The Structure of the AtomChapter 4.2

Bell Ringer1. The ancient Greek philosopher Democritus coined what word for a tiny piece of

matter that cannot be divided?a) Elementb) Electronc) Atomd) Molecule

2. Dalton’s theory (~1800; based on behavior of gasses) included all but one of the following points. Which is not from Dalton?

a) All elements are composed of atoms.b) Most of an atom’s mass is in its nucleus.c) Compounds contain atoms of more than one element.d) In a specific compound, atoms of different elements always combine in the same

way.3. J. J. Thomson’s gas tube electrical experiments (~1897) provided first evidence of:

a) Atomsb) Nucleusc) Subatomic particlesd) Elements

4. Rutherford’s gold foil experiment (~1914) provided evidence for one of these:a) Negative and positive charges are spread evenly throughout an atom.b) Alpha particles have a positive charge.c) Gold is not as dense as previously thought.d) There is a dense, positively charged mass in the center of an atom.

Properties of Subatomic Particles

J.J. Thomson was the first to detect subatomic particles (later named electrons)

Electron: › Negatively charged subatomic particle.› Found in the space outside the nucleus.› Very light mass (~1/2000 of Hydrogen

atom)› Charge of 1-.

Thomson’s ExperimentsIn his experiments, J. J. Thomson used a sealed tube containing a very small amount of gas.

Thomson’s Model of the Atom

Sealed tube filled with gas at low pressure

Glowing beam

Metal disk Source of electric current

Metal disk

Metal disk Source of electric current

Positive plate

Negative plate

Properties of Subatomic Particles

Rutherford found positive charge varies among the elements. › Based on experiments with gold and

other elements. Proton:

› Positively charged subatomic particle› Found in the nucleus of an atom. › Charge of 1+. › Each nucleus contains at least one

proton.

The Gold Foil Experiment

Rutherford’s Atomic Theory

Beam of alpha particles

Source of alpha particles

Slit

Deflected particle

Undeflected particle

Screen

Gold atoms

Alpha particles

Alpha particles

Nucleus

Properties of Subatomic Particles

James Chadwick 1932 - the English physicist carried out an experiment to show that neutrons exist.› Radioactive decay produced particles about

the mass of a proton but without any charge.› Proved the particles produced were neutral

because a charged object did not deflect their paths.

3) Neutron:› Neutral (no charge) subatomic particle › Found in the nucleus of an atom.› Mass ~ equal to that of a proton.

Everything we know about subatomic particles is based on how the particles behave in experiments.

› Scientists still do not have an instrument that can show the inside of an atom.

Comparing Subatomic Particles

Comparing Subatomic Particles

Mass• Protons and neutrons have almost the same

mass.Mass of ~2000 electrons = the mass of one

proton.

Charge• An electron charge (-1) is equal in size but the

opposite sign of a proton charge (+1). • Neutrons have no charge.

Location in the atom • Nucleus - Protons and neutrons • Outside the nucleus - electrons

Subatomic Particles - Protons

Atoms of different elements have different numbers of protons.

Atomic Number = Number of protons in an atom of that element.

For any given element: All atoms have same atomic number.

› Hydrogen has 1 proton; atomic number = 1. › Helium has 2 two protons; atomic number = 2› Etc.

Each element has a unique atomic number.

Atomic Number and Mass NumberEach element has a different atomic number.

›A The atomic number of sulfur (S) is 16.›B The atomic number of iron (Fe) is 26.›C The atomic number of silver (Ag) is 47.

Atomic Number and Mass Number Atoms are neutral when:

• Each positive charge in an atom is balanced by a negative charge. i.e. # of protons = # of electrons Each proton has an electron to offset its charge

Examples:• Hydrogen atomic number = 1

Each neutral hydrogen atom has 1 proton & 1 electron.

• Sulfur atomic number = 16 Each neutral sulfur atom has 16 protons & 16

electrons.

Atomic Number and Mass Number Mass Number = Sum of protons +

neutrons. To find # of neutrons (when mass # &

atomic # are known): # of neutrons = mass # - atomic #

Example: Aluminum with mass # = 27 Mass # = 27 Atomic # = 13. 27 - 13 protons = 14 neutrons

Atomic Number and Mass Number

Charged Atoms

An atom becomes charged when it looses or gains an electron (e-).› Positive Charge = loss of e-’s

H becomes H+1

› Negative Charge = gaining of e-’s H becomes H-1

Electrons are the only subatomic particle that can be gained or lost.

Isotopes Isotopes = same element; different #

neutrons

Isotopes of an element:1)Same # of protons

• i.e. same atomic number

2)Different # of neutrons• Different atomic mass because of the

neutrons

Isotopes Isotopes are referred to by mass

number› For example, oxygen has 3 isotopes:

“oxygen 16”, “oxygen 17”, and “oxygen 18”

All three oxygen isotopes have same reactions

› Examples: All combine with hydrogen to form

water All combine with iron to form rust.

Isotopes

Most elements: Physical and chemical properties of all isotopes are very similar.

Hydrogen is an exception. ›Hydrogen-1 no neutrons (most H is hydrogen-1) ›Hydrogen-2 has one neutron›Hydrogen-3 has two neutrons.

Because a hydrogen-1 atom has only one proton, adding a neutron doubles its mass.

Heavy Water - hydrogen-2 atoms & oxygen-16

Hydrogen-2 atoms twice the mass of Hydrogen-1› Properties of heavy water are different from the

properties of ordinary water.

Isotopes

Assessment Questions

1) Of the subatomic particles that form the atom, the one with the smallest mass is:

a) Neutron. b) Electronc) Protond) Nucleus

Assessment Questions

2) Of the subatomic particles that form the atom, the one with the positive charge is:

a) Neutron. b) Electronc) Protond) Nucleus

Assessment Questions

3) Of the subatomic particles that form the atom, the one with the neutral charge is:

a) Neutron. b) Electronc) Protond) Nucleus

Assessment Questions

4) Of the subatomic particles that form the atom, which ones are found in the nucleus?

a) Neutron. b) Electronc) Protond) None

Assessment Questions

5) In which way do isotopes of an element differ? a. number of electrons in the atomb. number of protons in the atomc. number of neutrons in the atomd.net charge of the atom