Distinguishing Between Atoms Atomic Number: equals the number of protons in the nucleus of an atom of that element Atoms are electrically neutral, therefore the number of electrons must equal the number of protons How is the number of electrons for a neutral atom of a given element related to the atomic number of that element? The atomic number is equal to the number of electrons

Atoms of the First Ten Elements Composition of the Nucleus Name Symbol Atomic Number Composition of the Nucleus Mass Number of electrons Protons Neutrons Hydrogen H 1 Helium He 2 4 Lithium Li 3 7 Beryllium Be 5 9 Boron B 6 11 Carbon C 12 Nitrogen N 14 Oxygen O 8 16 Fluorine F 10 19 Neon Ne 20

Element block

Review 7 The element Nitrogen has an atomic number of what? 7, number of protons equals the number of electrons The element Nitrogen has an atomic number of what? How many protons and how many electrons are in a neutral nitrogen atom?

How many protons and electrons are in each of the following atoms? Calcium 19,19 19,20 20,20 20,19 Fluorine 9,8 9,9 8,9 8,8 Aluminum 12,13 12,12 13,12 13,13

Complete the Table Element Atomic Number Protons Electrons K 19 5 16 23

Mass Number: the total number of protons and neutrons in an atom Mass # = protons + neutrons Example: Oxygen 16 = 8 + n n = 8

The following is shorthand for writing the # of protons and neutrons, you don’t need to write the # of electrons because they are the same as the # of protons The large "X" represents where you will find the atom's elemental symbol.  The mass number, which is given the symbol "A", is located in the upper left-hand corner.  The atomic number (also called nuclear charge), which is given the symbol "Z", is found in the lower left-hand corner. Number of neutrons  =  mass number  −  atomic number

Number of neutrons = mass number − atomic number In the case of neon (Ne): Number of neutrons  =  20- 10 = 10 In the case of carbon (C): Number of neutrons  =  12- 6 = 6

More Examples 3 9 4 10 Elemental Symbol # of Protons (equal to Z) # of Neutrons (equal to A-Z) 4 10 # of electrons (equal to Z modified by charge number)

How many protons, electrons, and neutrons are in the following atoms? Atomic Number Mass Number (a) Beryllium (Be) 4 9 (b) Neon (Ne) 10 20 (a) Sodium (Na) 11 23

How many neutrons are in each atom? 8 16 24 32 S 16 32 48

Express the composition of the following atom in shorthand form Carbon-12 12 C 6 6 C 12 18 C Fluorine-19 9 F 19 28 F 9 19 F

Isotopes: atoms that have the same number of protons but different numbers of neutrons Isotopes have different mass numbers because they have different numbers of neutrons They are chemically alike because they have identical numbers of protons & electrons, which are the subatomic particles responsible for chemical behavior.

Example: Hydrogen has three isotopes; each isotope has 1 proton in its nucleus.

Carbon-12 Carbon-13 ___ ___ ___C ___C Two isotopes of carbon are carbon 12 and carbon 13. Write the symbol for each isotope using superscripts and subscripts to represent mass # and atomic number Carbon-12 Carbon-13 ___ ___ ___C ___C

Determine the number of protons, neutrons, and electrons for each of the following isotopes of Neon 22 10 11 Neon-21 21 Neon-22 22 10 12

Properties of Subatomic Particles Atomic Mass Properties of Subatomic Particles Particle Symbol Relative electrical charge Relative mass (Mass of proton =1) Actual mass (g) Electron e- 1- 1/1840 9.11x10-28 Proton P+ 1+ 1 1.67x10-24 Neutron n◦ Actual mass of a proton or a neutron is very small (1.67 x10-24 g) Since about the 1920s, it has been possible to determine these masses (protons & neutrons) by using a mass spectrometer

This is impractical to work with, so a reference isotope as a standard is used. The chosen isotope is C-12. It was assigned a mass of EXACTLY 12 atomic mass units or amu Carbon-12 has 6 protons and 6 neutrons, making up its mass Since the mass of an atom mainly depends on the number of protons and neutrons, shouldn’t the atomic mass of an element be a whole number? That’s usually not the case, Cl is 35.453 amu The reason for this involves the relative abundance of the naturally occurring isotopes of the element. Most occur as a mixture of two or more isotopes Each isotope has a fixed mass and a natural percent abundance

The atomic mass is a weighted average of all naturally occurring isotopes http://www.sisweb.com/referenc/source/exactmaa.htm

Example: Chlorine 35 Cl and 37 Cl 17 17 76% 24% Three more 35 Cl for every one 37 Cl 17 17 (Remember: the # of protons and neutrons = atomic mass) % abundance: # of protons = 17 + 17+ 17+ 17 = 68 # of neutrons = 18 +18 +18 +20 = 74 68 + 74 = 35.5 amu 4 Therefore, the Atomic Mass is a weighted average mass of the atoms in a naturally occurring sample of the element

Review Which isotope of copper is more abundant: copper – 63 or copper – 65? (the atomic mass of copper is 63.546 amu) Copper-63 Boron has two isotopes: boron-10 and boron-11, which is more abundant (atomic mass is 10.81 amu)? Boron -11

Element X has two natural isotopes. The isotope with a mass of 10 Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element. 10X = 10.012 x 0.1991 = 1.993 amu 11X = 11.009 x 0.8009 = 8.817 amu 10.810 amu What is this element? Boron

Calculate the atomic mass of Bromine Calculate the atomic mass of Bromine. The two isotopes of bromine have atomic masses and relative abundances of 78.92 amu (50.69%) and 80.92 amu (49.31%) 79.91 amu