Atomic Structure: The Nucleus

1. Atomic Structure: The Nucleus 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. Democritus I call them ATOMS The very first Atomic theory was put forward by around 400 BC! Democritus believed all matter was made up of tiny particles that could not be split up into anything smaller, and that between atoms was nothing but space. Greek Dictionary atomos: Cannot be cut.

3. 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!!!

4. 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

5. 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

6. The first atomic theory John Dalton proposed the first atomic theory in 1803, for the first time based on actual experiments instead of ideas! The main points • Atoms (think snooker balls!) are indivisible and indestructible • Atoms of the same element have the same mass and chemical properties • When atoms react they join together to form COMPOUNDS

7. The Plum Pudding Atom JJ Thomson discovered the electron in 1897. It was the first evidence that the atom itself must have some kind of structure. Thomson’s plum pudding model was widely accepted. However it did not explain all the things that were known about atoms and elements at the time. It was superceeded by Rutherford’s model of the atom 12 years later. He first thought that the hydrogen atom must contain around 2000 electrons to account for its mass! As atoms are neutral he suggested that the electrons were embedded in a positive cloud, with the electrons being like plums baked into a positively charged pudding!

8. Rutherford’s Experiment • In 1909 Geiger and Marsden were working for Rutherford at Manchester University. • They firedα particles at a thin gold foil. • The particles were scattered, but a tinyamount were bounced backwards. • The plum pudding model could not account for this….

9. Rutherford’s Atom • Rutherford was amazed at the results from the experiments he said “it was like firing a cannon at tissue paper and it bouncing off!” • The only way he could make this work was if there was a tiny dense nucleus with a positive charge, surrounded by orbiting electrons

10. Rutherford Strikes again! • Rutherford carried out more experiments with alpha particles. • When he bombarded nitrogen with α particles, he detected hydrogen nuclei had been formed (and an isotope of oxygen) • The only place the hydrogen nucleus could have come from was the nitrogen gas. • He deduced that the hydrogen nucleus must be an ELEMENTARY PARTICLE the proton! α H O

11. The Neutron • Rutherford knew that the electron did not really contribute to the mass of the atom. • The positively charged nucleus contained protons, but the mass of the proton was not enough to make up the known masses of the atoms. • He decided that there must be another particle in the nucleus, uncharged, about the same mass as the proton to account for the masses of the elements, but other scientists did not agree with him until…..

12. James Chadwick …… (another of Rutherford’s team!) fired α particles at Beryllium atoms. • No radiation was detected BUT something emitted by the beryllium atoms caused paraffin wax to emit protons. • Rutherford said something undetectable emitted from the beryllium atoms must be causing the protons to be knocked out of the paraffin. Like an invisible man in a crowd, seen as he bumps into other people. • THE NEUTRON was discovered! (in 1932)

13. 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.

14. 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!)

15. 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

16. 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.

17. 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?