Atomic Structure CHAPTER 4

Defining the Atom ✴ An atom is the smallest particle of an element that still has the chemical properties of that element ✴ The Greek philosopher Democritus (460 BC –370 BC) was among the first to suggest the existence of atoms ✴ Reasoned that atoms were indivisible and indestructible

Dalton’s Atomic Theory ✴ The modern process of discovery with atoms began with John Dalton (1766–1864) ✴ Used experimental methods, and transformed Democritus’s ideas on atoms into a scientific theory ✴ Studied the ratios in which elements combine in chemical reactions (Dalton’s Atomic Theory)

Daltons Atomic Theory 1.All elements are composed of tiny indivisible particles called atoms. Atoms of element A 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of element A Atoms of element B

Dalton’s Atomic Theory 3.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4.Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations Atoms of one element are never changed into atoms of another element Compound made by chemically combining atoms of elements A and B Mixture of atoms of elements A and B

Sizing up the Atom ✴ Every drop, no matter its size has the same properties ✴ A pure copper coin the size of a penny contains about 2.4 × atoms ✴ 24,000,000,000,000,000,000,000 atoms

Sizing up the Atom ✴ Individual atoms can be observed with an instrument known as a scanning electron microscope Toilet Paper Foot of a House Fly

Structure of the Atom

Subatomic Particles ✴ There are THREE types of subatomic particles: 1. Protons 2. Neutrons 3. Electrons

Electrons ✴ In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron ✴ Electrons are negatively charged subatomic particles Cathode Ray Tube

Protons and Neutrons ✴ In 1886, Eugene Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays ✴ Protons are positively charged subatomic particles ✴ In 1932, the English physicist James Chadwick (1891– 1974) confirmed the existence of the neutron ✴ Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton

Properties of Subatomic Particles ParticleSymbol Relative charge Relative mass (mass of proton = 1) Actual mass (g) Electrone–e– 1–1/  10 –28 Protonp+p  10 –24 Neutronn0n  10 –24 The table below summarizes the properties of these subatomic particles:

How are these three subatomic particles (protons, neutrons, and electrons) put together in an atom?

✴ Most scientists thought that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material ✴ J.J Thomson’s model was known as the plum pudding model ✴ Short-lived model

Ernest Rutherford’s Gold Foil Experiment In the experiment, a narrow beam of alpha particles was directed at a very thin sheet of gold ✴ Rutherford’s results were that most alpha particles went straight through, or were slightly deflected ✴ A small fraction of the alpha particles bounced off the gold foil at very large angles

Rutherford’s Atomic Model (The Nuclear Atom) 1. Proposed that the atom is mostly empty space 2. All the positive charge and almost all of the mass are concentrated in a small positively charged region (nucleus) 3. Protons and neutrons are located in the positively charged nucleus ✴ The electrons are distributed around the nucleus and occupy almost all the volume of the atom

✴ According to this model, the nucleus is tiny and densely packed compared with the atom as a whole ✴ If an atom were the size of a football stadium, the nucleus would be about the size of a marble

REVIEW 1. What subatomic particle it located in the nucleus and positively charged? 2. Where are the neutrons located and what is their charge? 3. What subatomic particle occupies almost all the volume of the atom? What is its’ charge? 4. Who discovered the electron? 5. Who is responsible for coming up with the plum pudding model? 6. Who discovered the nucleus and what two subatomic particles make up the nucleus?

Reminder..... ✴ An atom is the smallest particle of an element that still has the chemical properties of that element Distinguishing Among Atoms (What makes one element different from another?)

Atomic Number The atomic number identifies the element (like a social security number identifies you) ✴ An element’s atomic number is the number of protons in the nucleus of an atom of that element ✴ Atomic number = protons

Mass Number The total number of protons and neutrons in an atom is called the mass number Mass number = protons + neutrons

Atoms are electrically neutral therefore... In a neutral atom, protons = electrons ✴ Protons, neutrons, and electrons can be determined from the atomic number and mass number

PRACTICE How many protons, neutrons, and electrons are in each of the atoms: a. Be b. Ne c. Na d. Boron e. Fluorine f. Aluminum

Isotopes Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons ✴ Same number of protons, different mass number

IsotopeProtonsNeutronsElectrons Hydrogen-1 Hydrogen-2 Hydrogen-3 For Example: Hydrogen has 3 isotopes The correct way to represent an isotope is to write the chemical name followed by the mass number (protons + neutrons)

✴ Their color or size doesn’t change the fact that they are all dogs Just because the number of neutrons is changing does not change which element it is because the atomic number does not change

How are the atoms of one element different from the atoms of another element? How are isotopes of the same element different? REVIEW QUESTIONS:

The mass that is listed on the periodic table is an average atomic mass ✴ It is a weighted average of the atomic masses of naturally occurring isotopes Each isotope of an element has a fixed mass (amu) and a natural % abundance

Calculating Average Atomic Mass Chlorine-35 has an amu of with an abundance of % while Chlorine-37 has an amu of and an abundance of % Carbon-12 ( amu) makes up 98.93% of all of the carbon atoms, while carbon-13 ( amu) is about 1.07% abundant. What is the average atomic mass of Carbon?

PRACTICE TIME!