Question

in the modern periodic table elements are arranged by: *

atomic mass

atomic number

valence electrons

number of isotopes

which is a halogen? *

helium (he)

chlorine (cl)

oxygen (o)

cerium (ce)

Explanation:

• For the first question: In the modern periodic table, elements are arranged by atomic number. This is a fundamental concept in chemistry. Atomic number is defined as the number of protons in an atom's nucleus.
• For the second question: Halogens are elements in Group 17 of the periodic table. Chlorine (Cl) is a halogen. Helium (He) is a noble gas (Group 18), oxygen (O) is in Group 16, and cerium (Ce) is a lanthanide (a type of rare - earth element).

Answer:

• First question: atomic number
• Second question: Chlorine (Cl)