atom of that element –You can find this number by looking at your periodic table In a neutral atom, the number of protons is equal to the number of electrons

Atomic Number

Practice Finding # of protons and electrons

Finding the number of Neutrons The total number of protons and neutrons in an atom is called the mass number. –The number of neutrons in an atom is the difference between the mass number and atomic number. –How do I find the mass number? If given, add the protons and neutrons together If not, round the mass from the periodic table to the nearest whole number. This will provide the mass number of the most common isotope.

Finding Protons, Neutrons, Electrons You may just be given the name of an element and be expected to look it up on the periodic table. You may be given a symbol with mass numbers and atomic numbers beside it.

Writing and Interpreting Chemical Symbols

Mass of an atom Nucleus- contains most of the mass of an atom –Protons & Neutrons contain most of an atom’s mass Mass of proton = mass of neutron = 1amu –Electrons- 1/2000 th of the mass of a proton

Atomic structure Grams are not used to measure mass of atoms because atoms are too small –Amu is used instead –Amu = atomic mass unit Amu – 1/12 th the mass of a carbon-12 atom, which contains 6 protons and 6 neutrons

Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. –Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

Isotopes Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons. 4.3

Isotopes To indicate the identity of an isotope, write the element name – mass # –Carbon-12 Carbon-14 –Isotopes can also be indicated using the method we saw earlier

Calculating the Average atomic mass of an element Average atomic mass- a weighted average of the masses of the isotopes of an element Average atomic mass = (% x mass) + (% x mass) + (% x mass)….

Example of average atomic mass for carbon mass number 13 exact weightpercent abundance 1212.0000 amu98.90 1313.003355 amu1.10 (12.0000amu) (0.9890) + (13.003355amu) (0.0110) = 12.011 amu

Check your answers C has an average atomic mass of 12.011 amu. This is closer to 12 than to 13, which means most isotopes must be carbon- 12. This matches our known data, which say that 98.90% of carbon atoms are C- 12.

How do we know the mass of isotopes?? Use a mass spectrometer

Results from mass spectrometer

Throughout this section, we’ve been using the periodic table A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. –A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

Periodic Table Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element. –This occurs because they have a different number of outer shell (or valence) electrons

The Periodic Table—A Preview A Period 4.3

Periodic Table Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties. –This occurs because they have the same number of valence (or outer shell) electrons

The Periodic Table—A Preview A Group or Family 4.3