Atoms – a closer look at elements

1. Atoms – a closer look at elements As mentioned before, atoms are composed of protons, neutrons, and electrons

2. Review from the earlier notes • Protons are positive, have a mass roughly the same as a neutron, found in the nucleus, define an element • Neutrons have no charge, have a mass roughly the same as a proton, found in the nucleus, are responsible for isotopes • Electrons are negative, very small mass, smallest subatomic particle, found outside of the nucleus, are responsible for ions (atoms with a charge, add number to electron total if negative, subtract if positive).

3. So, what does it mean to define an element? • An element is defined as the number of protons that it has in its nucleus. • Therefore, the number of protons equals the element’s Atomic Number. • The atomic number is the same for all atoms of the same element.

4. More about the Atomic number • Equals the number of electrons for NEUTRAL atom • Determines an elements placement on the periodic table • Represented by letter Z • So Z = protons = electrons (if neutral, if not see above note)

5. Let’s look at a Periodic Table • How many protons does titanium (Ti) have? • Antimony has 51 protons, what is its atomic number? What abbreviation is given to Antimony? • Magnesium (Mg) is a neutral atom, how many electrons does it have?

6. Mass number (A) • Sum of the protons and neutrons of an atom • So, Mass number = protons + neutrons • To find neutrons given protons and mass number: • Neutrons = mass number - protons 108 Ag This is called a nuclear symbol 47 Top is mass number Bottom atomic number Note: Both numbers on left

7. Practice Problem • What is the mass number of an element with 10 protons and 10 neutrons? What element are we dealing with? How would you represent this symbolically? • 20 amu • Ne • Ne 20 10

8. More Practice • How many protons, neutrons and electrons does the following neutral atoms have? • Fe • 26 protons, 30 neutrons, 26 electrons • Xe • 54 protons, 77 neutrons, 54 electrons 56 26 131 54

9. Two More – now with a charge 16 protons 16 neutrons 18 electrons 20 protons 21 neutrons 19 electrons

10. Group Work – Make and complete this chart on the white boards

11. Look at the masses on the Periodic Table • So, if the atomic mass is equal to the sum of protons and neutrons, why isn’t the mass a whole number on all Periodic Tables of the Elements? Even though an atom has to have a certain number of protons, the number of neutrons can vary slightly. These different versions of the same atom are called… Isotopes

12. Isotopes • Same number of protons but different number of neutrons • Examples are Carbon -13 verse Carbon -12

13. Average Atomic Masses Relative atomic mass: average masses of isotopes average atomic mass is also known as Atomic weight (AW). Atomic weights are listed on the periodic table. *Don’t confuse these with mass numbers*

14. Naturally occurring Carbon is98.892 % 12C + 1.108 % 13C.Find the average atomic mass • Step 1: Multiply the mass number of each isotope by its percentage • Step 2: Add the numbers together • Step 3: Divide by 100 • Step 4: Check your answer. Mass should be between highest and lowest mass

15. An Isotope Example - Chlorine Let’s look at Chlorine, which has an atomic mass of 35.5 Atomic # = ___, which means that there are ___ protons. 17 17 There are two naturally occurring isotopes, Cl-35 and Cl-37 This means that Cl-35 has ___ neutrons while Cl-37 has ___ neutrons. 18 20 Cl-35 occurs naturally 75.8% of the time Cl-37 occurs naturally 24.2% of the time (35 x 75.8 + 37 x 24.2)/100 = 35.5

16. Try this one • Calculate the average mass of the isotopes of Uranium with: • 50.0% at 239 amu • 29.4% at 235 amu • 20.6% at 238 amu • 237.62 amu

17. Homework • Page 108: 30-37 • Page 253: 43 and 44