Atomic Structure: The Nucleus Chemical idea 2.1

1. Atomic Structure: The NucleusChemical idea 2.1 Learning Objectives Describe structure of atoms Explain and use the terms atomic mass and atomic number HSW: The development of the models (Atomic theory)

2. Atoms • Atoms are incredible small (atomic radius of hydrogen is about 1 billionth of a metre, its nucleus is 100 000 times smaller!!) so their mass is also very small! • Most of an atom is empty space. • If the nucleus of an atom was the size of a football, its electron cloud would fill the stadium!!!

3. Making Atoms As atoms are so small the actual amount of their masses and charges are silly numbers!! Instead we use RELATIVE masses and charges (i.e. the mass/charge relative to the proton. Relative masses/charges DON’T have units

4. Elements • All atoms are made up of the same types of particles. • Different types of atoms (ELEMENTS) contain different numbers of subatomic particles. • The nucleus of an atom can be described using ATOMIC NUMBER and MASS NUMBER

5. Describing the Atom • Another member of Rutherford’s team (Henry Moseley) had been using cathode rays (high energy electrons) to study atoms. • From the results from these experiments Rutherford was able to calculate the positive charge of a nucleus. • He called this the ATOMIC NUMBER (Z). Atomic number = number of protons in the atom • It is also known as PROTON Number.

6. Atomic number (Z) Tells us: • What the element is (atoms of the same element have the same atomic number) • The element’s position in the periodic table (it is arranged in order of atomic number) • The number of electrons in the ATOM (not ions!)

7. Mass number (A) The mass of an atom is almost entirely due to the mass of the neutrons and protons in the nucleus. Mass number = total number of protons and neutrons in the nucleus It is sometimes called NUCLEON number. Mass number A X Element symbol Atomic number Z

8. Isotopes Are different forms of the SAME element Same ELEMENT => atomic number SAME,(same number of PROTONS ). BUT mass number different, => different number of NEUTRONS.

9. Isotopes • E.g. 1) Hydrogen has 3 isotopes, protium, deuterium and tritium. • Can you guess the number of neutrons in each isotope? • E.g. 2) Carbon has 3 isotopes, carbon-12, carbon-13 and carbon-14 • How many neutrons does each isotope have?

10. Isotopic Abundance • Most elements exist as a MIXTURE of isotopes • Relative atomic mass (r.a.m.) = average of masses of isotopes taking into account their abundance • e.g. Chlorine Bromine 35Cl = 75% 79Br = 50% 37Cl = 25% 81Br = 50%

11. Calculating R.A.M • E.g. for chlorine (35 x 75) + (37 x 25) = 2625 + 925 100 100 = 3550 100 = 35.5

12. Now try problems for CI2.1