Chemistry: Atoms

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Atoms and the

Periodic Table

Early Ideas About Matter- The ancient Greeks tried to explain matter, but the scientific study of the atom began with John Dalton in the early 1800’s.

Defining the Atom-Defining the Atom-An atom is made of a nucleus containing An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus protons and neutrons; electrons move around the nucleus of an atom. of an atom.

An atom is the smallest quantity of matter that still retains the properties of matter.

An element is a substance that cannot be broken down into two or more simpler substances by any means.Examples: gold, oxygen, helium

A DVD collection can be separated into smaller numbers until you have just one DVD left. But a single DVD cannot be separated into smaller pieces that are still DVDs.

Atoms can also be divided smaller and smaller until eventually only a single atom remains. Dividing it any smaller would give pieces that are no longer an atom.

ElectronsElectrons (-) charge no mass located outside the nucleus

ProtonsProtons (+) charge 1 amu located inside the nucleus

NeutronsNeutrons no charge 1 amu located inside the nucleus

Particles in the Atom

ElectronsThe particles that Thomson detected were later named electrons. An electron is a negatively charged subatomic particle that is found in the space outside the nucleus. Each electron has a charge of 1.

Properties of Subatomic Particles

Protons Based on experiments with elements other than gold, Rutherford concluded that the amount of positive charge varies among elements. A proton is a positively charged subatomic particle that is found in the nucleus of an atom. Each proton is assigned a charge of 1+. Each nucleus must contain at least one proton.

Properties of Subatomic Particles

NeutronsIn 1932, the English physicist James Chadwick carried out an experiment to show that neutrons exist. Chadwick concluded that the particles he produced were neutral because a charged object did not deflect their paths. A neutron is a neutral subatomic particle that is found in the nucleus of an atom. It has a mass almost exactly equal to that of a proton.

Properties of Subatomic Particles

Everything scientists 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

Here are some similarities and differences between protons, electrons, and neutrons.

• Protons and neutrons have almost the same mass. About 2000 electrons equal the mass of one proton.

• An electron has a charge that is equal in size to, but the opposite of, the charge of a proton. Neutrons have no charge.

• Protons and neutrons are found in the nucleus. Electrons are found in the space outside the nucleus.

Comparing Subatomic Particles

How Atoms Differ- The number of protons and the mass number define the type of atom.

Atomic Number• The atomic number of an element is the

number of protons in an atom of that element.

• Atomic number = number of protons = number of electrons.

• Mass number = atomic number + number of neutrons.

• All atoms of any given element have the same atomic number. Each hydrogen atom has one proton in its nucleus. Hydrogen is assigned the atomic number 1.

• 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 NumberAtoms are neutral, so each positive charge in an atom is balanced by a negative charge. That means the atomic number of an element also equals the number of electrons in an atom of that element.

• Hydrogen has an atomic number of 1, so a hydrogen atom has 1 electron.

• Sulfur has an atomic number of 16, so a sulfur atom has 16 electrons.

Atomic Number and Mass NumberMass NumberThe mass number of an atom is the sum of the protons and neutrons in the nucleus of that atom. To find the number of neutrons in an atom, you need the mass number of the atom and its atomic number.

The atomic number of aluminum is 13. An atom of aluminum that has a mass number of 27 has 13 protons and 14 neutrons

Structure of the Atom

There are two regionsThe nucleus

• With protons and neutrons

– Positive charge– Almost all the mass

Electron cloud– Most of the volume of an atom– The region where the electron can be found

Subatomic Particles

POSIT IVECHARG E

PROT ONS

NEUT RALCHARG E

NEUT RONS

NUCLEUS

NEG AT IVE CHARG E

ELECT RONS

AT OM

Most of the atom’s mass.

NUCLEUSNUCLEUS ELECTRONSELECTRONS

PROTONSPROTONS NEUTRONSNEUTRONS Negative Charge

PositiveCharge

NeutralCharge

ATOM

QUARKSAtomic Numberequals the # of...

equal in a neutral atom

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Subatomic Particles

• Proton - positive electrical charge (1) and mass of 1 amu

• Neutron - no electrical charge and mass of 1 amu

• Electron - negative electrical charge (-1) and negligible mass

Crude Atom Model

Atomic Structure (Refined)

• The electrons move continuously through differently shaped clouds of space (electron energy levels) that lie at specific distances from the nucleus (see figures)

Figure 3.5A

What are atoms made of?

Taylor 2010

Protons: positively charged particles that are housed in the nucleus of an atom and have significant mass

Neutrons: neutral particles that are housed in the nucleus. They act to hold the protons in place since like charges repel each other. Neutrons have significant mass

Electrons have negligible mass, have a negative charge and are allowed to roam freely in the electron cloud so they take up significant volume in the atom

Atomic Number, and Mass NumberAtomic Number, and Mass Number

Elemental symbolXZA

All atoms can be identified by the number of protons and neutrons they contain.

The atomic number (Z) is the number of protons in the nucleus.Atoms are neutral, so it’s also the number of electrons.Protons determine the identity of an element. For example, nitrogen’s atomic number is 7, so every nitrogen has 7 protons.

The mass number (A) is the total number of protons and neutrons.Protons and neutrons are collectively referred to as nucleons.

Mass number(number of protons + neutrons)

Atomic number(number of protons)

I can identify an atom and the number of its subatomic particles using a periodic table.

Complete the chart below. Use your periodic table and the following reminders:Atomic mass = Number of protons and neutrons in a nucleus.

Protons = Atomic numberNeutrons = Atomic mass – atomic number

Electrons = Atomic number