Elements, Atoms & the Atomic Structure

1. Elements, Atoms & the Atomic Structure

2. Elements and Atoms • Elements are the simplest pure substances, and they cannot be broken down into any other substances. • An atom is the smallest particle of an element (or unit of matter) that retains the identity of the substance.

3. What is an atom? • Because all matter is made up of elements, and all elements are made up of atoms, atoms are often called the building blocks of matter.

4. Atomic Structure • Atoms are composed of 2 regions: • Nucleus: the center of the atom that contains the mass of the atom • Electron cloud: region that surrounds the nucleus that contains most of the space in the atom Nucleus Electron Cloud

5. What’s in the Nucleus? • The nucleus contains 2 of the 3 subatomic particles: • Protons (p+) : positively charged subatomic particles that are identical. An atom is identified by the number of protons in its nucleus. All atoms of the same element have the same number of protons.

6. What’s in the Nucleus? • Neutrons (n0): neutrally charged subatomic particles that are identical. Sometimes, atoms of the same element can have different numbers of neutrons. These are called isotopes. Because neutrons have no charge, the overall atom is not changed by the extra neutrons.

7. What’s in the Electron Cloud? • The 3rd subatomic particle resides outside of the nucleus in the electron cloud. • Electron (e-): the subatomic particle with a negative charge and relatively no mass. They are also identical. The number and arrangement of the electrons in an atom determine its chemical properties.

8. What’s in the Electron Cloud? • Scientists theorize that electrons orbit the atom’s nucleus in energy levels. • An electron’s energy level determines its average distance from the nucleus.

9. Valence Electrons • An atom’s valence electrons are those electrons in the highest energy level and are held the most loosely. • The number of valence electrons in an atom of an element determines many properties of that element, including the ways in which the atom can bond with other atoms.

10. How do these particles interact? • Protons and neutrons live compacted in the tiny, positively charged nucleus accounting for most of the mass of the atom. • The negatively charged electrons are small and have a relatively small mass but occupy a large volume of space outside the nucleus.

11. How do these particles interact? • Since unlike charges attract each other, electrons and protons exert an attractive electrical force on each other. That is what holds electrons to the nucleus.

12. How do the subatomic particles balance each other? • In an uncharged atom: • The protons = the electrons • If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral or uncharged. • The neutrons have no charge; therefore they do not have to equal the number of protons or electrons.

13. How do we know the number of subatomic particles in an atom? • Atomic number: this number indicates the number of protons in an atom. • Ex: Hydrogen’s atomic number is 1 • So hydrogen has 1 proton • Ex: Carbon’s atomic number is 6 • So carbon has 6 protons **The number of protons identifies the atom. Ex. 2 protons = He, 29 protons = Cu

14. How do we know the number of subatomic particles in an atom? • Mass number: the number of protons and neutrons in the nucleus. • Ex: hydrogen can have a mass of 3. Since it has 1 proton it must have 2 neutrons • # of neutrons = mass # - atomic #

15. Determining the number of protons and neutrons • Li has a mass number of 7 and an atomic number of 3 • Protons = 3 (same as atomic #) • Neutrons= 7-3 = 4 (mass # - atomic #) • Ne has a mass number of 20 and an atomic number of 10 • Protons = 10 • Neutrons = 20 - 10= 10

16. What about the electrons? • The electrons are equal to the number of protons (in an uncharged/neutral atom). • So e- = p+ = atomic # • Ex: Na has a mass # of 23 and an atomic # of 11 • p+ = 11 • no = 12 • e- =11

17. electrons protons protons neutrons Where Do You Find “these” Numbers? Atomic Number Symbol Name Atomic Mass (this is an average, always round) Atomic number equals the number of _________or __________. Atomic mass equals the number of _________ + ___________.

18. Determine the number of subatomic particles in the following: • Cl has a mass # of 35 and an atomic # of 17 • p+ = ____, no = ____, e- = ____ • K has a mass # of 39 and an atomic # of 19 • P+ = ___, no = ___ e- = ____

19. Early Ideas about Atoms!!! • Democritus- a Greek Philosopher who lived about 440 B.C. • He thought you could“cut” down matter into pieces, until you reached the smallest possible piece • He named these pieces atoms which is Greek for “uncuttable”.

20. Early Ideas about Atoms!! • John Dalton- a British school teacher • In 1802 proposed an Atomic Theory. • He carried out experiments and concluded that atoms have different characteristics. Atomic Theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised. • To this day, scientists have found few exceptions to Dalton’s theories.

21. Dalton’s Atomic Theory • Atoms can’t be broken into smaller pieces. • Atoms of the same element are exactly the same. • Atoms of different elements are different • Atoms of 2 different elements combine to form compounds. • Atoms of each element have a unique mass. • The masses of the elements in a compound are always in a constant ratio. • Ex. Water- H2O 2 Hydrogen 1 Oxygen

22. J.J. Thomson • J.J. Thomson was a British scientist who found that atoms contain negatively charged particles in 1897. • Because scientists knew that atoms didn’t have an electrical charge, Thomson reasoned that atoms also contained some sort of positive charge. • Thomson described an atom as having negative charges scattered throughout a ball of positive charge.

23. Earnest Rutherford • In 1911, one of Thomson’s students found evidence that went against his model. • Rutherford’s research team aimed a beam of positively charged particles at a thin sheet of gold foil. They found that some particles passed through without problem, but some deflected strongly. • Since like charges repel, Rutherford inferred there had to be a cluster of positive charges in a tiny region (called a nucleus).

24. Earnest Rutherford • Scientists knew that electrons had almost no mass, so they reasoned that nearly all of an atom’s mass must be located in the tiny, positively charged nucleus. • Later it was suggested that the nucleus was made of one or more positively charged particles. • Rutherford named those particles – protons.

25. What is the structure of an atom? Bohr Model “Planetary Model” Bohr model – electrons are in specific energy levels Schrödinger Model “Electron Cloud Model”Electron cloud model – electrons are in a random cloud

26. James Chadwick • In 1932, British scientist James Chadwick discovered another particle in the nucleus of atoms. • His discovery completed the modern atomic model. • It was the hardest to detect because it had no charge. • This electrically neutral particle was called a neutron.