Name: __________________________________ Date:__________________________ P___R___S_____
Honors Chemistry / CPA Chemistry
Atom & Subatomic Particles
Objectives: 1) describe an atom.
2) describe the 3 major subatomic particles and their properties.
What is an atom?
Matter has both mass and volume, and it is made up of atoms. An atom is the smallest part of an element
that still retains the properties of that element. Atoms are basic building blocks of matter, and cannot be
chemically subdivided by ordinary means.
Structure of an Atom:
What are subatomic particles of an atom?
Atoms are composed of three types of particles (subatomic particles): protons (p+), neutrons (no), and
electrons (e-) that affect the properties of atoms.
Compare subatomic particles by
1) Particle mass:
 protons and neutrons have about the same mass.
 The mass of an electron is much less than the mass of protons and neutrons.
2) Particle charge:
 Protons have a positive charge
 Neutrons have no charge
 Electrons have a negative charge
 Net charge of the nucleus is positive and equal to the number of protons
Atoms are neutrally charged when number of electrons is the same as the number of protons.
3) Particle Location:
 Protons and neutrons are tightly bound in a tiny nucleus in the center of the atom
 Electrons move in complicated patterns in the space around the nucleus (electron cloud).
Identity of the Atom
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The number of protons determines the identity of an atom (an element)
Atoms of the same element have the same number of protons, but the number of neutrons may vary.
An atom of a given element may lose or gain electrons yet it still remains the same element.
Mass of the Atom
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The total number of protons and neutrons within its nucleus is a major determinant for the mass of
the atom
Reactivity of the Atom
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Chemical reactions occur because the electrons around the atoms are exchanged or shared.
Atomic Number
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The atomic number of an element is equal to the number of protons and is always the same for a
given element.
The atomic number of an element is a whole number which can be found on the periodic table.
Atomic
Number
Mass Number
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Atoms of the same element may have different numbers of neutrons.
The mass number of a particular atom is the sum of that atom’s protons and neutrons.
The mass number is not the same as the average atomic mass on the periodic table. The mass
number is always a whole number and must be given through words or a symbol.