Atomic Structure and Periodic Trends
Grade 10 · Science · Study Guide
Explore the structure of atoms and how atomic properties change across the periodic table.
Atoms are the fundamental building blocks of matter, consisting of a nucleus containing protons and neutrons, surrounded by electrons in energy levels called shells or orbitals. The number of protons defines the element and is called the atomic number, while the mass number is the sum of protons and neutrons. Electrons occupy regions of space called orbitals, organized into sublevels (s, p, d, f) and energy levels.
The periodic table arranges elements by increasing atomic number, revealing repeating patterns called periodic trends. These trends arise because as we move across a period, the nuclear charge increases while electrons are added to the same shell. Moving down a group adds new electron shells, increasing the atomic radius and reducing the effective nuclear pull on outer electrons.
Key trends include atomic radius (decreases across a period, increases down a group), ionization energy (increases across a period, decreases down a group), and electronegativity (increases across a period, decreases down a group). Understanding these trends allows chemists to predict how elements will behave in chemical reactions.
| Property | Across Period (left→right) | Down Group (top→bottom) |
|---|---|---|
| Atomic Radius | Decreases | Increases |
| Ionization Energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic Character | Decreases | Increases |
Key terms
- Atomic number
- The number of protons in the nucleus of an atom; defines the element.
- Mass number
- Total number of protons and neutrons in an atom's nucleus.
- Ionization energy
- The energy required to remove an electron from a neutral gaseous atom.
- Electronegativity
- A measure of an atom's tendency to attract electrons in a chemical bond.
- Atomic radius
- Half the distance between the nuclei of two identical bonded atoms; a measure of atomic size.
- Electron configuration
- The arrangement of electrons in an atom's orbitals, described by energy level and sublevel.
Flashcards
- Atomic number
- Number of protons; determines the element's identity.
- Isotope
- Atoms of the same element with different numbers of neutrons.
- Period
- A horizontal row of the periodic table; elements share the same number of electron shells.
- Group
- A vertical column of the periodic table; elements share similar chemical properties.
- Valence electrons
- Electrons in the outermost shell; determine chemical reactivity.
- Shielding effect
- Inner electrons reduce the effective nuclear charge felt by outer electrons.
Practice questions
Which of the following increases as you move from left to right across Period 3?
- Atomic radius
- Ionization energy
- Metallic character
- Number of electron shells
Show answer
Answer: Ionization energy
Ionization energy increases left to right across a period because nuclear charge increases while electrons are added to the same shell, pulling electrons closer and making them harder to remove.
The atomic radius of potassium (K) is larger than the atomic radius of lithium (Li).
Show answer
Answer: True
Potassium is below lithium in Group 1; down a group, atomic radius increases as new electron shells are added.
The number of protons in an atom's nucleus is called the ___ number.
Show answer
Answer: atomic
The atomic number equals the number of protons and uniquely identifies an element on the periodic table.
An atom of carbon-14 has 6 protons and how many neutrons?
- 6
- 8
- 14
- 12
Show answer
Answer: 8
Carbon-14 has mass number 14. Mass number = protons + neutrons. 14 - 6 protons = 8 neutrons.
Electronegativity generally decreases as you move down a group in the periodic table.
Show answer
Answer: True
Moving down a group, each element has more electron shells and greater shielding, reducing the nucleus's pull on bonding electrons, so electronegativity decreases.
Fun facts
- Hydrogen is the most abundant element in the universe, making up about 75% of all normal matter by mass.
- Francium, in the bottom-left of the periodic table, is so reactive it has never been collected in visible quantities.
- The periodic table was largely devised by Dmitri Mendeleev in 1869, who left gaps for undiscovered elements — and was proven right.
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