4.2 The Structure of

an Atom

Atom is the smallest particle of an

element. Atoms cannot be divided

into anything smaller.

What is an atom made of?

• Three subatomic particles:–Proton–Neutron–electron

+ PROTONS +• Positively charged subatomic particle• Found in the nucleus of an atom–Center of the atom

• Each proton is assigned a charge of 1+. • Each nucleus must contain at least one

proton.• Mass is equal to the mass of a neutron

- ELECTRONS -• negatively charged subatomic

particle• found in the space outside the

nucleus. – Electron cloud

• Each electron has a charge of 1.• Mass is much smaller than the mass

of protons & neutrons.– 2000 electrons equal the mass of one proton

NEUTRONS• Neutral subatomic particle–Neither negative or positive

• Found in the nucleus of an atom. –Center of the atom.

• Mass almost exactly equal to that of a proton.

Atomic Number• Equals the number of protons and

electrons• Each element has a unique atomic

number.Atomic Number13

AlAluminum 29.982

Mass Number• The mass number of an atom is the sum of

the protons and neutrons in the nucleus of that atom.

13Al

Aluminum 29.982 Mass Number

…How do you find the number of neutrons?

13Al

Aluminum 26.982 Mass Number

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

Isotopes

What is an Isotope?

• Atoms with the same number of protons and electrons but different numbers of neutrons• Many elements have two or more

isotopes–Stable isotopes–Unstable isotopes

Isotopes

• Can distinguish one form another by looking at the mass number:– Each number represents the mass number• Oxygen has 3 isotopes: Oxygen-16, Oxygen-17, and

Oxygen-18• Carbon has 3 isotopes: Carbon-12, Carbon-13, and

Carbon-14

Stable Isotopes

• An element whose nucleus does not spontaneously give off particles or energy

• Of the known chemical elements, 80 elements have at least one stable nuclei. – These comprise the first 82 elements

from hydrogen to lead, with the two exceptions, technetium (element 43) and promethium (element 61), that do not have any stable nuclei.

• Tin has ten stable isotopes

Unstable Isotope• An element whose nucleus decomposes, or

decays, by losing particles and energy.

• Radioactive• The energy or particles that are emitted from the nucleus is called radiation• 3 Types of radiation: Alpha, Beta,

Gamma• Used to determine the age of fossils

Example of Isotopes• Carbon has three isotopes and they are

carbon 12, carbon 13 and carbon 14. – Carbon 12 & 13 are stable isotopes – Carbon 14 is an unstable isotope

Isotope NotationHow to write isotopes

• 1st-Element name-mass number– Examples• Oxygen has 3 isotopes: Oxygen-16, Oxygen-17, and

Oxygen-18 • Carbon has 3 isotopes: Carbon-12, Carbon-13, and

Carbon-14

• 2nd-

Calculating Isotopes• You will use the mass number and the atomic number

of the isotope to determine the number of neutrons in the nucleus.• Neutrons=Mass number-atomic number

• Example:– Oxygen has 3 isotopes:

• Oxygen-16• Oxygen-17 Each of these numbers represents the mass number

• Oxygen-18

– Oxygen’s atomic number=8– How many neutrons does each isotope of oxygen

have?

Calculating IsotopesHow many neutrons does each isotope below have?