The Structure of the Atom Chapter 4 The Structure of the Atom

Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton. Figure 2.1 John Dalton (1766-1844)

Dalton’s Atomic Theory Elements are made up of atoms Atoms of each element are identical. Atoms of different elements are different. Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom. Chemical reactions are rearrangement of atoms. Atoms are not created or destroyed.

Thomson’s Model Discover the electron. (Cathode Tube) Said the atom was like plum pudding. A bunch of positive stuff, with the electrons embedded throughout.

Cathode Ray Experiment

Rutherford’s Experiment Used uranium to produce alpha particles. Aimed alpha particles at gold foil by drilling hole in lead block. Since the mass is evenly distributed in gold atoms alpha particles should go straight through. Used gold foil because atoms could be made very thin.

Florescent Screen Lead block Uranium Gold Foil

What he expected

Because, he thought the mass was evenly distributed in the atom.

What he got

Gold Foil Experiment

The Result Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space. Figure 2.12

Atomic Structure Atoms are composed of: 1. 2. 3.

Atomic Structure Particle Location Mass Proton Inside nucleus 1.673 x 10-24 g Neutron 1.675 x 10-24 g Electron Outside nucleus 9.109 x 10-28 g

Niels Bohr Discovered that electrons occupy specific rings around the nucleus, while orbiting around it. These specific rings were moving outwards based on energy.

Atomic Number Number of protons in the nucleus Can be found of top of the symbol Every element has a different atomic number since every element has a different number of protons in the nucleus (it is the atom’s fingerprint)

A NEUTRAL ATOM HAS THE SAME NUMBER OF PROTONS AND ELECTRONS

Mass Number Total number of protons and neutrons in the nucleus Cannot be found on the periodic table # of neutrons = mass # - atomic #

Shorthand Notation X – Symbol A – Mass number (protons + neutrons) Z – Atomic number (protons)

Isotopes ISOTOPES ARE ATOMS WITH THE SAME NUMBER OF PROTONS (AND ELECTRONS) BUT DIFFERENT NUMBER OF NEUTRONS

Isotopes The element hydrogen has 3 isotopes H ( __ proton, __ electron, __ neutron)

Isotopes ISOTOPES OF AN ELEMENT ARE CHEMICALLY THE SAME SINCE THE ATOMS’ CHEMICAL PROPERTIES ARE DETERMINED BY THE NUMBER OF ELECTRONS.

Average Atomic Mass This is an average of the isotopes’ masses Can be found below the symbol To calculate the atomic mass of an element: Atomic mass = (abundance x mass) + (abundance x mass) + … The units are amu, atomic mass unit

Average Atomic Mass Naturally occurring chlorine is 75.78% Cl-35, which has an atomic mass of 34.969 amu, and 24.22% Cl-37, which has an atomic mass of 36.966 amu. Calculate the average atomic mass of chlorine.

Ions Ions are atoms or groups of atoms that have a positive or negative charge Ions are formed when an atom or groups of atoms gains or loses ELECTRONS

Cations Metallic elements tend to form ions by LOSING ONE OR MORE ELECTRONS What charge will metallic elements have? Ions with a _________ charge are known as CATIONS

Anions Nonmetallic elements tend to form ions by GAINING ONE OR MORE ELECTRONS What charge will nonmetallic elements have? Ions with a _________ charge are known as ANIONS

REMEMBER THE NUMBER OF PROTONS IN THE NUCLEUS DETERMINES THE IDENTITY OF AN ATOM.

Chemical vs. Nuclear During a CHEMICAL reaction only electrons are exchanged, therefore the atom’s identity remains unchanged. During a NUCLEAR reaction an atom of an element may change into the atom of another elements.

Nuclear Reactions An atom with an unstable nucleus is a radioactive atom. A radioactive atom emits radiation because it is unstable. The unstable nucleus loses energy by emitting radiation in a spontaneous process called radioactive decay.

Types of Radiation Property Alpha Beta Gamma Composition Symbol Charge Helium Nucleus Electron High-energy electromagnetic radiation Symbol α β γ Charge 2+ 1- Penetration Low Medium High Shielding Paper, clothes Metal foil Lead, concrete (incomplete)