Chemestry - atoms

Q1. Why does the atomic number define an element?

A. Because the atomic number equals the number of protons, and changing the number of protons changes the element.

Q2. Why do electrons not significantly affect the mass of an atom?

A. Because electrons have very little mass compared to protons and neutrons.

Q3. Where is most of the mass of an atom located, and why?

A. In the nucleus, because it contains protons and neutrons which have much greater mass than electrons.

Q4. Can two atoms have the same mass number but be different elements? Explain.

A. Yes. If they have different numbers of protons, they are different elements even if their mass numbers are the same.

Q5. An atom has 17 protons and 18 neutrons. Identify the element and isotope.

A. Chlorine-35.

Q6. An atom has 13 electrons and a 3+ charge. How many protons does it have?

A. 16 protons.

Q7. Why does changing the number of neutrons not change the element?

A. Because neutrons do not affect the atomic number, which defines the element.

Q8. Define isotopes using two key conditions.

A. Same number of protons, different number of neutrons.

Q9. Why do isotopes of the same element have similar chemical properties?

A. Because they have the same electron configuration.

Q10. Explain why relative atomic mass is often not a whole number.

A. Because it is a weighted average of the masses of all naturally occurring isotopes.

Q11. Write the electron configuration for an element in Group 2, Period 4.

A. 2,8,8,2 (Calcium).

Q12. How does electron configuration determine the group of an element?

A. The number of valence (outer-shell) electrons equals the group number (for main-group elements).

Q13. Why are noble gases chemically unreactive?

A. Because they have full outer electron shells.

Q14. An element has the configuration 2,8,7. Predict its reactivity.

A. Highly reactive, because it needs one electron to complete its outer shell.

Q15. Why do metals usually form positive ions?

A. Because they lose electrons to achieve a full outer shell.

Q16. Why do non-metals usually form negative ions?

A. Because they gain electrons to achieve a full outer shell.

Q17. Explain why ions are more stable than atoms in many cases.

A. Because ions have a full outer shell of electrons.

Q18. Explain the relationship between period number and electron shells.

A. The period number equals the number of occupied electron shells.

Q19. Why do elements in the same group have similar chemical properties?

A. Because they have the same number of valence electrons.

Q20. Predict the group of an element with five valence electrons.

A. Group 15.