Atomic Structure and the Periodic Table
Grade 9 · Science · Study Guide
Discover how atoms are built and how the periodic table organizes all known elements by their properties.
Every piece of matter you can touch, taste, or breathe is made of atoms. An atom is the smallest unit of an element that retains the chemical properties of that element. At the center of every atom is a dense nucleus containing positively charged protons and neutral neutrons. Surrounding the nucleus are negatively charged electrons arranged in energy levels called electron shells. In a neutral atom, the number of protons always equals the number of electrons.
The atomic number of an element equals its number of protons — this is what makes one element different from another. Carbon always has 6 protons; oxygen always has 8. The mass number equals protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons, giving them different mass numbers but identical chemical behavior.
The periodic table is one of science's greatest organizational tools. Elements are arranged in order of increasing atomic number. Rows are called periods and columns are called groups or families. Elements in the same group share similar chemical properties because they have the same number of valence electrons — the electrons in the outermost shell. Metals occupy the left and center of the table, nonmetals the upper right, and metalloids form a staircase boundary between them.
| Particle | Charge | Location | Relative Mass |
|---|---|---|---|
| Proton | +1 | Nucleus | 1 amu |
| Neutron | 0 | Nucleus | 1 amu |
| Electron | -1 | Electron shells | ~1/1836 amu |
| Group Name | Group Number | Valence Electrons | Example |
|---|---|---|---|
| Alkali Metals | 1 | 1 | Sodium (Na) |
| Alkaline Earth Metals | 2 | 2 | Calcium (Ca) |
| Halogens | 17 | 7 | Chlorine (Cl) |
| Noble Gases | 18 | 8 | Argon (Ar) |
Key terms
- Atom
- The smallest particle of an element that retains the element's chemical properties, made of protons, neutrons, and electrons.
- Atomic Number
- The number of protons in the nucleus of an atom; uniquely identifies each element.
- Mass Number
- The total number of protons and neutrons in an atom's nucleus.
- Isotope
- Atoms of the same element that have the same number of protons but different numbers of neutrons.
- Valence Electrons
- Electrons in the outermost energy level of an atom; determine chemical reactivity.
- Period
- A horizontal row in the periodic table; elements in the same period have the same number of electron shells.
- Group/Family
- A vertical column in the periodic table; elements share the same number of valence electrons and similar properties.
Flashcards
- Atomic Number
- Number of protons in an atom; identifies the element.
- Nucleus
- Dense center of an atom containing protons and neutrons.
- Isotope
- Same element, different number of neutrons.
- Valence Electrons
- Outermost electrons; determine how an atom bonds.
- Noble Gases
- Group 18 elements with full valence shells; very unreactive.
- Metal
- Element that is shiny, conducts electricity, and is malleable; found on the left side of the periodic table.
Practice questions
What determines the identity of an element?
- Number of neutrons
- Number of protons
- Number of electrons
- Mass number
Show answer
Answer: Number of protons
The atomic number — the count of protons — uniquely identifies each element. Change the proton count and you have a different element.
An atom of carbon-14 has 6 protons. How many neutrons does it have?
- 6
- 8
- 14
- 20
Show answer
Answer: 8
Mass number (14) minus protons (6) equals neutrons (8). Carbon-14 is a radioactive isotope used in carbon dating.
Elements in the same group of the periodic table have similar chemical properties.
Show answer
Answer: True
Groups (vertical columns) share the same number of valence electrons, which drives similar chemical behavior — for example, all alkali metals react vigorously with water.
The electrons in the outermost shell of an atom are called ___ electrons.
Show answer
Answer: valence
Valence electrons occupy the outermost energy level and are the ones involved in forming chemical bonds.
Which subatomic particle has a negative charge?
- Proton
- Neutron
- Electron
- Nucleus
Show answer
Answer: Electron
Electrons carry a -1 charge. Protons carry +1 and neutrons carry 0. In a neutral atom, proton count equals electron count.
Fun facts
- If an atom were the size of a football stadium, the nucleus would be about the size of a marble at the center.
- Hydrogen is the most abundant element in the universe, making up about 75% of all normal matter by mass.
- The periodic table was independently organized by Dmitri Mendeleev and Lothar Meyer in 1869 — Mendeleev even left gaps for elements not yet discovered, and he was right.
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